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3.4: Speciation - Biology

3.4: Speciation - Biology


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The biological definition of species, which works for sexually reproducing organisms, is a group of actually or potentially interbreeding individuals. According to this definition, one species is distinguished from another by the possibility of matings between individuals from each species to produce fertile offspring. There are exceptions to this rule. Many species are similar enough that hybrid offspring are possible and may often occur in nature, but for the majority of species this rule generally holds. In fact, the presence of hybrids between similar species suggests that they may have descended from a single interbreeding species and that the speciation process may not yet be completed.

Given the extraordinary diversity of life on the planet there must be mechanisms for speciation: the formation of two species from one original species. Darwin envisioned this process as a branching event and diagrammed the process in the only illustration found in On the Origin of Species (Figure 11.4.1a). For speciation to occur, two new populations must be formed from one original population, and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists have proposed mechanisms by which this could occur that fall into two broad categories. Allopatric speciation, meaning speciation in “other homelands,” involves a geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation, meaning speciation in the “same homeland,” involves speciation occurring within a parent species while remaining in one location.

Biologists think of speciation events as the splitting of one ancestral species into two descendant species. There is no reason why there might not be more than two species formed at one time except that it is less likely and such multiple events can also be conceptualized as single splits occurring close in time.

Speciation through Geographic Separation

A geographically continuous population has a gene pool that is relatively homogeneous. Gene flow, the movement of alleles across the range of the species, is relatively free because individuals can move and then mate with individuals in their new location. Thus, the frequency of an allele at one end of a distribution will be similar to the frequency of the allele at the other end. When populations become geographically discontinuous that free-flow of alleles is prevented. When that separation lasts for a period of time, the two populations are able to evolve along different trajectories. Thus, their allele frequencies at numerous genetic loci gradually become more and more different as new alleles independently arise by mutation in each population. Typically, environmental conditions, such as climate, resources, predators, and competitors, for the two populations will differ causing natural selection to favor divergent adaptations in each group. Different histories of genetic drift, enhanced because the populations are smaller than the parent population, will also lead to divergence.

Given enough time, the genetic and phenotypic divergence between populations will likely affect characters that influence reproduction enough that were individuals of the two populations brought together, mating would be less likely, or if a mating occurred, offspring would be non-viable or infertile. Many types of diverging characters may affect the reproductive isolation (inability to interbreed) of the two populations. These mechanisms of reproductive isolation can be divided into prezygotic mechanisms (those that operate before fertilization) and postzygotic mechanisms (those that operate after fertilization). Prezygotic mechanisms include traits that allow the individuals to find each other, such as the timing of mating, sensitivity to pheromones, or choice of mating sites. If individuals are able to encounter each other, character divergence may prevent courtship rituals from leading to a mating either because female preferences have changed or male behaviors have changed. Physiological changes may interfere with successful fertilization if mating is able to occur. Postzygotic mechanisms include genetic incompatibilities that prevent proper development of the offspring, or if the offspring live, they may be unable to produce viable gametes themselves as in the example of the mule, the infertile offspring of a female horse and a male donkey.

If the two isolated populations are brought back together and the hybrid offspring that formed from matings between individuals of the two populations have lower survivorship or reduced fertility, then selection will favor individuals that are able to discriminate between potential mates of their own population and the other population. This selection will enhance the reproductive isolation.

Isolation of populations leading to allopatric speciation can occur in a variety of ways: from a river forming a new branch, erosion forming a new valley, or a group of organisms traveling to a new location without the ability to return, such as seeds floating over the ocean to an island. The nature of the geographic separation necessary to isolate populations depends entirely on the biology of the organism and its potential for dispersal. If two flying insect populations took up residence in separate nearby valleys, chances are that individuals from each population would fly back and forth, continuing gene flow. However, if two rodent populations became divided by the formation of a new lake, continued gene flow would be unlikely; therefore, speciation would be more likely.

Biologists group allopatric processes into two categories. If a few members of a species move to a new geographical area, this is called dispersal. If a natural situation arises to physically divide organisms, this is called vicariance.

Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west coast of the United States, two separate subspecies of spotted owls exist. The northern spotted owl has genetic and phenotypic differences from its close relative, the Mexican spotted owl, which lives in the south (Figure 11.4.2). The cause of their initial separation is not clear, but it may have been caused by the glaciers of the ice age dividing an initial population into two.1

Additionally, scientists have found that the further the distance between two groups that once were the same species, the more likely for speciation to take place. This seems logical because as the distance increases, the various environmental factors would likely have less in common than locations in close proximity. Consider the two owls; in the north, the climate is cooler than in the south; the other types of organisms in each ecosystem differ, as do their behaviors and habits; also, the hunting habits and prey choices of the owls in the south vary from the northern ones. These variances can lead to evolved differences in the owls, and over time speciation will likely occur unless gene flow between the populations is restored.

In some cases, a population of one species disperses throughout an area, and each finds a distinct niche or isolated habitat. Over time, the varied demands of their new lifestyles lead to multiple speciation events originating from a single species, which is called adaptive radiation. From one point of origin, many adaptations evolve causing the species to radiate into several new ones. Island archipelagos like the Hawaiian Islands provide an ideal context for adaptive radiation events because water surrounds each island, which leads to geographical isolation for many organisms (Figure 11.4.3). The Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, called the founder species, numerous species have evolved, including the eight shown in Figure 11.4.3.

Notice the differences in the species’ beaks in Figure 11.4.3. Change in the genetic variation for beaks in response to natural selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food source. The fruit and seed-eating birds have thicker, stronger beaks which are suited to break hard nuts. The nectar-eating birds have long beaks to dip into flowers to reach their nectar. The insect-eating birds have beaks like swords, appropriate for stabbing and impaling insects. Darwin’s finches are another well-studied example of adaptive radiation in an archipelago.

Speciation without Geographic Separation

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? A number of mechanisms for sympatric speciation have been proposed and studied.

One form of sympatric speciation can begin with a chromosomal error during meiosis or the formation of a hybrid individual with too many chromosomes. Polyploidy is a condition in which a cell, or organism, has an extra set, or sets, of chromosomes. Scientists have identified two main types of polyploidy that can lead to reproductive isolation of an individual in the polyploid state. In some cases a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition called autopolyploidy (Figure 11.4.4). The prefix “auto” means self, so the term means multiple chromosomes from one’s own species. Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of separating.

For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2n = 6, when they should be n = 3), the gametes now have twice as many chromosomes as they should have. These new gametes will be incompatible with the normal gametes produced by this plant species. But they could either self-pollinate or reproduce with other autopolyploid plants with gametes having the same diploid number. In this way, sympatric speciation can occur quickly by forming offspring with 4n called a tetraploid. These individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral species. The other form of polyploidy occurs when individuals of two different species reproduce to form a viable offspring called an allopolyploid. The prefix “allo” means “other” (recall from allopatric); therefore, an allopolyploid occurs when gametes from two different species combine. Figure 11.4.5 illustrates one possible way an allopolyploidy can form. Notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results.

The cultivated forms of wheat, cotton, and tobacco plants are all allopolyploids. Although polyploidy occurs occasionally in animals, most chromosomal abnormalities in animals are lethal; it takes place most commonly in plants. Scientists have discovered more than 1/2 of all plant species studied relate back to a species evolved through polyploidy.

Sympatric speciation may also take place in ways other than polyploidy. For example, imagine a species of fish that lived in a lake. As the population grew, competition for food also grew. Under pressure to find food, suppose that a group of these fish had the genetic flexibility to discover and feed off another resource that was unused by the other fish. What if this new food source was found at a different depth of the lake? Over time, those feeding on the second food source would interact more with each other than the other fish; therefore they would breed together as well. Offspring of these fish would likely behave as their parents and feed and live in the same area, keeping them separate from the original population. If this group of fish continued to remain separate from the first population, eventually sympatric speciation might occur as more genetic differences accumulated between them.

This scenario does play out in nature, as do others that lead to reproductive isolation. One such place is Lake Victoria in Africa, famous for its sympatric speciation of cichlid fish. Researchers have found hundreds of sympatric speciation events in these fish, which have not only happened in great number, but also over a short period of time. Figure 11.4.6 shows this type of speciation among a cichlid fish population in Nicaragua. In this locale, two types of cichlids live in the same geographic location; however, they have come to have different morphologies that allow them to eat various food sources.

Finally, a well-documented example of ongoing sympatric speciation occurred in the apple maggot fly, Rhagoletis pomonella, which arose as an isolated population sometime after the introduction of the apple into North America. The native population of flies fed on hawthorn species and is host-specific: it only infests hawthorn trees. Importantly, it also uses the trees as a location to meet for mating. It is hypothesized that either through mutation or a behavioral mistake, flies jumped hosts and met and mated in apple trees, subsequently laying their eggs in apple fruit. The offspring matured and kept their preference for the apple trees effectively dividing the original population into two new populations separated by host species, not by geography. The host jump took place in the nineteenth century, but there are now measureable differences between the two populations of fly. It seems likely that host specificity of parasites in general is a common cause of sympatric speciation.

Section Summary

Speciation occurs along two main pathways: geographic separation (allopatric speciation) and through mechanisms that occur within a shared habitat (sympatric speciation). Both pathways force reproductive isolation between populations. Sympatric speciation can occur through errors in meiosis that form gametes with extra chromosomes, called polyploidy. Autopolyploidy occurs within a single species, whereas allopolyploidy occurs because of a mating between closely related species. Once the populations are isolated, evolutionary divergence can take place leading to the evolution of reproductive isolating traits that prevent interbreeding should the two populations come together again. The reduced viability of hybrid offspring after a period of isolation is expected to select for stronger inherent isolating mechanisms.

Multiple Choice

Which situation would most likely lead to allopatric speciation?

A. A flood causes the formation of a new lake.
B. A storm causes several large trees to fall down.
C. A mutation causes a new trait to develop.
D. An injury causes an organism to seek out a new food source.

A

What is the main difference between dispersal and vicariance?

A. One leads to allopatric speciation, whereas the other leads to sympatric speciation.
B. One involves the movement of the organism, whereas the other involves a change in the environment.
C. One depends on a genetic mutation occurring, whereas the other does not.
D. One involves closely related organisms, whereas the other involves only individuals of the same species.

B

Which variable increases the likelihood of allopatric speciation taking place more quickly?

A. lower rate of mutation
B. longer distance between divided groups
C. increased instances of hybrid formation
D. equivalent numbers of individuals in each population

B

Free Response

Why do island chains provide ideal conditions for adaptive radiation to occur?

Organisms of one species can arrive to an island together and then disperse throughout the chain, each settling into different niches, exploiting different food resources and, evolving independently with little gene flow between different islands.

Two species of fish had recently undergone sympatric speciation. The males of each species had a different coloring through which females could identify and choose a partner from her own species. After some time, pollution made the lake so cloudy it was hard for females to distinguish colors. What might take place in this situation?

It is likely the two species would start to reproduce with each other if hybridization is still possible. Depending on the viability of their offspring, they may fuse back into one species.

Footnotes

  1. 1 Courtney, S.P., et al, “Scientific Evaluation of the Status of the Northern Spotted Owl,” Sustainable Ecosystems Institute (2004), Portland, OR.

Glossary

adaptive radiation
a speciation when one species radiates out to form several other species
allopatric speciation
a speciation that occurs via a geographic separation
dispersal
an allopatric speciation that occurs when a few members of a species move to a new geographical area
speciation
a formation of a new species
sympatric speciation
a speciation that occurs in the same geographic space

New piece of the puzzle increases understanding of speciation

Speciation is important because it increases biodiversity. A thesis from the University of Gothenburg examines the speciation process in multiple marine species where different populations of the same species might evolve into two completely new species.

When two populations of a species become isolated, their genes no longer intermix and over time, the two populations become increasingly different from each other. What is known as a reproduction barrier has then been formed because the two different populations no longer mate with each other even if they would meet again.

For a long time, researchers proposed that new species could be formed only if two populations were separated by a physical barrier over a very long period of time, for hundreds of thousands of generations or more.

New species can form without physical isolation

Today, there are many examples of species being formed without isolation, such as during ongoing genetic exchange. This exchange should prevent two populations to become different and so, understanding how reproductive barriers can still develop is an intriguing question for speciation researchers.

Samuel Perini, researcher at the Department of Marine Sciences and author of the new thesis, has studied what happens in species with populations that are genetically different and meet at a contact zone, a boundary area between the two populations.

"I have investigated reproductive barriers that exist between two different forms of Littorina saxatilis, an intertidal marine snail, and I have analyzed data on reproductive barriers found in several marine species around the mouth of the Baltic Sea," says Perini.

Surrounding marine environment plays a role

In a review of reproductive barriers in 23 different species, including cod, herring and plaice, Samuel Perini found large genetic differences between the Baltic Sea populations and the North Sea populations.

"These differences are maintained partly because the populations survive differently in different salinities and partly because their reproduction is separated in time or space, or both."

For the Littorina saxatilis snail, which is common in the Atlantic along the coasts of both Europe and North America, two different populations or ecotypes have formed under ongoing genetic exchange, according to previous research. One population is known as the "Crab" ecotype and the other population is known as the "Wave" ecotype.

Crab snails live in and are adapted to portions of the rocky shore with large stones and crabs, while Wave snails live on portions of the rocky shore with rock slabs exposed to waves. Crab snails and Wave snails meet at the boundary of these two habitats but genetic and phenotypic differences are still maintained between the two populations. Adaptations to the Crab and Wave habitat is strongly driven by natural selection and survival in the non-native environment is low. Hence, natural selection reduces genetic exchange between Crab and Wave snail populations because it decreases the opportunity for a Crab snail to survive and reproduce in the Wave habitat with a Wave snail of the opposite sex (and vice versa).

Size matters

The size of the intertidal marine snail is important for adaptation to the different environments. Large snails are selected for in environments where there are crabs, and small snails are favoured in environments exposed to waves.

"My studies show that the size of intertidal marine snails is important not only for survival but also for mating. I show in my thesis that mating is more common between snails of similar sizes and that small males have more matings. Both of these factors help to counteract gene exchange between the large Crab snails and the smaller Wave snails when they meet both inside and outside the contact zones."


11.4 Speciation

The biological definition of species, which works for sexually reproducing organisms, is a group of actually or potentially interbreeding individuals. According to this definition, one species is distinguished from another by the possibility of matings between individuals from each species to produce fertile offspring. There are exceptions to this rule. Many species are similar enough that hybrid offspring are possible and may often occur in nature, but for the majority of species this rule generally holds. In fact, the presence of hybrids between similar species suggests that they may have descended from a single interbreeding species and that the speciation process may not yet be completed.

Given the extraordinary diversity of life on the planet there must be mechanisms for speciation : the formation of two species from one original species. Darwin envisioned this process as a branching event and diagrammed the process in the only illustration found in On the Origin of Species (Figure 11.14a). For speciation to occur, two new populations must be formed from one original population, and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists have proposed mechanisms by which this could occur that fall into two broad categories. Allopatric speciation , meaning speciation in “other homelands,” involves a geographic separation of populations from a parent species and subsequent evolution. Sympatric speciation , meaning speciation in the “same homeland,” involves speciation occurring within a parent species while remaining in one location.

Biologists think of speciation events as the splitting of one ancestral species into two descendant species. There is no reason why there might not be more than two species formed at one time except that it is less likely and such multiple events can also be conceptualized as single splits occurring close in time.

Speciation through Geographic Separation

A geographically continuous population has a gene pool that is relatively homogeneous. Gene flow, the movement of alleles across the range of the species, is relatively free because individuals can move and then mate with individuals in their new location. Thus, the frequency of an allele at one end of a distribution will be similar to the frequency of the allele at the other end. When populations become geographically discontinuous that free-flow of alleles is prevented. When that separation lasts for a period of time, the two populations are able to evolve along different trajectories. Thus, their allele frequencies at numerous genetic loci gradually become more and more different as new alleles independently arise by mutation in each population. Typically, environmental conditions, such as climate, resources, predators, and competitors, for the two populations will differ causing natural selection to favor divergent adaptations in each group. Different histories of genetic drift, enhanced because the populations are smaller than the parent population, will also lead to divergence.

Given enough time, the genetic and phenotypic divergence between populations will likely affect characters that influence reproduction enough that were individuals of the two populations brought together, mating would be less likely, or if a mating occurred, offspring would be non-viable or infertile. Many types of diverging characters may affect the reproductive isolation (inability to interbreed) of the two populations. These mechanisms of reproductive isolation can be divided into prezygotic mechanisms (those that operate before fertilization) and postzygotic mechanisms (those that operate after fertilization). Prezygotic mechanisms include traits that allow the individuals to find each other, such as the timing of mating, sensitivity to pheromones, or choice of mating sites. If individuals are able to encounter each other, character divergence may prevent courtship rituals from leading to a mating either because female preferences have changed or male behaviors have changed. Physiological changes may interfere with successful fertilization if mating is able to occur. Postzygotic mechanisms include genetic incompatibilities that prevent proper development of the offspring, or if the offspring live, they may be unable to produce viable gametes themselves as in the example of the mule, the infertile offspring of a female horse and a male donkey.

If the two isolated populations are brought back together and the hybrid offspring that formed from matings between individuals of the two populations have lower survivorship or reduced fertility, then selection will favor individuals that are able to discriminate between potential mates of their own population and the other population. This selection will enhance the reproductive isolation.

Isolation of populations leading to allopatric speciation can occur in a variety of ways: from a river forming a new branch, erosion forming a new valley, or a group of organisms traveling to a new location without the ability to return, such as seeds floating over the ocean to an island. The nature of the geographic separation necessary to isolate populations depends entirely on the biology of the organism and its potential for dispersal. If two flying insect populations took up residence in separate nearby valleys, chances are that individuals from each population would fly back and forth, continuing gene flow. However, if two rodent populations became divided by the formation of a new lake, continued gene flow would be unlikely therefore, speciation would be more likely.

Biologists group allopatric processes into two categories. If a few members of a species move to a new geographical area, this is called dispersal . If a natural situation arises to physically divide organisms, this is called vicariance .

Scientists have documented numerous cases of allopatric speciation taking place. For example, along the west coast of the United States, two separate subspecies of spotted owls exist. The northern spotted owl has genetic and phenotypic differences from its close relative, the Mexican spotted owl, which lives in the south (Figure 11.15). The cause of their initial separation is not clear, but it may have been caused by the glaciers of the ice age dividing an initial population into two. 5

Additionally, scientists have found that the further the distance between two groups that once were the same species, the more likely for speciation to take place. This seems logical because as the distance increases, the various environmental factors would likely have less in common than locations in close proximity. Consider the two owls in the north, the climate is cooler than in the south the other types of organisms in each ecosystem differ, as do their behaviors and habits also, the hunting habits and prey choices of the owls in the south vary from the northern ones. These variances can lead to evolved differences in the owls, and over time speciation will likely occur unless gene flow between the populations is restored.

In some cases, a population of one species disperses throughout an area, and each finds a distinct niche or isolated habitat. Over time, the varied demands of their new lifestyles lead to multiple speciation events originating from a single species, which is called adaptive radiation . From one point of origin, many adaptations evolve causing the species to radiate into several new ones. Island archipelagos like the Hawaiian Islands provide an ideal context for adaptive radiation events because water surrounds each island, which leads to geographical isolation for many organisms (Figure 11.16). The Hawaiian honeycreeper illustrates one example of adaptive radiation. From a single species, called the founder species, numerous species have evolved, including the eight shown in Figure 11.16.

Notice the differences in the species’ beaks in Figure 11.16. Change in the genetic variation for beaks in response to natural selection based on specific food sources in each new habitat led to evolution of a different beak suited to the specific food source. The fruit and seed-eating birds have thicker, stronger beaks which are suited to break hard nuts. The nectar-eating birds have long beaks to dip into flowers to reach their nectar. The insect-eating birds have beaks like swords, appropriate for stabbing and impaling insects. Darwin’s finches are another well-studied example of adaptive radiation in an archipelago.

Concepts in Action

Click through this interactive site to see how island birds evolved click to see images of each species in evolutionary increments from five million years ago to today.

Speciation without Geographic Separation

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? A number of mechanisms for sympatric speciation have been proposed and studied.

One form of sympatric speciation can begin with a chromosomal error during meiosis or the formation of a hybrid individual with too many chromosomes. Polyploidy is a condition in which a cell, or organism, has an extra set, or sets, of chromosomes. Scientists have identified two main types of polyploidy that can lead to reproductive isolation of an individual in the polyploid state. In some cases a polyploid individual will have two or more complete sets of chromosomes from its own species in a condition called autopolyploidy (Figure 11.17). The prefix “auto” means self, so the term means multiple chromosomes from one’s own species. Polyploidy results from an error in meiosis in which all of the chromosomes move into one cell instead of separating.

For example, if a plant species with 2n = 6 produces autopolyploid gametes that are also diploid (2n = 6, when they should be n = 3), the gametes now have twice as many chromosomes as they should have. These new gametes will be incompatible with the normal gametes produced by this plant species. But they could either self-pollinate or reproduce with other autopolyploid plants with gametes having the same diploid number. In this way, sympatric speciation can occur quickly by forming offspring with 4n called a tetraploid. These individuals would immediately be able to reproduce only with those of this new kind and not those of the ancestral species. The other form of polyploidy occurs when individuals of two different species reproduce to form a viable offspring called an allopolyploid. The prefix “allo” means “other” (recall from allopatric) therefore, an allopolyploid occurs when gametes from two different species combine. Figure 11.18 illustrates one possible way an allopolyploidy can form. Notice how it takes two generations, or two reproductive acts, before the viable fertile hybrid results.

The cultivated forms of wheat, cotton, and tobacco plants are all allopolyploids. Although polyploidy occurs occasionally in animals, most chromosomal abnormalities in animals are lethal it takes place most commonly in plants. Scientists have discovered more than 1/2 of all plant species studied relate back to a species evolved through polyploidy.

Sympatric speciation may also take place in ways other than polyploidy. For example, imagine a species of fish that lived in a lake. As the population grew, competition for food also grew. Under pressure to find food, suppose that a group of these fish had the genetic flexibility to discover and feed off another resource that was unused by the other fish. What if this new food source was found at a different depth of the lake? Over time, those feeding on the second food source would interact more with each other than the other fish therefore they would breed together as well. Offspring of these fish would likely behave as their parents and feed and live in the same area, keeping them separate from the original population. If this group of fish continued to remain separate from the first population, eventually sympatric speciation might occur as more genetic differences accumulated between them.

This scenario does play out in nature, as do others that lead to reproductive isolation. One such place is Lake Victoria in Africa, famous for its sympatric speciation of cichlid fish. Researchers have found hundreds of sympatric speciation events in these fish, which have not only happened in great number, but also over a short period of time. Figure 11.19 shows this type of speciation among a cichlid fish population in Nicaragua. In this locale, two types of cichlids live in the same geographic location however, they have come to have different morphologies that allow them to eat various food sources.

Finally, a well-documented example of ongoing sympatric speciation occurred in the apple maggot fly, Rhagoletis pomonella, which arose as an isolated population sometime after the introduction of the apple into North America. The native population of flies fed on hawthorn species and is host-specific: it only infests hawthorn trees. Importantly, it also uses the trees as a location to meet for mating. It is hypothesized that either through mutation or a behavioral mistake, flies jumped hosts and met and mated in apple trees, subsequently laying their eggs in apple fruit. The offspring matured and kept their preference for the apple trees effectively dividing the original population into two new populations separated by host species, not by geography. The host jump took place in the nineteenth century, but there are now measureable differences between the two populations of fly. It seems likely that host specificity of parasites in general is a common cause of sympatric speciation.


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3.4: Speciation - Biology

1. Mendel discovered the principles of inheritance with experiments in which large numbers of pea plants were crossed.

  • Mendel cross fertilised parental generation pure breeding individuals with contrasting traits
  • The resulting F1 generation displayed only one of the two versions of the trait seen in the P generation
  • Mendel allowed the resulting F1 generation to self -fertilize
  • The resulting F2 generation produced both of the parental versions of the trait, in 3:1 ratios

Terms are used to describe genes and their interactions:

  • genotype: the alleles possessed by an organism
  • phenotype: the characteristics of an organism
  • dominant allele: an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state
  • recessive allele: an allele which only has an effect on the phenotype when present in the homozygous state
  • codominant alleles: pairs of alleles that both affect the phenotype when present in a heterozygote
  • locus: the particular position on homologous chromosomes of a gene
  • homozygous: having the two identical alleles of a gene
  • heterozygous: having two different alleles of a gene
  • carrier: an individual that has a recessive allele of a gene that does not have an effect on their phenotype
  • test cross: testing a suspected heterozygote by crossing with a known homozygous recessive

2. Skill: Construction of Punnett grids for predicting the outcomes of monohybrid genetic crosses.

3. Gametes are haploid so contain only one allele of each gene.

4. The two alleles of each gene separate into different haploid daughter nuclei during meiosis.

5. Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.

6. Dominant alleles mask the effects of recessive alleles but co-dominant alleles have joint effects.

Application: Inheritance of ABO blood groups.

The expected notation for ABO blood group alleles is:

7. Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic diseases are due to dominant or co-dominant alleles.

Application: Inheritance of cystic fibrosis and Huntington’s disease.

8. Some genetic diseases are sex-linked. The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes.

sex chromosomes control gender in humans.

some genes are present on the X chromosome and absent from the shorter Y chromosome in humans

Application: Red-green colour blindness and hemophilia as examples of sexlinked inheritance.

  • a human female can be homozygous or heterozygous with respect to sex-linked genes.
  • female carriers are heterozygous for X-linked alleles.
  • heterozygous female carriers do not show the disease
  • but can pass it on to half of their male offspring
    • hemophilia: X H , X h , Y

    Guidance: Alleles carried on X chromosomes should be shown as superscript letters on an upper case X, such as X h .

    9. Skill: Comparison of predicted and actual outcomes of genetic crosses using real data.

    10. Skill: Analysis of pedigree charts to deduce the pattern of inheritance of genetic diseases.

    • Roman numeral identifies generation number
    • Arabic numeral identifies individual within a generation, numbering from left to right
    • square = male circle = female
    • shaded = affected by genetic disorder or trait

    11. Many genetic diseases have been identified in humans but most are very rare.

    Genetic Disorder Prevalence

    12. Radiation and mutagenic chemicals increase the mutation rate and can cause genetic diseases and cancer.

    Application: Consequences of radiation after nuclear bombing of Hiroshima and accident at Chernobyl.


    Unit 3&4 Resources

    On this page, you will find a number of downloadable resources for Unit 3&4 Biology. Suggested commercial resources are also listed below.

    COMMERCIAL RESOURCES

    You do not need all of these as you have your textbook, your notes and your teacher.

    1. Leading Edge

    The Leading Edge resources include cram notes for each area of study as well as notes on hypothesis formulation – this is one of the key examinable skills across each unit so you should definitely have a look at it. There are also exam tips and some samples to compare poor and excellent answers to some sample exam questions. A full length practice exam is also included.

    These 8 tests increase in the amount of content they cover as you go through. Test 8 covers the whole course, and there is a practice examination as well.

    3. Neap SmartStudy

    There are two parts to the Neap SmartStudy resource. There are AOS resources and three practice exams. The AOS resources contain both multiple choice and short answer questions, however, the short answer questions are grouped into specific topics to aid with your revision.

    4. Checkpoints

    The Checkpoints books are collection of previous VCAA examination questions as well as sets of questions to guide you through the key concepts for each area of study.

    5. Revise VCE Biology in a Month

    Fill the gap type revision and few examples of exam questions for each topic.

    DOWNLOADABLE RESOURCES

    UNIT 3: SIGNATURES OF LIFE

    Checkpoints:

    Immunology: Immunology Presentation – By Sarah Jones (Monash University)

    TSSM Test Yourself (Solutions):

    UNIT 4: CONTINUITY AND CHANGE

    Checkpoints:

    Genetics Quiz + Solutions: Quiz 1

    Unit 4- AOS 1 Test (Solutions): Test Answers

    NEW: TSSM Question 1 and 2 (from class)- Question 1 and 2 Solutions

    2014: Unit 4 STAV Exam (Solutions)- STAV 2014 (Unit 4) page 1 , STAV 2014 (Unit 4) page 2


    Inheritance 3.4

    This is a quiz of multiple choice style questions about the inheritance topic 3.4They are self-marking questions, so you can click on "check" to see whether you have the answer correct.Each question has a helpful note written by an examiner. Great for revision.Teachers can control access to this quiz for their groups in the "student access" section.Students - If this is an assignment - remember to click the "SUBMIT".

    To access the entire contents of this site, you need to log in or subscribe to it.


    The National Science Foundation funded this research (DEB 1257784/DEB 1257669). The Ohio Supercomputer Center allocated resources to support part of this study (PAS1184). Support for A.M. was provided in part by a graduate fellowship at OSU funded by CONACyT (Reg. 217900 CVU 324588).

    We thank museum curators who lent us tissue samples of specimens under their care: AMNH, American Museum of Natural History, New York, USA ASNHC, Angelo State Natural History Collection, Texas, USA CIIDIR-Durango, Colección de Mamíferos, Centro Interdisciplinario de Investigación para el Desarrollo Integral Regional, Durango, México UABC, Colección de Mamíferos, Universidad Autónoma de Baja California, México UMMZ, University of Michigan Museum Zoology. We thank Edwin Rice for assistance on DNA extractions and Troy Kieran for advice on library preparations. We thank members of the Carstens and O’Meara labs, and students in the first PHRAPL workshop for conversations related to this work.


    Lotsy, J. P. Evolution by Means of Hybridization (Martinus Nijhoff, The Hague, 1916)

    Mayr, E. Animal Species and Evolution (Harvard Univ. Press, Cambridge, Massachusetts, 1963)

    Stebbins, G. L. Processes of Organic Evolution (Prentice-Hall, Englewood Cliffs, New Jersey, 1971)

    Grant, V. Plant Speciation (Columbia Univ. Press, New York, 1981)

    Barton, N. H. The role of hybridization in evolution. Mol. Ecol. 10, 551–568 (2001). Medline

    Coyne, J. A. & Orr, H. A. Speciation (Sinauer Associates, Sunderland, Massachusetts, 2004)

    Anderson, E. & Stebbins, G. L. Hybridization as an evolutionary stimulus. Evolution 8, 378–388 (1954)

    Arnold, M. L. Natural Hybridization and Evolution (Oxford Univ. Press, Oxford, 1997)

    Seehausen, O. Hybridization and adaptive radiation. Trends Ecol. Evol. 19, 198–207 (2004).

    Dowling, T. E. & Secor, C. L. The role of hybridization and introgression in the diversification of animals. Annu. Rev. Ecol. Syst. 28, 593–620 (1997)

    Bullini, L. Origin and evolution of animal hybrid species. Trends Ecol. Evol. 9, 422–426 (1994)

    Grant, P. R., Grant, B. R. & Petren, K. Hybridization in the recent past. Am. Nat. 166, 56–57 (2005).

    Mallet, J. A species definition for the modern synthesis. Trends Ecol. Evol. 10, 294–299 (1995)

    Butlin, R. Speciation by reinforcement. Trends Ecol. Evol. 2, 8–12 (1987)

    Ortíz-Barrientos, D., Counterman, B. A. & Noor, M. A. F. The genetics of speciation by reinforcement. PLoS Biol. 2, e416 (2004)

    Tunner, H. G. & Nopp, H. Heterosis in the common European water frog. Naturwissenschaften 66, 268–269 (1979).

    Fisher, R. A. The Genetical Theory of Natural Selection (Clarendon Press, Oxford, 1930)

    Mallet, J. Hybridization as an invasion of the genome. Trends Ecol. Evol. 20, 229–237 (2005).

    Husband, B. C. Constraints on polyploid evolution: a test of the minority cytotype exclusion principle. Proc. R. Soc. Lond. B 267, 217–223 (2000)

    Wright, S. The roles of mutation, inbreeding, crossbreeding and selection in evolution. Proc. XI Int. Congr. Genet. Hague 1, 356–366 (1932)

    Otto, S. P. & Whitton, J. Polyploid incidence and evolution. Annu. Rev. Genet. 34, 401–437 (2000).

    Ramsey, J. & Schemske, D. W. Neopolyploidy in flowering plants. Annu. Rev. Ecol. Syst. 33, 589–639 (2002)

    Astaurov, B. L. Experimental polyploidy in animals. Annu. Rev. Genet. 3, 99–126 (1969)

    Muller, H. J. Why polyploidy is rarer in animals than in plants. Am. Nat. 59, 346–353 (1925)

    Mable, B. K. ‘Why polyploidy is rarer in animals than in plants’: myths and mechanisms. Biol. J. Linn. Soc. 82, 453–466 (2004)

    Soltis, D. E., Soltis, P. S. & Tate, J. A. Advances in the study of polyploidy since plant speciation. New Phytol. 161, 173–191 (2004)

    Brochmann, C. et al. Polyploidy in arctic plants. Biol. J. Linn. Soc. 82, 521–536 (2004)

    Abbott, R. J. & Lowe, A. J. Origins, establishment and evolution of new polyploid species: Senecio cambrensis and S. eboracensis in the British Isles. Biol. J. Linn. Soc. 82, 467–474 (2004)

    Ainouche, M. L., Baumel, A. & Salmon, A. Spartina anglica C. E. Hubbard: a natural model system for analysing early evolutionary changes that affect allopolyploid genomes. Biol. J. Linn. Soc. 82, 475–484 (2004)

    Buerkle, C. A., Morris, R. J., Asmussen, M. A. & Rieseberg, L. H. The likelihood of homoploid hybrid speciation. Heredity 84, 441–451 (2000).

    Rieseberg, L. H. Hybrid origins of plant species. Annu. Rev. Ecol. Syst. 28, 359–389 (1997)

    Rieseberg, L. H., Raymond, O., Rosenthal, D. M., Lai, Z. & Livingstone, K. Major ecological transitions in wild sunflowers facilitated by hybridization. Science 301, 1211–1216 (2003).

    Gross, B. L. & Rieseberg, L. H. The ecological genetics of homoploid hybrid speciation. J. Hered. 96, 241–252 (2005).

    Nolte, A. W., Freyhof, J., Stemshorn, K. C. & Tautz, D. An invasive lineage of sculpins, Cottus sp. (Pisces, Teleostei) in the Rhine with new habitat adaptations has originated from hybridization between old phylogeographic groups. Proc. R. Soc. Lond. B 272, 2379–2387 (2005)

    DeMarais, B. D., Dowling, T. E., Douglas, M. E., Minckley, W. L. & Marsh, P. C. Origin of Gila seminuda (Teleostei: Cyprinidae) through introgressive hybridization: implications for evolution and conservation. Proc. Natl Acad. Sci. USA 89, 2747–2751 (1992).

    Gompert, Z., Fordyce, J. A., Forister, M., Shapiro, A. M. & Nice, C. C. Homoploid hybrid speciation in an extreme habitat. Science 314, 1923–1925 (2006).

    Schwarz, D., Matta, B. M., Shakir-Botteri, N. L. & McPheron, B. A. Host shift to an invasive plant triggers rapid animal hybrid speciation. Nature 436, 546–549 (2005).

    Mavárez, J. et al. Speciation by hybridization in Heliconius butterflies. Nature 441, 868–871 (2006).

    Meyer, A., Salzburger, W. & Schartl, M. Hybrid origin of a swordtail species (Teleostei: Xiphophorus clemenciae) driven by sexual selection. Mol. Ecol. 15, 721–730 (2006).

    Labandeira, C. C. & Sepkoski, J. J. Insect diversity in the fossil record. Science 261, 310–315 (1993).

    Patterson, N., Richter, D. J., Gnerre, S., Lander, E. S. & Reich, D. Genetic evidence for complex speciation of humans and chimpanzees. Nature 441, 1103–1108 (2006).

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    Barton, N. H. How did the human species form? Curr. Biol. 16, R647–R650 (2006).

    Seehausen, O., van Alphen, J. J. M. & Witte, F. Cichlid fish diversity threatened by eutrophication that curbs sexual selection. Science 277, 1808–1811 (1997)

    Grant, B. R. & Grant, P. R. High survival of Darwin’s finch hybrids — effects of beak morphology and diets. Ecology 77, 500–509 (1996)


    Speciation Graphics For each graphic shown below , Identify the type of speciation and write a short caption describing the events occurring in the process .

    the concave shape of wave rock is a very good evidence that supports the evolution of earth due to chemical weathering. if we talk about structure of wave rock, studies have estimated that it is around 2.7 billion years old. it also serves as the foundation of evidence that it was the place where australian continent began to form.

    how the structure of wave rock was created?

    studies suggest that structure of wave rock was created when the surrounded area was started to weather due to which deposition of material started on the part of land which now contains wave rock.

    the structure of wave rock is considered to be one of the best-known landforms in australia. at the height of around 15 cm, there can be found flared slopes which formulate the margins of hyden rock. it is now assumed by geologists that wave rock was created by subsurface water-induced weathering and not by glaciers, waves or wind.

    due to deposition of material over the period of long time, the wave rock became tougher and tougher due to weathering from surrounding terrain. this makes it a physical evidence that supports the evolution of earth due to chemical weathering.


    3.7.3 Evolution may lead to speciation (A-level only)

    Individuals within a population of a species may show a wide range of variation in phenotype. This is due to genetic and environmental factors. The primary source of genetic variation is mutation. Meiosis and the random fertilisation of gametes during sexual reproduction produce further genetic variation.

    Predation, disease and competition for the means of survival result in differential survival and reproduction, ie natural selection.

    Those organisms with phenotypes providing selective advantages are likely to produce more offspring and pass on their favourable alleles to the next generation. The effect of this differential reproductive success on the allele frequencies within a gene pool.

    The effects of stabilising, directional and disruptive selection.

    Evolution as a change in the allele frequencies in a population.

    Reproductive separation of two populations can result in the accumulation of difference in their gene pools. New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring. In this way, new species arise from existing species.

    Allopatric and sympatric speciation.

    The importance of genetic drift in causing changes in allele frequency in small populations.

    Students should be able to:

    • explain why individuals within a population of a species may show a wide range of variation in phenotype
    • explain why genetic drift is important only in small populations
    • explain how natural selection and isolation may result in change in the allele and phenotype frequency and lead to the formation of a new species
    • explain how evolutionary change over a long period of time has resulted in a great diversity of species.

    Students could apply their knowledge of sampling to the concept of genetic drift.

    Students could devise an investigation to mimic the effects of random sampling on allele frequencies in a population.

    Students could use computer programs to model the effects of natural selection and of genetic drift.


    Watch the video: Ανάλυση καρυότυπου: Πώς γίνεται; Τι πληροφορίες δίνει; (January 2023).