A tree of tree frogs around the Black Sea

Speciation, the process by which one species evolves into two or more, is a major focus of ongoing debate, particularly regarding the geographic context in which it occurs. Geographic models of speciation tend to fall into discrete categories, typically referred to as allopatric, parapatric and sympatric speciation, according to whether two groups evolve reproductive isolation while geographically isolated, differentiated but connected by gene flow, or completely co‐occurring. Yet molecular studies indicate that full development of reproductive isolation can take very long compared with the timescale at which climatic oscillations occur, such that the geographic context of differentiating forms might change often during the long process to full species. Studies of genetic relationships across the ranges of organisms with low‐dispersal distances have the potential to reveal these complex histories. In a particularly elegant example in this issue, Dufresnes et al. ( 2016 ) use genetic variation and ecological niche modelling to show that a ring of populations of the eastern tree frog (Hyla orientalis) surrounding the Black Sea had a complex history of geographic differentiation. Alternating phases of geographic fragmentation and phases of gene flow between neighbouring populations have produced a pattern of gradual genetic change connecting the western, southern and eastern sides of the ring, with the northwestern and northeastern forms being most differentiated. In the north, a population in Crimea appears to have been produced through mixture of the two extreme forms. The overall genetic relationships are reminiscent of those found in ring species, which have been used as prime demonstrations of the process of speciation. The difference, however, is that the terminal forms appear to have mixed rather than be reproductively isolated, although more research is needed to infer whether there might be some reproductive isolation on the northern side of the ring.     

Sympatric speciation without borders?

The biogeography of speciation remains a controversial issue and the process of allopatric speciation reigns. Sympatric speciation differs from allopatric speciation in terms of geographic setting and the role of selection in bringing about reproductive isolating mechanisms, making it a particularly fascinating and controversial subject for evolutionary biologists. Mayr (1947) explained the difference eloquently: for allopatric speciation, populations spatially diverge and then become reproductively isolated; for sympatric speciation, populations first become reproductively isolated and then diverge. Because of this, sympatric speciation is difficult to show empirically and most evolutionary biologists agree that strict ecological, evolutionary, and geographic criteria must be met ( Coyne & Orr 2004 ). In this issue, Crow et al. (2010) challenge us to expand the definition of sympatric speciation by studying species of marine fishes that they propose have arisen by sympatric speciation in a setting that does not appear to conform to the usual geographical criteria.     

Sympatric Speciation in the Post “Modern Synthesis” Era of Evolutionary Biology

Sympatric speciation is among the most controversial and challenging concepts in evolution. There are a multitude of definitions of speciation alone, and when combined with the biogeographic concept of sympatric range overlap, consensus on what sympatric speciation is, whether it happens, and its importance, is even more difficult to achieve. Providing the basis upon which to define and judge sympatric speciation, the Modern Evolutionary Synthesis (Huxley in Evolution: the modern synthesis. MIT Press, Cambridge, 1942) led to the conclusion that sympatric speciation is an inconsequential process in the generation of species diversity. In the post Modern Synthesis era of evolutionary biology, the PCR revolution and associated accumulation of DNA sequence data from natural populations has led to a resurgence of interest in sympatric speciation, and more importantly, the role of natural selection in lineage diversification. Much effort is currently being devoted to elucidating the processes by which the constituents of an initially panmictic population can become reproductively isolated and evolve some level of reproductive incompatibility without the complete cessation of gene flow due to geographic barriers. The evolution of reproductive isolation solely due to natural selection is perhaps the most controversial manner by which sympatric speciation occurs, and it is that which we focus upon in this review. Mathematical model simulations indicate that even strict definitions of sympatric speciation are possible to satisfy, empirical data consistent with sympatric divergence are accumulating, but irrefutable evidence of sympatric speciation in natural populations remains elusive. Genomic investigations are advancing our ability to identify genetic patterns caused by natural selection, thereby advancing our understanding of the power of natural selection relative to gene flow. Overall, sympatric lineage divergence, especially at the sub-species level, may have led to a substantial portion of biodiversity.     

Song variation in an avian ring species

Abstract.— Divergence of mating signals can occur rapidly and be of prime importance in causing reproductive isolation and speciation. A ring species, in which two reproductively isolated taxa are connected by a chain of intergrading populations, provides a rare opportunity to use spatial variation to reconstruct the history of divergence. I use geographic variation in the song of a likely ring species, the greenish warbler ( Phylloscopus trochiloides ) to reconstruct the microevolutionary steps that occurred during divergence of a trait that is often important in speciation in birds. Populations of a western Siberian ( P. t. viridanus ) and an eastern Siberian ( P. t. plumbeitarsus ) form of the greenish warbler meet, but do not interbreed in central Siberia; these forms are connected by a chain of interbreeding populations extending in a ring to the south around the treeless Tibetan Plateau. I show that: (1) song structure differs greatly between the two Siberian forms, which share the same habitat; (2) song structure changes gradually around the ring; (3) singing behavior is relatively simple in the Himalayas, but becomes increasingly complex to the north, both to the west and east of the Tibetan Plateau; and (4) song varies along independent axes of complexity in the western and eastern south-north clines. By comparing geographic variation in singing behavior and ecological variables, I distinguish among possible causes of song divergence, including selection based on the acoustic environment, stochastic effects of sexual selection, and selection for species recognition. I suggest that parallel south-to-north ecological gradients have caused a greater intensity of sexual selection on song in northern populations and that the stochastic effects of sexual selection have led to divergence in song structure.     

Biosystematic and evolutionary statuses of two sympatric populations ofAphytis mytilaspidis [Hym.: Aphelinidae]

Studies on 2 sympatric morphologically identical populations ofAphytis mytilaspidis Le Baron frpm Greece were conducted on their mating behavior, reproductive isolation, hybridization in the laboratory, host preferences and adult survivorships at different temperatures. These studies revealed that the 2 populations were partially reproductively isolated and qualify at least for the rank of strong “semispecies” with respect to one another. Marked differences were also recorded in their host preferences and adult survivorships at different temperatures. Gene flow among these populations in nature is likely to occur only in one direction. Differences in host preference may further reduce gene flow by providing ecological reproductive isolation. Also hybrid sterility is indicated which if complete would prove these 2 forms to be sibling species. Based on the experimental evidence, an hypothesis is presented that these 2 populations represent a case of incipient if not complete sympatric speciation, with the presumed descendant population (R-69-120) possessing a broader host range and better adult longevity, in comparison to the proposed progenitor population (R-63-63-4).     

Phenotypic divergence and reproductive isolation between sympatric forms of Japanese threespine sticklebacks

The threespine stickleback ( Gasterosteus aculeatus ) species complex is well suited for identifying the types of phenotypic divergence and isolating barriers that contribute to reproductive isolation at early stages of speciation. In the present study, we characterize the patterns of genetic and phenotypic divergence as well as the types of isolating barriers that are present between two sympatric pairs of threespine sticklebacks in Hokkaido, Japan. One sympatric pair consists of an anadromous and a resident freshwater form and shows divergence in body size between the forms, despite the lack of genetic differentiation between them. The second sympatric pair consists of two anadromous forms, which originated before the last glacial period and are currently reproductively isolated. These two anadromous forms have diverged in many morphological traits as well as in their reproductive behaviours. Both sexual isolation and hybrid male sterility contribute to reproductive isolation between the anadromous species pair. We discuss the shared and unique aspects of phenotypic divergence and reproductive isolation in the Japanese sympatric pairs compared with postglacial stickleback species pairs. Further studies of these divergent species pairs will provide a deeper understanding of the mechanisms of speciation in sticklebacks.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 671–685.     

Long- term effects of previous experience determine nutrient discrimination abilities in birds

Background

The evolution of reproductive traits, such as hybrid incompatibility (postzygotic isolation) and species recognition (prezygotic isolation), have shown their key role in speciation. Theoretical modeling has recently predicted that close linkage between genes controlling pre- and postzygotic reproductive isolation could accelerate the conditions for speciation. Postzygotic isolation could develop during the sympatric speciation process contributing to the divergence of populations. Using hybrid fitness as a measure of postzygotic reproductive isolation, we empirically studied population divergence in perch (Perca fluviatilis L.) from two genetically divergent populations within a lake.

Results

During spawning time of perch we artificially created parental offspring and F₁ hybrids of the two populations and studied fertilization rate and hatching success under laboratory conditions. The combined fitness measure (product of fertilization rate and hatching success) of F₁ hybrids was significantly reduced compared to offspring from within population crosses.

Conclusion

Our results suggest intrinsic genetic incompatibility between the two populations and indicate that population divergence between two populations of perch inhabiting the same lake may indeed be promoted by postzygotic isolation.     

Reproductive Isolation Among <Emphasis Type="Italic">Drosophila arizonae</Emphasis> from Geographically Isolated Regions of North America

A long-standing goal of speciation research is to describe how reproductive isolating barriers develop, when they arise along the ‘speciation continuum’, and to measure the strength with which they restrict gene flow. Drosophila arizonae and D. mojavensis are a recently diverged sister species pair distributed from the southwestern United States through southern Mexico. While incipient speciation in D. mojavensis has been studied for decades, relatively little attention has been directed toward D. arizonae, despite the fact that previous studies have revealed evidence for significant genetic differentiation among populations separated by geographic barriers. Here, we examine the potential for both pre- and post-mating reproductive isolation in D. arizonae from geographically isolated parts of North America. We find evidence for strong premating isolation between flies from northern mainland Mexico and southern mainland Mexico, but no evidence for postmating isolation in any cross. This study highlights the utility of the D. arizonae system for further investigation into the early evolution of premating isolation, and reinforces the potential of the D. arizonae/D. mojavensis system as a whole for studying the evolution of reproductive isolation at a range of divergence times.     

Behavioural response of a migratory songbird to geographic variation in song and morphology

Introduction

Sexually selected traits contribute substantially to evolutionary diversification, for example by promoting assortative mating. The contributing traits and their relevance for reproductive isolation differ between species. In birds, sexually selected acoustic and visual signals often undergo geographic divergence. Clines in these phenotypes may be used by both sexes in the context of sexual selection and territoriality. The ways conspecifics respond to geographic variation in phenotypes can give insights to possible behavioural barriers, but these may depend on migratory behaviour. We studied a migratory songbird, the Stonechat, and tested its responsiveness to geographic variation in male song and morphology. The traits are acquired differently, with possible implications for population divergence. Song can evolve quickly through cultural transmission, and thus may contribute more to the establishment of geographic variation than inherited morphological traits. We first quantified the diversity of song traits from different populations. We then tested the responses of free-living Stonechats of both sexes to male phenotype with playbacks and decoys, representing local and foreign stimuli derived from a range of distances from the local population.

Results

Both sexes discriminated consistently between stimuli from different populations, responding more strongly to acoustic and morphological traits of local than foreign stimuli. Time to approach increased, and time spent close to the stimuli and number of tail flips decreased consistently with geographic distance of the stimulus from the local population. Discriminatory response behaviour was more consistent for acoustic than for morphological traits. Song traits of the local population differed significantly from those of other populations.

Conclusions

Evaluating an individual’s perception of geographic variation in sexually selected traits is a crucial first step for understanding reproductive isolation mechanisms. We have demonstrated that in both sexes of Stonechats the responsiveness to acoustic and visual signals decreased with increasing geographic distance of stimulus origin. These findings confirm consistent, fine discrimination for both learned song and inherited morphological traits in these migratory birds. Maintenance or further divergence in phenotypic traits could lead to assortative mating, reproductive isolation, and potentially speciation.

     

Multilocus model of sympatric speciation II. Two characters

The method used in the previous paper (Kondrashov, 1983Theor. Pop. Biol.27, 000-000) is applied to population polymorphic at two quantitative characters. Sympatric speciation is found to be possible in the case when difference in two characters is necessary for reproductive isolation. The influence of various factors on the process of sympatric speciation is studied and selection forces necessary for its completion are found. Speciation occurs more readily under the action both of disruptive selection in separate characters and of selection against individuals with “unbalanced” phenotypes. This type of selection is also most realistic when various phenotypes make use of different niches. The results obtained allow the supposition that the possibility of sympatric speciation is not reduced to a few cases when reproductive isolation between the forming species develops due to minor genetic differences. It is also shown that if one of the characters is not directly involved in the processes concerning speciation then the forming species do not differ in the character. Relative frequencies of the intermediate phenotypes are found for the terminal stage of speciation.     

Molecular and morphological delimitation of Australian <Emphasis Type="Italic">Triops</Emphasis> species (Crustacea: Branchiopoda: Notostraca)—large diversity and little morphological differentiation

Australia has a very rich and diverse large branchiopod fauna with approximately 140 described or provisionally delimited species, but only one species of Triops, Triops australiensis (Spencer and Hall 1895), is currently recognized. Previous studies identified extensive genetic diversity within T. australiensis that suggested the presence of cryptic species. Herein, we employed an integrative approach to taxonomy to delimit putative species, integrating COI and EF1α sequence data and morphological data. Putative species were initially delimited based on COI by two computational approaches (GMYC and ABGD). The results were interpreted in the light of several species concepts, with particular emphasis on reproductive isolation. Twenty to 27 genetic lineages were delimited. Of these, up to 26 represent species following an evolutionary or phylogenetic species concept. Eighteen are biological species, though reproductive isolation could not be unambiguously established for allopatric species or species without known males. The level of co-occurrences was exceptionally high for Triops, with up to three syntopic and six sympatric species. Species delimitation was impeded by extensive overlap between intraspecific variability and interspecific variation in the genetic as well as morphological datasets. Without prior delimitation of putative species via COI, morphological delimitation would have been impossible. A potential explanation for the morphological variability is the retention of ancestral polymorphisms over long periods of time and across multiple speciation events without subsequent differentiation.     

Testing evolutionary hypotheses for phenotypic divergence using landscape genetics

Understanding the evolutionary causes of phenotypic variation among populations has long been a central theme in evolutionary biology. Several factors can influence phenotypic divergence, including geographic isolation, genetic drift, divergent natural or sexual selection, and phenotypic plasticity. But the relative importance of these factors in generating phenotypic divergence in nature is still a tantalizing and unresolved problem in evolutionary biology. The origin and maintenance of phenotypic divergence is also at the root of many ongoing debates in evolutionary biology, such as the extent to which gene flow constrains adaptive divergence ( Garant et al. 2007 ) and the relative importance of genetic drift, natural selection, and sexual selection in initiating reproductive isolation and speciation ( Coyne & Orr 2004 ). In this issue, Wang & Summers (2010) test the causes of one of the most fantastic examples of phenotypic divergence in nature: colour pattern divergence among populations of the strawberry poison frog (Dendrobates pumilio) in Panama and Costa Rica ( Fig. 1 ). This study provides a beautiful example of the use of the emerging field of landscape genetics to differentiate among hypotheses for phenotypic divergence. Using landscape genetic analyses, Wang & Summers were able to reject the hypotheses that colour pattern divergence is due to isolation‐by‐distance (IBD) or landscape resistance. Instead, the hypothesis left standing is that colour divergence is due to divergent selection, in turn driving reproductive isolation among populations with different colour morphs. More generally, this study provides a wonderful example of how the emerging field of landscape genetics, which has primarily been applied to questions in conservation and ecology, now plays an essential role in evolutionary research.

Figure 1 Open in figure viewer PowerPoint Divergent colour morphs observed among populations of the strawberry poison frog, Dendrobates pumilio. Frogs are from San Cristobal (upper left), Cerro Brujo (upper right), Bastimentos (lower right), and Agua (lower left).     

Habitat preference and flowering‐time variation contribute to reproductive isolation between diploid and autotetraploid Anacamptis pyramidalis

Tetraploid lineages are typically reproductively isolated from their diploid ancestors by post‐zygotic isolation via triploid sterility. Nevertheless, polyploids often also exhibit ecological divergence that could contribute to reproductive isolation from diploid ancestors. In this study, we disentangled the contribution of different forms of reproductive isolation between sympatric diploid and autotetraploid individuals of the food‐deceptive orchid Anacamptis pyramidalis by quantifying the strength of seven reproductive barriers: three prepollination, one post‐pollination prezygotic and three post‐zygotic. The overall reproductive isolation between the two cytotypes was found very high, with a preponderant contribution of two prepollination barriers, that is phenological and microhabitat differences. Although the contribution of post‐zygotic isolation (triploid sterility) is confirmed in our study, these results highlight that prepollination isolation, not necessarily involving pollinator preference, can represent a strong component of reproductive isolation between different cytotypes. Thus, in the context of polyploidy as quantum speciation, that generates reproductive isolation via triploid sterility, ecological divergence can strengthen the reproductive isolation between cytotypes, reducing the waste of gametes in low fitness interploidy crosses and thus favouring the initial establishment of the polyploid lineage. Under this light, speciation by polyploidy involves ecological processes and should not be strictly considered as a nonecological form of speciation.     

Strong cryptic prezygotic isolation despite lack of behavioral isolation between sympatric host races of the leaf beetle Lochmaea capreae

One of the major goals in speciation research is to understand which isolation mechanisms form the first barriers to gene flow. This requires examining lineages that are still in the process of divergence or incipient species. Here, we investigate the presence of behavioral and several cryptic barriers between the sympatric willow and birch host races of Lochmaea capreae. Behavioral isolation did not have any profound effect on preventing gene flow. Yet despite pairs mating indiscriminately, no offspring were produced from the heterospecific matings between birch females and willow males due to the inability of males to transfer sperm to females. We found evidence for differences in genital morphology that may contribute to failed insemination attempts during copulation. The heterospecific matings between willow females and birch males resulted in viable offspring. Yet fecundity and hatchability was remarkably reduced, which is likely the result of lower efficiency in sperm transportation and storage and lower survival of sperm in the foreign reproductive tract. Our results provide evidence for the contribution of several postmating‐prezygotic barriers that predate behavioral isolation and act as primary inhibitors of gene flow in this system. This is a surprising, yet perhaps often overlooked feature of barriers acting early in sympatric speciation process.     

Locus- and population-specific selection and differentiation between incipient species of Anopheles gambiae

Anopheles gambiae, the primary mosquito vector of malaria in sub-Saharan Africa, is divided into 2 sympatric incipient species known as M form and S form. Recent genomic analysis of each form revealed that differentiation between forms is clustered into 3 unlinked regions of the genome. Here, we expand the investigation of these "genomic islands of speciation" to multiple populations, including all of the genes across one of the islands. Differentiation between the M and S forms in 2 of the islands is complete across all individuals in all populations, confirming that the M and S forms are reproductively isolated taxa. Differentiation at the third island (on chromosome 2R) is limited to Cameroon populations. There is reduced variation in the M form in Cameroon at this location and increased divergence to the outgroup Anopheles arabiensis, supporting an association of adaptation with reproductive isolation.     

Why was Darwin’s view of species rejected by twentieth century biologists?

Historians and philosophers of science agree that Darwin had an understanding of species which led to a workable theory of their origins. To Darwin species did not differ essentially from ‘varieties’ within species, but were distinguishable in that they had developed gaps in formerly continuous morphological variation. Similar ideas can be defended today after updating them with modern population genetics. Why then, in the 1930s and 1940s, did Dobzhansky, Mayr and others argue that Darwin failed to understand species and speciation? Mayr and Dobzhansky argued that reproductively isolated species were more distinct and ‘real’ than Darwin had proposed. Believing species to be inherently cohesive, Mayr inferred that speciation normally required geographic isolation, an argument that he believed, incorrectly, Darwin had failed to appreciate. Also, before the sociobiology revolution of the 1960s and 1970s, biologists often argued that traits beneficial to whole populations would spread. Reproductive isolation was thus seen as an adaptive trait to prevent disintegration of species. Finally, molecular genetic markers did not exist, and so a presumed biological function of species, reproductive isolation, seemed to delimit cryptic species better than character-based criteria like Darwin’s. Today, abundant genetic markers are available and widely used to delimit species, for example using assignment tests: genetics has replaced a Darwinian reliance on morphology for detecting gaps between species. In the 150th anniversary of The Origin of Species, we appear to be returning to more Darwinian views on species, and to a fuller appreciation of what Darwin meant.     

Sympatric reinforcement of reproductive barriers between <Emphasis Type="Italic">Neotinea tridentata</Emphasis> and <Emphasis Type="Italic">N. ustulata</Emphasis> (Orchidaceae)

Reinforcement is the process by which selection favors traits that decrease mating between two incipient species in response to costly mating or the production of maladapted hybrids, causing the evolution of greater reproductive isolation between emerging species. I have studied a pair of orchids, Neotinea tridentata and N. ustulata, to examine the level of postmating pre- and post-zygotic isolating mechanisms that maintain these species, and the degree to which the boundary may still be permeable to gene flow. In this study, I performed pollen tube growth rate experiments and I investigated pre- and post-zygotic barriers by performing hand pollination experiments in order to evaluate fruit set, embryonate seed set and seed germination rates by intra- and interspecific crosses. Fruit set, the percentage of embryonate seeds and germinability of interspecific crosses were reduced compared to intraspecific pollinations, showing significant differences between sympatric and allopatric populations. While in allopatric populations the post-pollination isolation index ranged between 0.40 and 0.11, in sympatric populations orchid pairs showed total isolation due to post-pollination prezygotic barriers, guaranteed at the level of pollen–stigma interactions. Indeed, in sympatric populations, pollen tubes reached the ovary after 24 h in only 8 out of 45 plants; in the remaining cases, the pollen tubes did not enter the ovary, and thus no fruit set occurred. This pair of orchids is characterized by postmating pre-zygotic reproductive isolation in sympatric populations that prevents the formation of hybrids. This mechanism of speciation, starting in allopatry and triggering the reinforcement mechanisms of reproductive isolation in secondary sympatry, is the most likely explanation for the pattern of evolutionary transitions found in this pair of orchids.     

Floral odour chemistry defines species boundaries and underpins strong reproductive isolation in sexually deceptive orchids

Background and Aims

The events leading to speciation are best investigated in systems where speciation is ongoing or incomplete, such as incipient species. By examining reproductive barriers among incipient sister taxa and their congeners we can gain valuable insights into the relative timing and importance of the various barriers involved in the speciation process. The aim of this study was to identify the reproductive barriers among sexually deceptive orchid taxa in the genus Chiloglottis.

Methods

The study targeted four closely related taxa with varying degrees of geographic overlap. Chemical, morphological and genetic evidence was combined to explore the basis of reproductive isolation. Of primary interest was the degree of genetic differentiation among taxa at both nuclear and chloroplast DNA markers. To objectively test whether or not species boundaries are defined by the chemistry that controls pollinator specificity, genetic analysis was restricted to samples of known odour chemistry.

Key Results

Floral odour chemical analysis was performed for 600+ flowers. The three sympatric taxa were defined by their specific chiloglottones, the semiochemicals responsible for pollinator attraction, and were found to be fully cross-compatible. Multivariate morphometric analysis could not reliably distinguish among the four taxa. Although varying from very low to moderate, significant levels of genetic differentiation were detected among all pairwise combinations of taxa at both nuclear and chloroplast loci. However, the levels of genetic differentiation were lower than expected for mature species. Critically, a lack of chloroplast DNA haplotype sharing among the morphologically indistinguishable and most closely related taxon pair confirmed that chemistry alone can define taxon boundaries.

Conclusions

The results confirmed that pollinator isolation, mediated by specific pollinator attraction, underpins strong reproductive isolation in these taxa. A combination of large effective population sizes, initial neutral mutations in the genes controlling floral scent, and a pool of available pollinators likely drives diversity in this system.     

Reproductive character displacement of female mate preferences for male cuticular hydrocarbons in Drosophila subquinaria

Several lines of evidence implicate sexual isolation in both initiating and completing the speciation process. Although its existence is straightforward to demonstrate, understanding the evolution of sexual isolation requires identifying the underlying phenotypes responsible so that we can determine how these have diverged. Here, we study geographic variation in female mate preferences for male sexual displays in the fly Drosophila subquinaria. Female D. subquinaria that are sympatric with its sister species D. recens discriminate strongly against both D. recens and allopatric conspecific males, whereas females from allopatric populations do not. Furthermore, female mate preferences target at least in part a suite of cuticular hydrocarbons (CHCs) in males and geographic variation in CHCs mirrors the pattern of mate discrimination. In this study, we quantify female mate preferences for male CHCs from populations that span the geographic range of D. subquinaria. We find that the direction of linear sexual selection varies significantly between populations that are sympatric versus allopatric with D. recens in a pattern of reproductive character displacement. Differences in preference partially align with existing differences in CHCs and patterns of sexual isolation, although discrepancies remain that suggest the involvement of additional traits and/or more complex, nonlinear preference functions.     

Adaptive radiation of barbs of the genus Labeobarbus (Cyprinidae) in an East African river

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