18.2E: Sympatric Speciation - Biology

18.2E: Sympatric Speciation - Biology

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Sympatric speciation occurs when two individual populations diverge from an ancestral species without being separated geographically.

Learning Objectives

  • Give examples of sympatric speciation

Key Points

  • Sympatric speciation can occur when one individual develops an abnormal number of chromosomes, either extra chromosomes ( polyploidy ) or fewer, such that viable interbreeding can no longer occur.
  • When the extra sets of chromosomes in a polyploid originate with the individual because their own gametes do not undergo cytokinesis after meiosis, the result is autopolyploidy.
  • When individuals of two different species reproduce to form a viable offspring, such that the extra chromosomes come from two different species, the result is an allopolyploid.
  • Once a species develops an abnormal number of chromosomes, it can then only interbreed with members of the population that have the same abnormal number, which can lead to the development of a new species.

Key Terms

  • sympatric speciation: the process through which new species evolve from a single ancestral species while inhabiting the same geographic region
  • autopolyploid: having more than two sets of chromosomes, derived from the same species, as a result of redoubling
  • allopolyploid: having multiple complete sets of chromosomes derived from different species

Sympatric Speciation

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? The answer is yes. The process of speciation within the same space is called sympatric speciation. The prefix “sym” means same, so “sympatric” means “same homeland” in contrast to “allopatric” meaning “other homeland.” A number of mechanisms for sympatric speciation have been proposed and studied.

One form of sympatric speciation can begin with a serious chromosomal error during cell division. In a normal cell division event, chromosomes replicate, pair up, and then separate so that each new cell has the same number of chromosomes. However, sometimes the pairs separate and the end cell product has too many or too few individual chromosomes in a condition called aneuploidy.

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, or the inability to interbreed with normal individuals, of an individual in the polyploidy 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. 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. However, 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: 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. 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, it takes place most commonly in plants. (Animals with any of the types of chromosomal aberrations described here are unlikely to survive and produce normal offspring. ) Scientists have discovered more than half of all plant species studied relate back to a species evolved through polyploidy. With such a high rate of polyploidy in plants, some scientists hypothesize that this mechanism takes place more as an adaptation than as an error.

18.2E: Sympatric Speciation - Biology

Sympatric speciation has been contentious since its inception, yet is increasingly recognized as important based on accumulating theoretical and empirical support. Here, we present a compelling case of sympatric speciation in a taxon of marine reef fishes using a comparative and mechanistic approach. Hexagrammos otakii and H. agrammus occur in sympatry throughout their ranges. Molecular sequence data from six loci, with complete sampling of the genus, support monophyly of these sister species. Although hybridization occurrs frequently with an allopatric congener in an area of slight distributional overlap, we found no F1 hybrids between the focal sympatric taxa throughout their coextensive ranges. We present genetic evidence for complete reproductive isolation based on SNP analysis of 382 individuals indicating fixed polymorphisms, with no shared haplotypes or genotypes, between sympatric species. To address questions of speciation, we take a mechanistic approach and directly compare aspects of reproductive isolation between allopatric and sympatric taxa both in nature and in the laboratory. We conclude that the buildup of reproductive isolation is strikingly different in sympatric vs. allopatric taxa, consistent with theoretical predictions. Lab reared hybrids from allopatric species crosses exhibit severe fitness effects in the F1 or backcross generation. No intrinsic fitness effects are observed in F1 hybrids from sympatric species pairs, however these treatments exhibited reduced fertilization success and complete pre‐mating isolation is implied in nature because F1 hybrid adults do not occur. Our study addresses limitations of previous studies and supports new criteria for inferring sympatric speciation.

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Interactions among quantitative traits in the course of sympatric speciation

Sympatric speciation, the origin of two or more species from a single local population, has almost certainly been involved in formation of several species flocks 1,2,3,4 , and may be fairly common in nature 5 . The most straightforward scenario for sympatric speciation requires disruptive selection favouring two substantially different phenotypes, and consists of the evolution of reproductive isolation between them followed by the elimination of all intermediate phenotypes 6 . Here we use the hypergeometric phenotypic model 7,8,9,10 to show that sympatric speciation is possible even when fitness and mate choice depend on different quantitative traits, so that speciation must involve formation of covariance between these traits. The increase in the number of variable lociaffecting fitness facilitates sympatric speciation, whereas the increase in the number of variable loci affecting mate choice has the opposite effect. These predictions may enable more cases of sympatric speciation to be identified.

EECG Embarkation: Investigating the genomic architecture of speciation via reinforcement

**The AGA grants EECG Research Awards each year to graduate and post-doctoral researchers who are at a critical point in their research, where additional funds would allow them to conclude their research project and prepare it for publication. EECG awardees also get the opportunity to hone their science communication and write three posts over their grant tenure for the AGA Blog. In the first in the series, our EECG awardees write about their research and their interests as an ’embarkation’.

About the Author: Dr. Rachel Moran is a Postdoctoral Associate in Dr. Suzanne McGaugh’s lab at the University of Minnesota and a 2021 EECG Awardee. Her research integrates behavioral and genomic approaches to study sexual selection, local adaptation, and speciation in fishes. To learn more about Rachel’s work you can visit her website and follow her on twitter @Rachel_L_Moran.

A major goal in evolutionary biology is to understand how lineages diverge in the face of gene flow. Gene flow and recombination have traditionally been thought of as homogenizing forces, acting to break up co-adapted gene complexes that differ between diverging lineages (Turelli et al. 2001). Within the past decade, an increase in the availability of genomic data has made it possible to look for signatures of recent and ancient introgression in natural populations (reviewed in Abbott et al. 2016, Taylor & Larson 2019). Surprisingly, this work has demonstrated that gene flow and hybridization (i.e., gene flow that occurs between members of two different species) often play an important role in species diversification.

Reinforcement is one type of speciation with gene flow that can act to finalize reproductive isolation in sympatry after secondary contact. Reinforcement occurs when natural selection directly favors the evolution of prezygotic barriers (e.g., behavioral isolation) in response to the presence of postzygotic barriers (e.g., genetic incompatibilities) (Butlin 1987, Howard 1993). In this manner, hybridization can actually promote the process of speciation.

Figure 1. Range map for the orangethroat darter (Etheostoma spectaible) and the rainbow darter (Etheostoma caeruleum). Males of both species are shown. © R Moran

My research examines the genetic basis of reinforcement in a diverse group of North American stream fishes called darters. Much of my work has focused on two wide-ranging species that overlap in several areas of sympatry, the orangethroat darter (Etheostoma spectabile) and the rainbow darter (Etheostoma caeruleum) (Figure 1). I found that males of these species have strong mating preferences for conspecific females (versus females from the other species), but only in populations where they naturally co-occur in sympatry. If fish from allopatric populations of both species – which have never encountered the other species in the wild – are brought together in the lab, males do not discriminate between females of the two species (Moran & Fuller 2018). This pattern of enhanced behavioral isolation in sympatry compared to allopatry is a classic signature of reinforcement, and is also referred to as reproductive character displacement (Figure 2). (As an aside, although the terminology used to describe character displacement and reinforcement in the literature has historically been inconsistent, the current consensus appears to be that a pattern of reproductive character displacement can come about through many underlying mechanisms/selective pressures, and reinforcement/selection against hybridization is just one of those possible mechanisms.)

Figure 2. Reproductive character displacement results in exaggerated trait differences between two species in areas of sympatry relative to allopatry. This can reduce the frequency of maladaptive interspecific interactions in sympatry. © R Moran

My work in darters has also shown that reinforcement drives the pattern of reproductive character displacement in this system. Hybrids suffer from negative fitness consequences in the form of skewed sex ratios in F1s and heightened mortality in backcrosses (Moran et al. 2018). In Moran et al. (2018), we found that F1 hybrid crosses between orangethroat and rainbow darters result in highly male-skewed sex ratios in both cross directions. Parental crosses do not deviate from a 1:1 male to female sex ratio. Interestingly, females do not appear to be missing from hybrid clutches, as F1 and parental crosses do not differ in total clutch size or number of offspring that survived it to adulthood. This suggests that F1 individuals that are genetically female may appear phenotypically as male, potentially due to genetic incompatibilities at the sex determining region (SDR). Such an incompatibility could act as a postzygotic barrier between species.

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Genetics Speciation And The Founder Principle

Synopsis : Genetics Speciation and the Founder Principle written by Luther Val Giddings, published by Oxford University Press on Demand which was released on 01 July 1989. Download Genetics Speciation and the Founder Principle Books now! Available in PDF, EPUB, Mobi Format. This book describes the genetic mechanisms that govern the development and evolution of animals and plants. In particular, the book focuses on animal and plant species evolving in isolated habitats and species colonizing new territories. This approach--studying "founder" populations--enables geneticists to more readily identify some of the evolutionary pressures affecting the speciation process. The Founder Principle in population genetics was elucidated in large part by Hampton Carson in classic studies of Hawaiian fruit flies (Drosophila). The editors of this volume have commissioned seventeen chapters by an internationally recognized group of geneticists who discuss the principle in relation to plant speciation, chromosomal evolution, molecular evolution and development, sexual selection, and genetic changes in natural populations.