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Evolution and the levels of selection

Evolution and the levels of selection



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Reading Okasha's "Evolution and the levels of selection" he talks about "the levels of selection problem." There is a bit of a problem with this opening chapter because, while he talks about why the levels of selection problem is a problem, he doesn't define what the levels of selection problem is.

The opening line from the book…

"The levels of selection problem is one of the most fundamental in evolutionary biology, for it arises directly from the underlying logic of Darwinism. The problem can be seen as the upshot of three factors… "

I think what he means by the "levels of selection problem" is that selection acts on many levels, and what is adaptive at one level may be maladaptive at another. Therefore we can not define selection as acting at one single level, and studying it as such is likely to lead to incorrect conclusions - studying selection is not a simple process. Can anyone provide a firm definition of what Okasha, and the general community on the matter, mean by the "levels of selection problem"?

(Note: The factors are the abstract nature of the principles of selection, the hierarchical nature of biological organisation, and the process of adaptation via natural selection).


Reeve and Keller in the first chapter of Levels of Selection in Evolution (Princeton Univ. Press,1999) seem to address this question in the second item in the following paragraph.

The purpose of this volume is to sample current theoretical and empirical research on (1) how natural selection among lower-level biological units (e.g., organisms) creates higher-level units (e.g., societies), and (2) given that multiple levels exist, how natural selection at one biological level affects selection at lower or higher levels [boldface added]. These two problems together constitute what Leigh (chap. 2) calls the "fundamental problem of ethology." Indeed, as Leigh further suggests, they could be viewed jointly as the "fundamental problem of biology," when genes and organisms are also included as adjacent levels in the biological hierarchy. This generalization has the desirable property of immediately removing the long-standing conceptual chasm between organismal and molecular biologists.

The authors suggest definitions of fitness that account for hierarchical relationships (the level of selection problem) by considering the effect of a trait (nepotism among social insects) on "adjacent" biological levels, such as individual and colony.

By measuring what they call 'absolute fitness force' they propose to eclipse what they see as spurious debate over the fundamental level of selection (genes vs. individuals, say).

A somewhat surprising aspect of their discussion emerges here:

"It is still embarrassingly common to read inaccurate statements… that frogs have to produce many eggs to ensure the survival of the species because tadpoles suffer extremely high rates of predation, or that wolves have evolved ritualized displays to establish dominance hierarchies because physical combats would be too disadvantageous for the species. These naive statements betray a widespread and persistent misunderstanding of the level at which natural selection most commonly operates."

Their point is that such statements focus on the wrong level of selection (group), and that stronger selection may occur at the individual level; their overall argument being that both levels should be factored in.

I cannot present this as a 'community definition' but it may be a representative example.


9.2: Darwin, Wallace, and the Theory of Evolution by Natural Selection

  • Contributed by Suzanne Wakim & Mandeep Grewal
  • Professors (Cell Molecular Biology & Plant Science) at Butte College

The Grand Canyon, shown in Figure (PageIndex<1>), is an American icon and one of the wonders of the natural world. It is also a record of the past. Look at the rock layers in the picture. If you were to walk down a trail to the bottom of the canyon, with each step-down, you would be taking a step back in time. That&rsquos because lower layers of rock represent the more distant past. The rock layers and the fossils they contain show the prehistory of the region and its organisms over a 2-billion-year time span. Although Charles Darwin never visited the Grand Canyon, he saw rock layers and fossils in other parts of the world. They were one inspiration for his theory of evolution. Darwin&rsquos theory rocked the scientific world. In this concept, you will read why.

Figure (PageIndex<1>): Grand Canyon


Introduction

Climate change affects various aspects of biodiversity across the planet (e.g., [1, 2]). In particular, shifts in phenotypic distributions within populations are widely reported, for a variety of morphological, phenological, or life-history traits [2–4]. Surprisingly, however, little is still known about the relative contributions of mechanisms underlying these shifts [5]. Within a population, phenotypic distributions may change due to a change in population structure (e.g., age structure or sex ratio), due to phenotypic plasticity (within or between individuals), and due to genetic change [6–8]. The exact mixture of mechanisms driving phenotypic change will determine the future of a population facing a prolonged change in environmental conditions [9], for several reasons. First, the consequences of changing population structure are variable and may be idiosyncratic (e.g., [8, 10]). Second, phenotypic plasticity can provide an efficient way to cope with a changing environment, but its effect may be short-lived and even maladaptive [11–13]. Third, genetic evolution, when driven by natural selection, can improve population growth rate, potentially contributing to long-term population persistence [12].

In wild populations, the respective contributions of plasticity versus evolution remain unknown for the vast majority of documented phenotypic changes [14, 15] (note that by evolution we mean genetic change, here and in the rest of the manuscript). To date, most of the evidence for evolutionary responses to climate change comes from plants [16]. In contrast, despite numerous examples of phenotypic changes apparently related to climate, there have been surprisingly few examples demonstrating unambiguously that a vertebrate population is evolving in response to climate change (see discussions in [17–20]). This lack of evidence may, in part, be due to the question not being prioritized [14, 15]. However, it probably also reflects the substantial challenges inherent in testing for adaptive evolution, in terms of requirements for appropriate data and statistical methods. For wild populations in which experimental manipulations are not feasible, the most plausible means of testing for the genetic basis of phenotypic changes is to use long-term pedigree data to test for changes in “breeding values,” the estimated genetic merit of individuals as ascertained from the phenotypes of their relatives [21]. This needs to be done with care, as trends in predicted breeding values can be confounded with environmental trends unless appropriately controlled for [22], and the precision of estimates of evolutionary rates can be inflated if the correlation structure of breeding value estimates is not properly handled [23]. To our knowledge, among the studies of wild vertebrate populations that properly account for uncertainty in breeding value predictions, only 3 have found evidence of genetic change underlying phenotypic change in line with selection pressures changing with climate: plumage colouration in collared flycatchers [20], and body size in Siberian Jays [24] and snow voles [25]. However, only with more empirical studies explicitly testing for evolution will it become possible to say whether the current lack of evidence also reflects a generally slow rate of adaptation to environmental change in natural populations [26].

Climate change may have impacts on numerous aspects of an organism’s biology, but phenology (i.e., the seasonal timing of life-history events) appears to be particularly affected [3, 27–29]. Dramatic changes of phenologies in response to earlier onset of spring are particularly well documented in mid- and high-latitude passerines, where breeding times are occurring earlier in numerous populations and species [18, 30]. The study of avian systems in particular has shown that a fine-tuning of phenology to the climate is crucial in determining individual fitness. Mistiming between mean breeding date and a fitness optimum that shifts with climate may re-shape selective pressures and hence potentially reduce population growth rate [31, 32], although establishing the link between individual-level and population-level processes is challenging [33, 34]. The effects of climate change on mammalian phenology are less well documented and less clear than those of birds [29] and may be more complex because mammals’ long gestation times likely make their breeding phenology sensitive to climate across a longer timeframe [17]. Furthermore, despite the extensive evidence for phenotypic shifts in phenology, the few studies that test for a genetic basis to changes in phenology in wild populations have not found evidence of genetic changes [35–38]. One possible exception is the change of egg hatching date in winter moths [39], for which a common garden experiment suggested a contribution of genetic change.

In a population of red deer (Cervus elaphus, Linnaeus 1758) on the Isle of Rum, Northwest Scotland, parturition date has advanced at a rate of 4.2 days per decade since 1980, a change that has been linked to temperatures and other weather conditions in the year preceding parturition, especially around the time of conception [40, 41]. Previous studies of this population have shown that phenotypic plasticity in response to temperature and population structure explain a substantial proportion (23%) of the advance in parturition dates [41] and also that within-individual plasticity is sufficient to explain the population-level relationship between temperature and parturition date [42]. However, the documented plasticity does not explain the majority of the observed phenotypic change over time, leaving room for processes that have not been investigated as of yet. It is plausible that evolution plays a role because the observed phenotypic change is qualitatively consistent with a genetic response to selection: parturition date is heritable in this population [43] and also under selection for earlier dates [44].

In this study, we use quantitative genetic animal models [21, 45] to estimate the rate of evolution in parturition date and the contribution of plastic and demographic processes to the observed shift in phenology in the Rum red deer study population. We start by considering the response to selection that might be expected from the observed strength of selection and (narrow-sense) heritability of parturition date, based on a simple “breeder’s equation” prediction [46]. One of the most striking conclusions from the recent application of quantitative genetic theory in evolutionary ecology has been the failure of univariate “breeder’s equation” predictions to capture trait dynamics in wild populations [47, 48]. This may be for multiple reasons, foremost of which is likely to be the unrealistic assumption that only the focal trait is relevant. We therefore also consider a multivariate breeder’s equation [49] and ask how selection on offspring size and the genetic correlation between parturition date and size alters the expected evolutionary response. However, there is a second, less well-explored reason for the failure of the theory: predicted genetic responses to selection are often compared to observed rates of phenotypic change rather than of genetic change. Phenotypic changes are generally affected not only by genetic changes but also by numerous nongenetic processes and therefore may poorly reflect underlying genetic changes. As the central analysis of this work we use trends in breeding values and the secondary theorem of natural selection (STS) to estimate the rate of evolution in parturition date. We then test whether the estimated rate of evolution is compatible with the response to selection predicted by either the univariate or multivariate “breeder’s equation,” or with genetic drift. We also consider the effect of nongenetic processes contributing to phenotypic change and quantify the role of phenotypic plasticity in response to warming temperatures and of changes in population structure.


Group selection

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Group selection, in biology, a type of natural selection that acts collectively on all members of a given group. Group selection may also be defined as selection in which traits evolve according to the fitness (survival and reproductive success) of groups or, mathematically, as selection in which overall group fitness is higher or lower than the mean of the individual members’ fitness values. Typically the group under selection is a small cohesive social unit, and members’ interactions are of an altruistic nature. Examples of behaviours that appear to influence group selection include cooperative hunting, such as among lions and other social carnivores cooperative raising of young, such as in elephants and systems of predatory warning, such as those used by prairie dogs and ground squirrels.

The study of group selection has played an important role in informing other theories of selection and has shed light on the significance of altruistic behaviours observed in animals, including humans. However, it has been controversial since its introduction in the 19th century by British naturalist Charles Darwin. Often, selfless behaviours jeopardize the acting individuals’ fitness, possibly lowering their chances for leaving behind offspring. Darwin realized that this presented a problem for his theory of natural selection, for which the bearing and survival of offspring was a vital determinant of evolutionary success.

In the early 20th century, Darwin’s observations of group behaviour were explored by others in studies that focused on the evolution of certain physical traits and behaviours that appear to benefit social groups. But toward the middle of that century, following the rise of neo-Darwinism, in which Darwin’s theory of natural selection was synthesized with genetics (the modern evolutionary synthesis), the idea that selection acted on groups was largely dismissed. Many evolutionary biologists agreed that adaptation through selection at the level of the individual and the gene was of greater consequence than selection at the group level.


Lesson Plan on Evolution and Natural Selection

Instructional goals: The purpose of this lesson is for students to understand that evolution a long process, and is the result of a species need to adapt to survive in the environment it lives in. The activity also introduces students to the work done by Charles Darwin on his voyage to the Galapagos islands, 1831-1836. The driving question guiding this lesson plan is: Why do plant and animal appearances change over a long period of time?

Grade level: Grade 7, 8

Duration: 75-150 minutes

Instructional Materials:
1. Picture of Darwin’s Finches (see Appendix A)
2. Worksheet from Eureka B Worksheet Encyclopedia 10

QEP POLs for secondary cycle 1 relevant to the concept of Evolution:
1. Describes the stages in the evolution of living organisms
2. Explains the natural selection process

Children’s preconceptions relevant to the concept of Evolution: Adaptations arise in response to an environmental challenge and that this environmental pressure is what drives evolution.

Assessment Items to explore or uncover students’ preconceptions around the concept of Evolution:

Question 1. Present day giraffes have long necks because:
A. They stretch them to reach the trees for food.
B. Their ancestors adapted to have long necks over time.
C. Giraffes with the longest necks are the strongest and most perfect.
D. Their neck length increases their body temperature.
E. Their neck length increases their speed.

(Retrieved from MOSART, Life Science Survey Test, Item form # 921, Q21)

Question 2. During the course of evolution, it is often the case that a structure, such as a functional eye, is lost in an animal’s body. This is because .
A. It is no longer actively used.
B. Mutations accumulate that disrupt its function.
C, It interferes with other traits and functions.
D. The cost to maintain it is not justified by the benefits it brings

(Adapted from Biological Concepts Instrument, Q12)

Question 3. How can a catastrophic global event influence the course of evolution?
A. Unwanted versions of genes are removed.
B. New genes are generated.
C. Only some species may survive the event.
D. There are short term effects that disappear over time.

(Adapted from Biological Concepts Instrument, Q4)

Activity – Evolution and Natural Selection

Through this inquiry-based activity, titled Evolution and Natural Selection, students will understand that species adapt/evolve to survive in their specific habitats. In the following sections, we described the procedure of the activity and provide relevant discussion questions for teachers using this lesson plan.

Step 1: Eliciting Student Thinking/ Intuitive Models:
Began a discussion by telling the class that all dog breeds are descended from wolves and asking the class “If you had a bunch of wolves and wanted a Chihuahua, how would you create one?” In groups, students discussed the question. The teacher moves from group to group, eliciting answers from randomly selected students. Possible answers from students are:

  • Chihuahuas can be bred from wolves by selectively breeding small wolves with short faces and wiry tan hair for many generations.
  • Raising wolves in a warm environment so they will not need heavy fur and providing them with plenty of food so the wolves become less aggressive and develop smaller teeth. (Revealing the misconceptions that environment causes individuals and traits evolve from use or disuse).

Introduce the main activity by talking about Darwin’s voyage to the Galapagos Islands in 1831-1836: When Darwin arrived at the Galapagos, he found finches with small-sized beaks that fed mostly on insects living on Santa Maria Island finches with large-sized beaks that fed mostly on hard seeds on Pinta Island and finches with medium sized beaks that fed mostly on fruits on San Cristobal Island.
Next, have students look at the pictures of finches and ask: (1) What finch eats which food? and (2) Why do you think the beak sizes and shapes are specific to one island?. Looking at the photos and making the connection between food source, adaptation (beaks size), students come to realize that over time species adapt to survive.

Step 2: Collecting and Making Sense of Data:
Teacher asks students: What would happen if a volcanic eruption occurred and hard seeds became the only available food on Isabella Island (finches with large-sized beak are the only ones that can survive)?.
Then, teacher tells students that several finches moved to neighboring islands looking for new food. Students Teacher can also ask students what type of finches will survive on each island (the majority of finches on Pinta Island will have a large-sized beak, the majority of finches on San Cristobal Island will have a medium-sized beak, and the majority of finches on Santa Maria Island will have a small-sized beak).

Note: Teachers can create a table on a board and tally students’ response.

Teacher asks students to compare the distribution of finches on Pinta, San Cristobal and Santa Maria islands after the volcanic eruption, with what Darwin found (the finch populations are exactly the same). This question is leading students back to the concept that if there is huge disruption in an organism’s habitat, that the species who survive do so because they posses an adaptation that enhances survival. Of the finches that moved San Cristobal Island, only those with a medium-sized beak will survive because their beaks are best adapted to the food available on the island - fruit. Of the finches that moved to Santa Maria Island, only those with a small-sized beak will survive because their beak is suited to eating insects, the predominant food on the island. Finches that moved to Pinta Island would thrive if they had large-sized beaks, which are best suited to eating the hard seeds found on Pinta Island.

Step 3: Developing evidence-based explanations:
Teacher asks students to predict the relationships among the different types of finches. Students might suggest the following:

  • The different finches were always different (incorrect).
  • The finches are now a different species but they had a common ancestry. If students suggest this explanation, ask what might have happened that caused one species to change into many different species? They are different from each other because of an event that separated the populations - the isolation result of living on different islands with different food supplies.

After this discussion, ask students what might have brought about the beak-size changes (the volcanic eruption that changed the availability of the food on Isabella Island caused this example of evolutionary change). Discussion can focus on the following:

Ø Pressing for explanations: For example: Why do you think that beak-size changed? Can you share with me more about what you are thinking?

Ø Orienting students to each other's’ thinking. This could help teachers to find similarities and differences among students’ responses. For example: Do you agree or disagree with what Students A said? And why?

Note: Teachers can write down students’ responses on a board. Teachers can use different colours of chalks and markers for same ideas. If similar ideas are brought forward, teachers can use the same colours.

Step 4: Extension and discussion of antibiotic resistance:
A very serious application of evolution is happening to antibiotic resistant bacteria. The purpose of this extension activity is to heighten student awareness about an increasingly serious health problem our society is facing now and into the future that is evolution in action. A driving question guiding this discussion is: What does natural selection predict about the evolution of antibiotic resistance in E. coli?
Students will be invited to discuss this driving question in groups. They will also be invited to write their answers on the board. Specifically, questions for groups are as follow:

  • How many of you have used antibiotics in the past year?
  • Antibiotics have been around since 1933. How did people fight infections prior to the discovery of antibiotics?
  • Do you think people always need antibiotics when they are sick? Have an earache? Have mononucleosis?
  • When dealing with some mononucleosis is bacterial, antibiotics would work. Other times it is viral, so antibiotics do not work. But invariably, antibiotics are prescribed just in case. What do you think is the problem here?
  • Would antibiotics cure a cold (The answer is so because colds are caused by a virus. But people still want antibiotics)? What do you think is the problem here?
  • Why would some bacteria become resistant to antibiotics? (If a small number of bacteria survive in the presence of antibiotics, they reproduce and pass this advantageous trait to their offspring.)
  • The average generation time for bacteria is 20-30 minutes. How would this be a factor in bacteria developing resistance to antibiotics?

Note: Antibiotic resistance in E. coli. E. coli are rod-shaped bacteria that live in the human digestive tract. Most strains are harmless, but some cause infections that doctors may treat with penicillin or other antibiotics. Some E. coli have an enzyme that can break down penicillin and other antibiotics. Some E. coli are now resistant to antibiotics.
Note: Use of free multimedia software that examines evolution and natural selection: http://learn.genetics.utah.edu/content/selection/recipe/http://learn.gen.

Step 5: Evaluation:
The goal of this lesson is to engage students by providing them with a research topic of their choosing that focuses on evolution and adaptation. In order to evaluate students’ learning outcome, teachers can invite students to write a report on natural selection, using any organism of interest. Students should include an explanation of how natural selections works to demonstrate their understanding of evolution and natural selection. Possible examples might include:


Frequency-dependent Selection

Figure 2. A yellow-throated side-blotched lizard is smaller than either the blue-throated or orange-throated males and appears a bit like the females of the species, allowing it to sneak copulations. (credit: “tinyfroglet”/Flickr)

Another type of selection, called frequency-dependent selection, favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection). An interesting example of this type of selection is seen in a unique group of lizards of the Pacific Northwest. Male common side-blotched lizards come in three throat-color patterns: orange, blue, and yellow. Each of these forms has a different reproductive strategy: orange males are the strongest and can fight other males for access to their females blue males are medium-sized and form strong pair bonds with their mates and yellow males (Figure 2) are the smallest, and look a bit like females, which allows them to sneak copulations. Like a game of rock-paper-scissors, orange beats blue, blue beats yellow, and yellow beats orange in the competition for females. That is, the big, strong orange males can fight off the blue males to mate with the blue’s pair-bonded females, the blue males are successful at guarding their mates against yellow sneaker males, and the yellow males can sneak copulations from the potential mates of the large, polygynous orange males.

In this scenario, orange males will be favored by natural selection when the population is dominated by blue males, blue males will thrive when the population is mostly yellow males, and yellow males will be selected for when orange males are the most populous. As a result, populations of side-blotched lizards cycle in the distribution of these phenotypes—in one generation, orange might be predominant, and then yellow males will begin to rise in frequency. Once yellow males make up a majority of the population, blue males will be selected for. Finally, when blue males become common, orange males will once again be favored.

Negative frequency-dependent selection serves to increase the population’s genetic variance by selecting for rare phenotypes, whereas positive frequency-dependent selection usually decreases genetic variance by selecting for common phenotypes.


5 Examples of Natural Selection of Evolution on Earth | Biology

Some of the examples of natural selection of the evolution on earth are as follows:

A. Genetic basis of adaptation in Industrial melanism (Fig. 7.41):

In the early part of nineteenth century (1830s), there was not much industrial growth in England (e.g. Birmingham) and there was mainly a white- winged peppered moth Bistort betularia which was well adapted to defend itself from the predatory birds.

It used to rest during daytime on light-coloured, lichen covered trunk of oak tree. But with the industrial growth in 1920, the smoke particles released from the chimneys of the industries killed the lichens and darkened the tree trunks.

So white-winged Biston betularia became more distinct to the predatory birds. Then a dominant gene mutation appeared in some members of the moth population. This gene mutation produced a dark winged melanic moth which had better chances of survival than grey-coloured moth.

The first melanic form of peppered moth was observed in 1845. These dark coloured moths by differential reproduction produced a dark coloured melanic species, Biston carbonaria which formed 99% of moth population by 1895.

This replacement of light- coloured moth by dark coloured melanic species due to industrial smoke was called industrial melanism. Thus, natural selection favoured the melanic moths to reproduce more successfully by differential reproduction for their adaptation in the industrial areas of England.

This shows that an evolutionary change always has a genetic basis and this genetic variation, when favoured by natural selection, enables the organisms to adapt to a particular environment which increase their chances of survival.

It was originally studied by R.A. Fischer and E.B. Ford. Industrial melanism was experimentally tested by a British ecologist, Bernard Kettelwell in the 1950s. He reared equal number of dark and light coloured peppered moths. He released one set of these moths in the polluted area (woods of Birmingham) and other set in an unpolluted area (in Dorset).

After some years, he could recapture 19 per cent of light moths and 40 per cent of dark moths from the polluted area, while could recapture only 12.5 per cent of light moths and 6 per cent of dark moths from unpolluted area. This result shows the patterns of differential survival of B. betularia in the polluted and unpolluted areas.

(i) Sooty areas offer great protection to melanic forms because of increased frequency of a dominant gene in industrial area.

(ii) In unpolluted area and rural areas where industrialization did not occur, the frequency of the gene responsible for light-coloured moths has more selective advantage.

(iii) In a mixed population, the better adapted individuals survive and increase in number but no variant is completely wiped out e.g., industrial pollution did not eliminate the gene for light-coloured moths completely.

It was also reported in many other European countries as well. Industrial melanism has been noticed in about 70 species of moths in England and in about 100 species in USA. But since 1956 after the passage of clean air legislation, the coal is being replaced by oil and electricity.

This had reduced the soot deposition on the tree trunks. Consequently, light-coloured moths have again increased in number with the reduction in pollution. This is called reverse evolution.

B. DDT-Resistant mosquitoes:

Mosquitoes are known to be the vectors of diseases like malaria and elephantiasis caused by Plasmodium and Wuchereria. Earlier the mosquito-population had more DDT-sensitive but less DDT-resistant mosquitoes. When DDT was not being used, the DDT-resistant remained dominated by DDT-sensitive mosquitoes.

But when the use of DDT as an insecticide started (introduced in 1940s), DDT-resistant mosquitoes had a competitive advantage over their counterparts. Their DDT-resistant property spread over more and more members of the population so now the mosquito population is dominated by DDT-resistant ones.

It is also an example of directional selection. Thus according to principle of Natural Selection, chemical insecticides would remain useful only for a limited time.

C. Sickle cell anaemia:

Sickle cell anaemia is characterised by:

(a) About 1-2% of total RBCs become sickle-shaped.

(b) Early rupturing of RBCs leading to severe anaemia.

(c) Normal haemoglobin Hb-A is replaced by defective haemoglobin Hb-S in which glutamic acid of β-chain is replaced by valine amino acid due to a single base substitution in a gene.

(d) O2-carrying capacity of Hb-S is less than that of Hb-A.

(e) Death of affected person by the age of puberty.

(ii) Cause. It is caused by a recessive autosomal gene mutation in homozygous condition (Hb S Hb S ). The heterozygotes (Hb A Hb S ) also have some sickle-shaped cells.

Persons with sickle cell anaemia are mostly found in those areas of tropical Africa where malaria is very common. It was reported that sickle-shaped cells of the heterozygote kill the malaria parasite. So the heterozygotes can resist the malaria infection in a much better way than the homozygous for the normal haemoglobin.

The loss of deleterious recessive genes through death of homozygotes is being balanced by successful reproduction of heterozygotes. Thus, the natural selection has preserved it along with the normal haemoglobin in the malaria affected areas. It is an example of balancing or stabilizing selection.

E. Antibiotic resistance in the microbes

(Fig. 7.44). Joshua Lederberg and Esther Lederberg showed the genetic basis of adaptations in bacteria by culturing bacterial cells by their plating experiment.

Lederberg’s’ experiment had following steps:

1. They inoculated bacteria on an agar plate and obtained a plate having several bacterial colonies. This plate was called “master plate”.

2. They formed several replicas from this master plate. For this, they took a sterilized velvet disc, mounted on a wooden block, which was gently pressed on the master plate. Some of the bacterial cells from each colony sticked to the velvet cloth.

3. Now by pressing this velvet on new agar plates, they obtained exact replicas of the master plate. This is so because the bacterial cells were transferred from one plate to another by the velvet.

4. Then they tried to make replicas on the agar plates containing an antibiotic penicillin. A few colonies were able to grow on the agar plate and were said to be penicillin-resistant, while other colonies did not grow on antibiotic penicillin medium and were said to be pencillin-sensitive colonies.

Thus there was a pre-adaptation in some bacterial cells to grow a medium containing the antibiotic penicillin. This pre-adaptation had developed in certain bacteria by chance gene-mutation and not in response to penicillin. This expressed only when such bacteria were exposed to penicillin. The new environment does not induce mutations it only selects the preadaptive mutations that occurred earlier.

Lederberg’s replica plating experiment provided a support to neo-Darwinism and proved that the penicillin-resistance adaptation in the bacterial cells originated due to selection of pre-existing mutant forms of bacteria by nature.

Penicillin-resistant bacteria cells had no advantage to multiply in an environment when there was no penicillin. But they had a competitive edge on others in penicillin contouring agar plates, so they multiplied rapidly and formed colonies in the penicillin containing medium.

F. Drug resistance in bacteria. L. Cavalli and G.A. Meccacaro (1952) demonstrated that colon bacteria – Escherichia coli are resistant to antibiotic drug – Chloramphenicol by 250 times to that tolerated by normal bacteria. Crossing over experiments confirm that resistance in bacteria is acquired by mutation and is inherited on Mendelian principles.

Excessive use of herbicides, pesticides, antibiotics, etc., has resulted in selection of resistant varieties in lesser time scale. These are examples of evolution by anthropogenic actions and prove that evolution is not a directed process but is a stochastic process based on chance events in nature and chance mutation in the organisms.


Principles of Evolution, Ecology and Behavior

Chapter 1. Introduction [00:00:00]

Professor Stephen Stearns: Biological evolution has two big ideas. One of them has to do with how the process occurs, and that’s called microevolution. It’s evolution going on right now. Evolution is going on in your body right now. You’ve got about 10 13th bacteria in each gram of your feces, and they have enough mutations in them to cover the entire bacterial genome. Every time you flush the toilet, you flush an entire new set of information on bacterial genomes down the toilets. It’s going on all the time.

Now, the other major theme is macroevolution. This process of microevolution has created a history, and the history also constrains the process. The process has been going on for 3.8 billion years. It has created a history that had unique events in it, and things happened in that history that now constrain further microevolution going on today.

That’s one of the tricky things about evolution. It has many different scales. My wife always gets frustrated with me. She says, “Well when did that happen?” I say, “Oh not too long ago, only about 20 million years.” And, you know, that’s what happens when you become an evolutionary biologist, you zoom in and out of deep time a lot. And this process of microevolution is going to be the first thing we examine. It’s the nuts and bolts. It’s what’s really created the patterns. But the patterns of macroevolution are also very important because they record the history of life on the planet and they constrain the current process.

So the evolution part of the course is set up basically with two introductory lectures. Then I’m going to spend six lectures talking about microevolutionary principles. So these are things that you can always return to if you are puzzled about a problem. Then there’ll be five lectures on how organisms are designed for reproductive success. This includes cool stuff like sexual selection, mate choice, that kind of stuff. I usually manage to give the sexual selection lecture just about on Valentine’s Day.

Then we’ll do macroevolutionary principles. This has to do both with speciation, how new species form, and with how biologists now analyze the tree of life to try to understand and infer the history of life on the planet. Then we’ll take a look at that history, looking at key events–and this includes both fossils and the diversity of organisms–and some abstract organizing principles about life. So all of those are part of how we can analyze the history of life on the planet.

And then, just before Spring Break, we will integrate micro and macroevolution. We’ll do it in two different ways. We’ll do it with co-evolution, where micro and macro come together, and we’ll also do it with evolutionary medicine, where both kinds of thinking are necessary really to understand disease and the design of the human body.

Chapter 2. History of Evolutionary Studies [00:03:22]

So where did this idea of evolution come from? Well, there are always ideas. You can go back to Aristotle and find elements of evolutionary thought in Aristotle. But really it’s a nineteenth century idea, and in order to see how it developed let’s go back to about 1790 or 1800 so at the end of the Century of the Enlightenment.

At that point, if you were to ask a well-educated person living in a Western culture how old the world is, they would say, “Oh thousands of years.” And if you were to ask them, “Well, where did all these species on the planet come from?” they would say they were all created just the way they look now and they’ve never changed. And if you asked them, “Have there ever been any species that went extinct?” they would say, “No, everything that was created is still alive and can be found somewhere on the planet.”

So when Alexander von Humboldt, who was certainly a creature of The Enlightenment, sets out to explore South America, he thinks that he might encounter some of those strange fossils, that the French have been turning up in the Paris Basin, on top of Tepuis in Venezuela. So he really thought that there was a lost world. Of course, Arthur Conan Doyle later wrote a novel about that. But these guys actually thought, “Hey, I go to Venezuela or I go to the Congo, I might meet a brontosaurus.” That was what they thought at that time.

They thought that adaptations were produced by divine intervention. They did not think that there was a natural process that could produce anything that was so exquisitely designed as your eye. We now know that your eye is in fact very badly designed, but it looked pretty good to them. Anybody here know why the eye is badly designed? What’s wrong with your eye?

Student: The blind spot.

Professor Stephen Stearns: It’s got a blind spot and–?

Student: [Inaudible]

Professor Stephen Stearns: It’s got–the nerves and the blood vessels are in front of the retina. The light has to go through the nerves and the blood vessels, to get to the retina. The octopus has a much better eye.

Okay, now by the time that Darwin published his book in 1859, people thought that the world is very, very old how old they weren’t sure. We now know about four-a-half billion, but at that point, based on the rate of erosion of mountains and on the saltiness of the ocean, assuming that the ocean had been accumulating salt continuously, and that it hadn’t been getting buried anywhere, which it does, people thought hundreds of millions of years. They weren’t yet in the billions range, but they thought hundreds of millions.

They knew that fossils probably represent extinct species. That was Cuvier’s contribution. He did it for mammal fossils in the Paris Basin. Geoffrey Saint-Hilaire had had a big debate with Cuvier about homology, and that was in 1830. By the way, it was one that many people throughout Europe followed very closely–this was a very, very key intellectual topic at the time–and it was about homology. Basically it was about the idea that Geoffrey Saint-Hillaire had had that if my hand has five fingers then–and a bat’s wing has five fingers and the fin of a porpoise has five fingers–that that indicates that we all got those five fingers from a common ancestor, and therefore we are related because we had a common ancestor.

So you could see that in 1830. That’s before Darwin publishes his book. Okay? Then of course we have the idea that adaptations are produced by natural selection and we owe that to Darwin. And I will run through the process he went through between 1838 and 1859 very briefly. This is one of the most important ideas about the nature of life, and therefore about the human condition, that’s ever been published, and I strongly recommend that, if you have a chance, read The Origin of Species. Darwin actually was quite a good writer. It’s Victorian prose, so it’s a little bit like reading Dickens. But it’s good stuff, he has a nice rolling style.

How did he come to it? Well Darwin was a med school dropout. Went to Edinburgh, didn’t like med school loved beetles and became passionate enough as a naturalist to become known, as a 22-year-old young man, as a guy who might be a good fellow to have on an expedition. And the British Admiralty was sending Fitzroy around the world to do nautical charts and Darwin got on the ship.

So at an age not very much greater, or perhaps even a bit younger than some of you, Darwin sets off. He’s 22 years old. He wants to know how species form. He has set himself that goal. So he’s ambitious. He’s set a clear goal. The goal is to solve one of the most pressing problems that biology has at that time: where do species come from?

Now the stimulus that he has is in part from Charles Lyell, the geologist, who had discovered deep time, and that convinced Darwin that there would’ve been enough time. He stops in Argentina. In the banks of a river in Argentina he can see giant fossil armadillos, and then right on top of that same bank he can see the current armadillos walking around, up on top of the bank. There they are the live ones are right above the fossil ones. They look the same but–I mean, they look similar–but they’re not the same. So there’s some connection there.

He gets on a horse in Chile and he rides up into the Andes and he sees marine fossils lifted thousands of feet above sea level clearly some dynamic process is going on that had lifted those marine fossils up. He doesn’t know about continental drift yet–right?–but there the fossils are.

In the harbor at Valparaiso he sees the effects of an earthquake that had happened just before they arrived. It was a big one. It was probably as large as the earthquake that recently caused the big tsunami in Indonesia–so it was probably an 8.5, 8.6 earthquake–and it had caused an uplift in the harbor of maybe 50 feet. So he began to see the world as dynamic. Things hadn’t always been the way they are.

Then he goes to the Galapagos, and please navigate the Galapagos website and have a look at some of these differences. The thing that Darwin noticed is that the mockingbirds are different on the different islands. If you go to the Galapagos what you’ll notice is that if you land on Espanola, the mockingbirds really want your water supply, and they will hop onto your head or your knee to try to get at your water supply. But, in fact, the mockingbirds all look a little bit different on the different islands, and that’s what Darwin noticed.

He could also see that that the marine iguanas look a bit different, and the land iguanas look different. Interestingly, he didn’t notice the differences in the finches, until he got back to England and gave his collection to the British Museum, and the ornithologists at the British Museum came in and said, “Hey Darwin, do you realize that the finches on these islands are different?” And that was when he began to really see how many differences could accumulate, how rapidly, when you take a migrant from Central America and put it on an isolated archipelago.

So he goes back to London. He’s been onboard ship for about four years. He has a problem with seasickness. He never again sets foot on a ship. He doesn’t want to go near the water after being four years on this ship. He had a few issues with the captain too, Fitzroy, but mainly it was that he had a very bad upset stomach onboard the Beagle.

He reads the Reverend Malthus on population growth. Malthus’s book had come out in 1798. Malthus said basically that populations grow exponentially but agriculture grows linearly. Therefore populations will always outstrip their resource base. This convinced Darwin that all organisms are in a competitive struggle for resources, and that that must inevitably be the case. He saw very clearly how powerful reproduction is at generating exponential population growth. We will come back to that in the ecology portion of the course.

And we now know that organisms are in competition really essentially not just over food resources, they are in competition over anything that will get their genes into the next generation. So that can be competition for mates. It can be competition for nesting sites, competition for food lots of different things. But at any rate this primed Darwin’s thinking. So he writes down the idea of natural selection. It comes to him in 1838 it’s in his notebooks in 1838.

Basically, I’ll run through natural selection in a minute. It’s a deceptively simple idea because the mechanism looks so simple, but the consequences are so wide ranging. Darwin recognized what the consequences were. And he didn’t publish immediately. He did other things. He went off and he worked five or six years on barnacles. He wrote down lots of ideas about things unrelated to natural selection, and he wasn’t really jogged out of this until a letter arrived in 1858 from Alfred Russel Wallace, a young British naturalist who had, in a fit of malarial fever, had the same idea, in Indonesia.

And Wallace knew that Darwin had been thinking about these things, and he sent Darwin a letter. And at that point Darwin, British gentleman as he was, had to decide whether he would do the sort of gracious, honorable thing and let Wallace have the idea, or do the honest thing, which, his colleagues knew, was that he had already had the idea. And what they decide upon is that they will do a joint publication.

So if you go to the Biological Journal of the Linnaean Society for 1858, which is in the Yale Library, you can look up the back to back papers by Alfred Russel Wallace and Charles Darwin in which the idea of Natural Selection is laid out. And then Darwin rushes his book into print. So he has been working on a book that was probably going to be about 1200 pages long, and instead he publishes an abstract of it, which he calls “The Origin of Species”, which is about 350 pages long. And it sells out on the first day, sold all 6000 copies on the first day, and has remained in print ever since.

That’s The Beagle. Darwin slept in a hammock in the captain’s cabin, at the back of the ship, which rocked horribly. And that’s essentially all I want to do about the development of the idea of Evolution. Basically what I did was I wanted to give you the feeling that there was somebody like you who went out and knew what a deep problem was, and happened to have the luck to get into a special situation where they were stimulated, and came up with an idea that changed the world. No reason it can’t happen again.

Chapter 3. Conditions for Natural Selection [00:15:59]

So now I’m going to give you a brief overview of microevolution and macroevolution. Here’s Natural Selection here’s Darwin’s idea. If, in a population, there is variation in reproductive success–what does that mean? Would everybody in the room raise their hand if they’re an only child? Look around. There are about five or six. How many of you come from families with two children? Lots. How many with three? Quite a few. How many with four? Quite a few, but not as many as there with only children. Anybody with five? Yes, a couple. Anybody with six? No. If we were, by the way, in the nineteenth century, at this point there would still be lots of hands going up.

What you’ve just seen is the amount of variation in reproductive success represented by the families in this room. Variation in reproductive success basically means that different families have different numbers of offspring, or different individuals have different numbers of offspring. Then there has to be some variation in a trait.

How many of you are under 5’5? Raise your hands. How many between 5’5 and 6 feet? How many over 6 feet? Lots of variation in height in this room. So we got lots of variation in reproductive success lots of variation in height. There has to be a non-zero correlation between reproductive success and the trait. On this particular trait there’s been some research. Turns out that taller men have more children. I don’t know whether that’s just an NBA effect or what that is but it turns out to be true in many societies.

So there is a non-zero correlation between the reproductive success and the trait. Then there has to be heritability for the trait. The heritability of height in humans is about 80%. So all of the conditions for natural selection on height are present in this room. All you have to do is go out and have kids and it will happen.

So if you’re ever in doubt about whether evolution is operating in a population, go back to these basic conditions. You can always decide whether it’s likely to be operating or not. We can turn natural selection off by violating any of these four points. If there’s no variation in reproductive success–for example, if there is lifetime monogamy and a one-child policy, there will be zero-variation in reproductive success if everybody just has one child of course some people will still have zero, but that’s about as close as you can get.

If there’s no variation in the trait–if the trait is like five fingers there are very few people with six fingers there are some, but very few. If there’s a non-zero correlation between reproductive success and the trait if there is a zero correlation between reproductive success and the trait. We’ll go into all the conditions for that. That results in neutral evolution. Okay? Then things just drift. Well have a whole lecture on that. Or if the trait is not heritable, if there’s no genetic component to it, then it won’t evolve.

So Natural Selection-I wonder why it’s doing that? Sorry- Natural Selection does not necessarily happen. It only happens under certain conditions. Essentially in this picture, this is what I’ve just told you about Natural Selection. If there’s variation in the trait, represented on the X-axis, and there’s variation in reproductive success, based on the Y-axis, and there is a correlation between the two, represented by the fact that I can just about draw a straight line between these points, Natural Selection will occur and it will push the trait to the right.

If all of these conditions, except the correlation, occur–so you have variation in the trait, variation in reproductive success but no correlation–then you get random drift. And these two situations result in radically different things. This situation produces adaptation, it produces all of the fantastic biology that you’re familiar with. It’s produced meiosis it’s produced your eye it’s produced your brain. It’s extremely powerful.

This situation on the right, the random drift situation, is what connects microevolution to phylogenetics, and it’s what allows us to use variation in DNA sequences to infer history. And I’ll get to that. That statement right now is opaque. Don’t expect that one to be transparent at this point. But two or three lectures from now I will go into that in detail and you will see that we need to have a process of drift in order to generate a kind of large-scale regularity that gives us timing and relationship in macroevolution.

So both are driven by variation in reproductive success. The difference is in whether there’s a correlation between the variation of the gene or the trait and the variation in reproductive success.

Chapter 4. The Power of Selection and Adaptation [00:21:25]

If we have strong selection, we can get pretty amazing things. I could illustrate adaptation a lot of different ways. I could do it say with the leaf cutting ants that were the first farmers they domesticated a fungus 50 million years ago and have been cultivating it ever since. That would be one way I could do it.

I could do it with the exquisite morphology of the deep sea glass sponges and how efficient they are at filtering stuff out of the water. I could do it with the design of a shark’s body. Lots of stuff.

I’ll do it with bats, in part because when I was a Yale undergrad I worked on bats in this building. We had a guy that did research on bats at that time. Now a lot of bats are insectivores, and they will hunt moths at night, in complete darkness. They do it with sonar.

The bat only weighs about say 50 to 100 grams, and it is making a sound that is as loud as a Metallica concert when you’re standing right next to the lead guitar’s speaker system. Okay? Or it’s as loud, if you like, as a Boeing 747 taking off from a runway. It’s this tiny little creature. It’s making an incredibly loud sound. It’s 130 decibels.

It does that because the intensity of sound, the amplitude of sound, decreases with the square of distance, and it needs to detect an echo coming back from the moth. The echo coming back from the moth–which by the way it can pick up at a distance of about 20 feet–is about a million times less loud, and it’s only coming in about one to two milliseconds later. So imagine, there you are, you’ve gone “woo”–except a lot louder than that–and milliseconds later you hear “click”, and you haven’t deafened yourself.

That’s exquisite. It has all kinds of physiology in its ear to hear the returning echo, and it can actually discern whether or not it’s looking at a kind of a fuzzy moth or a smooth beetle. The moth has all kinds of adaptations to try to get away from the bat. It hears the bat. The bat’s cruising around, the moth hears the bat. The moth goes into a desperate spiral, diving towards the ground–okay–the bat starts to swoop in. There is a mite that lives in the ear of moths. I think you begin to understand the problem that this mite has. If the moth gets caught, the mite will be eaten.

The mite’s solution? It only lives in one ear. If you collect moths and you look for mites in their ears, you will find that they are always only on one side. So the moth always has a clear ear so it can hear the bat. There’s stuff like this all through biology.

There’s another kind of a bat, called a Noctilio, hunts fish. A Noctilio basically detects ripples in the water surface, and then it swoops down and it gaffs the fish with its hind legs. It can detect a wire 1/10 th of a millimeter in diameter, sticking 1/10 th of a millimeter above the water surface. When I was taking care of bats, I’d never seen a Noctilio. I thought, “God, this must be the greatest bat in the world.”

About four years ago, on the Amazon, my wife and I went out in a canoe, at sunset, on a lake, just off the Amazon River. It was starting to get dark. All day long the kingfishers had been fishing on that lake, and during the day the lake had gotten covered with a lot of food that the fish wanted, but they were afraid of the kingfishers. As it got darker the kingfishers couldn’t hunt anymore and the whole surface of the lake dimpled with the fish coming up to eat the food.

So their timing was exquisite. They knew exactly how dark it had to get before they were safe. The fish came up and started to eat the food. At that point–it was just shortly after sunset–the bat falcons were still stationed around the lake. You could see, up on the trees, falcons sitting up on the limbs and making flights off of the limbs. About 15 minutes after the fish started to eat, it got dark enough so that the bat falcons couldn’t hunt anymore, and at that point Noctilio came out, and the water was covered with hundreds of bats that were catching the fish. They were catching the fish within a meter of us.

Now there are a couple of things about that story that I think, uh, I’d like to underline. One is that that entire community is exquisitely adapted. Every element in it knows when everything is going on and what the risks are, and what the costs and the benefits are. The other thing is that I had benefited from a liberal education, and when that bat came out, and was flying around a meter away from my canoe in the Amazon, my life was so much richer because I had been waiting to see it for 40 years. I had heard about it in a course at Yale. I knew where it fit in. I knew what kinds of adaptations it had, and boy was I happy to see it.

Chapter 5. Drift [00:27:09]

So adaptation can be impressive. Drift is something that actually appeals to the geeks among us. I have a geeky side too, okay? Drift isn’t such a morphologically or artistically beautiful thing. It’s a mathematically beautiful thing. Drift happens whenever there is no correlation between reproductive success and variation in a trait, and it produces patterns like this.

So here we start off with 20 populations, and we start them all with a gene frequency of 0.5, and we let meiosis–which is like flipping a fair coin–and we let variation and reproductive success take their course, and we just run these populations for 20 generations, and you can see that there’s just about an equally likely distribution of end-states out here. So we all start off at 0.5, and it gets noisy as we go along.

So this is an image of the process of drift, and if any of these populations happens to get up to 1, or down to 0, in terms of gene frequency, the process will stop, because those are absorbing states. If the frequency becomes 1, then everybody’s got it and there can’t be any change, and if the frequency becomes 0, then nobody’s got it and there can’t be any change. So that’s what’s meant by absorbing state.

Now to a first approximation, whole organism traits are the products of Natural Selection. Maybe not in the immediate past, but usually at some point in the history of life, a whole organism trait will have been under Natural Selection. So it will have been shaped and designed by this process. And to a first approximation, a lot of DNA sequences have been shaped by drift. So we see design in the whole organism and we see noise in the genome–to a rough cut lots of exceptions.

There are DNA sequences that have clear selective value in fact, there’s a whole literature on this now. If any of you want to write an essay on signatures of selection in the genome, you can find lots of stuff on that now, on how to recognize that a chunk of genome has recently been under selection. There are whole organism traits that have no apparent selective value for example, the chin.

The chin actually is the result of evolution, operating on development, to take a face, which is like that of a gorilla or a chimpanzee, which bulged out like this and essentially flattened it out so that we are vertically much flatter than a chimp or a gorilla, and as a result of this being pushed back, something that was there, but kind of covered up, stuck out.

So that’s where the chin came from. That doesn’t mean chins were selective. Now it may be that after they originated, that there could’ve been a little bit of sexual selection operating on them. But certainly the developmental process that originally produced them didn’t have to be adaptive. It could just be a byproduct of something that was going on, basically from the mouth up.

So the themes of microevolution are selection and drift. Natural selection is driven by variation in reproductive success. The strength of selection is measured by the correlation of variation in a trait with reproductive success. When there’s no correlation, there’s no systematic change, and then things just drift, okay?

Chapter 6. History of Life [00:31:11]

Now macroevolution the big scale process, the big picture. Well here are sort of the basic statements about macroevolution. If anybody asks you, “What does this fancy word macroevolution mean?”, tell them basically this is it. There’s one tree of life. Everything on the planet had a common origin. Everything is related to everything else, with the possible exception of the viruses, which are too small for us to decide their genomes are too small. The branch points in the tree, speciation events–that’s when new species were formed.

This history is marked by striking major events. There have been mass extinctions. There have been meteorite impacts. There have been major changes in the organization of the information structure of life. And the biological disciplines that you may encounter map onto this timeline. So actually different parts of biology study different parts of this process.

The tree looks like this. This is the large-scale tree. So at this scale, what you see here are the three kingdoms of life, which are the bacteria, the archaea, and the eukaryotes, up here the root’s at about 3.7 billion years, not million years. And at one point a purple bacterium got into the eukaryotes and became a mitochondrion, and at another point a cyanobacterium got into various plant lineages, three times, and became a chloroplast.

So that’s the large scale. And you’re probably searching around on that to find out where you, the most important thing in the universe are, and you’re way up here, on a little twig. Okay? Now if we blow that up and just look at the multi-cellular organisms, multi-cellularity looks like it originated around 800 million to a billion years ago. And these are the fungi, these are the things we call the plants, multi-cellular plants, and then off in this direction we have got a fairly complicated series of branches that end up with us being up here. Okay?

The things that are–this was done by Tom Pollard, at MCDB, about five years ago, and at that point the things in yellow had genomes that had been completely sequenced. Now there are hundreds of completely sequenced genomes. So for the first two billion years of life most of the action is down in the basal radiation. So going on with bacteria, archaea and eukaryote ancestor single-celled things. At that scale–we’re just way up at a small twig on the tip–and symbiotic events brought mitochondria and chloroplasts into eukaryotic cells.

Already this is telling you something interesting about yourself. You are a community of genomes. You are not a unitary genome. You’ve got that mitochondria in you. The main themes are basically that the speciation events that have occurred, particularly over the last billion years or so, have created a tree of life that describes the relationships of everything on the planet.

Systematic biology, phylogenetics, tries to infer the history of life by studying those relationships. And there’s a real deep issue here of how do we infer the tree? The tree–organisms don’t come with a barcode on their foreheads telling us who they are related to. We have to try to figure out who they’re related to, and when we understand the relationships, then we know the history, because the relationships define the history.

So we work with hypotheses about history, and we test these hypotheses against each other and try to come up with the one that’s most consistent with the data that we’ve got. And they give us a historical framework within which we can then interpret what’s happened. There are major events that have happened. Briefly these are they.

Life originates about 3.6 to 3.9 billion years ago. And, by the way, it seems to have originated fairly quickly. Within probably about 100 million years–see I’m being an evolutionary biologist again–within just a hundred million years, uh, after water could exist on the surface of the planet in liquid form–so following the meteorite bombardment, when the surface of the planet cools down enough for water to be liquid–life probably originates pretty quickly. And arguably, within the first hundred generations, the first parasites were around. So those things happened pretty quickly.

Then eukaryotes and meiosis, which is how a biologist refers to organized sex, happened about 1.5 to 2.5 billion years ago multi-cellularity, which gives us developmental biology, about a billion years ago. All the major body plans for animals appear to have, with the exception perhaps of the, uh, jellyfish and a few of their relatives, they all seem to have originated about 550 million years ago.

There was a near loss of life on the planet in the Permian mass extinction. We will study that later in the course. You’re welcome to write an essay on mass extinctions if you want to you know, big death is kind of exciting. It seems to have occurred basically by a process of poisoning of the oceans. The flowers radiate about between 65 and 135 million years ago.

Language is important because once language occurs, then we have an independent kind of information transmission from generation to generation we get cultural transmission. That’s probably about 60-100,000 years old at least with syntax and complicated information storage. Writing is only about 6000 years old. And of course the important stuff is quite recent.

So this is a view of life that goes from bacteria to dinosaurs to rock and roll and that all can be studied with evolutionary principles. How do the biological disciplines map onto this? Well microbiology and biochemistry try to study things that are common to all life. That means that the same chemical reactions that go on in bacteria go on in the human liver, and that’s about one-and-a-half to four billion years old. Okay?

Genetics and cell biology study stuff that follows the evolutionary invention of meiosis to a large degree. There is bacterial genetics, but eukaryotic genetics is something which is studying things that are about 1.5 billion years old. Developmental biology and general physiology, those are multi-cellular disciplines they depend upon the existence of a multi-cellular organism. That thing didn’t come along until about a billion years ago. Neurobiology, you need a complex–you need cephalization–you need to have a complex nervous system. That studies phenomena that are probably about 500 to 600 million years old. Same for behavior.

There are several anthropologists in the class. You guys are studying things that probably originated along our branch of the tree, within the last 15 to 20 million years. So there is a temporal assembly of biology, as a discipline, as well as there is of life, on the planet.

Chapter 7. Conclusion [00:39:33]

So the key concepts from this lecture are that there are two kinds of explanation in biology. One is the proximate or mechanical question, which is answered by studying how molecules and larger structures work. Those are basically physical and chemical explanations. And then there are the evolutionary questions, which is why does the thing exist why did it get designed this way? And that could be answered either through selection or through history or the best way to do it is to use both and combine those explanations.

The thing that distinguishes biology from physics and chemistry is Natural Selection. This is not a principle that you can find in a physics textbook or in a chemistry textbook. This is something that is a general principle that actually applies to lots of things besides biology, but it’s not contained within physics and chemistry. And there is a pattern in biology that unites biology with geology and astronomy, and that’s history. So there is an important element of historical thought in evolutionary biology, as well as the more abstract action of natural selection on designing organisms for reproductive success and shaping changes and gene frequencies.

Now I want to end the lecture by telling you something astonishing. I won’t always be able to tell you something astonishing in every lecture. But one of the great privileges of teaching Introductory Biology, or being in an Intro Bio class, is that there are certain big things that never get discussed again. Okay? This is one of them. We are continuous with non-life.

Here’s how I’m going to convince you of that. Think of your mother. Now think of her mother. Now think of your mother’s mother’s mother. Now I want you to go through a process like you’ve done in math where you do an inductive proof you just go back. Just let that process go. Okay? Back you go in time. Speed it up now. Okay? We’re back at ten million. Now we’re at a hundred million. Now we’re at a billion years. Now we’re at 3.9 billion years. Every step of the way there has been a parent. 3.9 billion years ago something extremely interesting happens. You pass through the origin of life, and there’s no parent anymore. At that point you are connected to abiotic matter.

Now this means that not only does the tree of life connect you to all the living things on the planet, but the origin of life connects you to the entire universe. That’s a deep thought. Every element in your body, which is heavier than iron, and you need a number of them, was synthesized in a nova, uh, supernova. The planet that you’re sitting on is a secondary recycling of supernova material, and your bodies are constructed of that stuff and they use it in some of their most important processes.

So the vision that evolutionary biology gives you is not only the practical one of how to think about and analyze how and why questions in biology, it’s also a more general statement about the human condition, and I hope it’s one that you’ll have time to reflect on. Next time we’ll do basic genetics.


Major Books

The study of natural selection began with Darwin’s Origin of Species (Darwin 2010, originally published in 1859), but fell out of favor again until the “Modern Synthesis” the main books from the synthesis that dealt with field studies of natural selection were Dobzhansky’s Genetics and the Origin of Species (Dobzhansky 1937), Mayr’s Systematics and the Origin of Species (Mayr 1942), and Stebbins’s Variation and Evolution in Plants (Stebbins 1950). The next major books studies of selection in the field came with Ford’s Ecological Genetics (Ford 1964) and Endler’s Natural Selection in the Wild (Endler 1986). The most recent books on natural selection with some focus on field studies is Bell’s Selection: The Mechanism of Evolution (Bell 2008). For more on the early work on natural selection see the separate Oxford Bibliographies in Evolutionary Biology article “Natural Selection.”

Bell, G. 2008. Selection: The mechanism of evolution. 2d ed. Oxford: Oxford Univ. Press.

In some ways an update of Endler 1986, but quite different in its approach and broader in its scope—this book reviews field studies of selection as well as studies of natural selection in the laboratory and artificial selection by humans.

Darwin, C. 2010. On the origin of species by means of natural selection. New York: Modern Library.

Originally published in 1859 (London: John Murray). Laid out abundant evidence for Darwin’s mechanism of adaptive evolution, that is, natural selection.

Dobzhansky, Th. 1937. Genetics and the origin of species. New York: Columbia Univ. Press.

Argues that population genetics, including selection, is consistent with genetic differences between species.

Endler, J. A. 1986. Natural selection in the wild. Princeton, NJ: Princeton Univ. Press.

Reviews studies of selection in the field to that point in time, which now included more complex traits the book includes a quantitative analysis of the strength of selection from estimates available, foreshadowing many later syntheses (see Syntheses).

Ford, E. B. 1964. Ecological genetics. London: Methuen.

This book presents better evidence for natural selection actually occuring in the field than had been previously available, focusing on simple polymorphic traits. Four editions were published, the last in 1975.

Mayr, E. 1942. Systematics and the origin of species. New York: Columbia Univ. Press.

Takes Dobzhansky’s argument down one level to show that differences among populations within animal species could arise through population genetic processes, again including selection.

Stebbins, G. L. 1950. Variation and evolution in plants. New York: Columbia Univ. Press.

Presents evidence that Mayr’s arguments apply equally well to plants.

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Want to start a brawl at an evolution conference? Just bring up the concept of group selection: the idea that one mixed bag of individuals can be &ldquoselected&rdquo as a group over other heterogeneous groups from the same species. Biologists who would not hesitate to form a group themselves to combat creationism or intelligent design might suddenly start a pie fight to defend the principle that &ldquoit&rsquos every man for himself.&rdquo

Yet Charles Darwin himself argued for group selection. He postulated that moral men might not do any better than immoral men but that tribes of moral men would certainly &ldquohave an immense advantage&rdquo over fractious bands of pirates. By the 1960s, however, selection at the group level was on the outs. Influential theorist George Williams acknowledged that although group selection might be possible, in real life &ldquogroup-related adaptations do not, in fact, exist.&rdquo

Richard Dawkins of the University of Cambridge, whose writings have reached millions, maintains that selection might not even reach such a high level of biological organization as the individual organism. Instead, he claims, selection operates on genes&mdashthe individual is the embodiment of the selection of thousands of selfish genes, each trying to perpetuate itself.

In the past few decades, however, group selection has made a quiet comeback among evolutionary theorists. E. O. Wilson of Harvard University and David Sloan Wilson (no relation) of Binghamton University are trying to give group selection full-fledged respectability. They are rebranding it as multilevel selection theory: selection constantly takes place on multiple levels simultaneously. And how do you figure the sum of those selections in any real-world circumstance? &ldquoWe simply have to examine situations on a case-by-case basis,&rdquo Sloan Wilson says.

But the Wilsons did offer some guidelines in the December 2007 issue of Quarterly Review of Biology. &ldquoAdaptation at any level,&rdquo they write, &ldquorequires a process of natural selection at the same level, and tends to be undermined by natural selection at lower levels.&rdquo

Experiments with actual groups illustrate the point. Pseudomonas fluorescens bacteria quickly suck all the dissolved oxygen out of a liquid habitat, leaving a thin habitable layer near the surface. But some bacteria spontaneously develop a beneficial mutation. These group-saving individuals secrete a polymer that enables bunches of individuals to form floating mats. As a mat, all the bacteria survive, even though most of them expend no metabolic energy producing the polymer. But if the freeloaders get greedy and reproduce too many of their kind, the mat sinks and everybody dies, altruists and freeloaders alike. Among these bacteria, then, groups that maintain enough altruists to float outcompete groups with fewer altruists than that minimum number. The former groups survive, grow and split up into daughter groups. Thus, altruistic individuals can prosper, despite the disadvantage of expending precious resources to produce the polymer.

Perhaps the biggest change that group selection brings to evolutionary theory is its implication for so-called kin selection. What looks like group selection, some theorists argue, can actually be understood as genetic relatedness. Evolutionist J.B.S. Haldane pithily explained kin selection: &ldquoI would lay down my life for two brothers or eight cousins.&rdquo In this view, altruistic bacteria in the Pseudomonas mats are saving close relatives, thereby ensuring the survival of most of the genes they themselves also carry.

Turning that argument on its head, the Wilsons assert that kin selection is a special case of group selection. &ldquoThe importance of kinship,&rdquo they note, &ldquois that it increases genetic variation among groups.&rdquo The individuals within any one group are much more like one another and much less like the individuals in any other group. And that diversity between groups presents clearer choices for group selection. Kinship thus accentuates the importance of selection at the group level as compared with individual selection within the group.

The Wilsons think evolutionists must embrace multilevel selection to do fruitful research in sociobiology&mdash&ldquothe study of social behavior from a biological perspective.&rdquo When doing so, other investigators can keep in mind the Wilsons&rsquo handy rule of thumb: &ldquoSelfishness beats altruism within groups. Altruistic groups beat selfish groups.&rdquo

Note: This article was originally printed with the title, "What's Good for the Group".


Watch the video: Tim Tyler: Okasha, Evolution and the Levels of Selection (August 2022).