THE BIOLOGY OF SPECIATION

Since Darwin published the “Origin,” great progress has been made in our understanding of speciation mechanisms. The early investigations by Mayr and Dobzhansky linked Darwin's view of speciation by adaptive divergence to the evolution of reproductive isolation, and thus provided a framework for studying the origin of species. However, major controversies and questions remain, including: When is speciation nonecological? Under what conditions does geographic isolation constitute a reproductive isolating barrier? and How do we estimate the “importance” of different isolating barriers? Here, we address these questions, providing historical background and offering some new perspectives. A topic of great recent interest is the role of ecology in speciation. “Ecological speciation” is defined as the case in which divergent selection leads to reproductive isolation, with speciation under uniform selection, polyploid speciation, and speciation by genetic drift defined as “nonecological.” We review these proposed cases of nonecological speciation and conclude that speciation by uniform selection and polyploidy normally involve ecological processes. Furthermore, because selection can impart reproductive isolation both directly through traits under selection and indirectly through pleiotropy and linkage, it is much more effective in producing isolation than genetic drift. We thus argue that natural selection is a ubiquitous part of speciation, and given the many ways in which stochastic and deterministic factors may interact during divergence, we question whether the ecological speciation concept is useful. We also suggest that geographic isolation caused by adaptation to different habitats plays a major, and largely neglected, role in speciation. We thus provide a framework for incorporating geographic isolation into the biological species concept (BSC) by separating ecological from historical processes that govern species distributions, allowing for an estimate of geographic isolation based upon genetic differences between taxa. Finally, we suggest that the individual and relative contributions of all potential barriers be estimated for species pairs that have recently achieved species status under the criteria of the BSC. Only in this way will it be possible to distinguish those barriers that have actually contributed to speciation from those that have accumulated after speciation is complete. We conclude that ecological adaptation is the major driver of reproductive isolation, and that the term “biology of speciation,” as proposed by Mayr, remains an accurate and useful characterization of the diversity of speciation mechanisms.     

The genic view of the process of speciation

The unit of adaptation is usually thought to be a gene or set of interacting genes, rather than the whole genome, and this may be true of species differentiation. Defining species on the basis of reproductive isolation (RI), on the other hand, is a concept best applied to the entire genome. The biological species concept (BSC; 84 ) stresses the isolation aspect of speciation on the basis of two fundamental genetic assumptions – the number of loci underlying species differentiation is large and the whole genome behaves as a cohesive, or coadapted genetic unit. Under these tenets, the exchange of any part of the genomes between diverging groups is thought to destroy their integrity. Hence, the maintenance of each species’ genome cohesiveness by isolating mechanisms has become the central concept of species. In contrast, the Darwinian view of speciation is about differential adaptation to different natural or sexual environments. RI is viewed as an important by product of differential adaptation and complete RI across the whole genome need not be considered as the most central criterion of speciation. The emphasis on natural and sexual selection thus makes the Darwinian view compatible with the modern genic concept of evolution. Genetic and molecular analyses of speciation in the last decade have yielded surprisingly strong support for the neo‐Darwinian view of extensive genetic differentiation and epistasis during speciation. However, the extent falls short of what BSC requires in order to achieve whole‐genome ‘cohesiveness’. Empirical observations suggest that the gene is the unit of species differentiation. Significantly, the genetic architecture underlying RI, the patterns of species hybridization and the molecular signature of speciation genes all appear to support the view that RI is one of the manifestations of differential adaptation, as 34 , Chap. 8) suggested. The nature of this adaptation may be as much the result of sexual selection as natural selection. In the light of studies since its early days, BSC may now need a major revision by shifting the emphasis from isolation at the level of whole genome to differential adaptation at the genic level. With this revision, BSC would in fact be close to Darwin’s original concept of speciation.     

Transitions between self-compatibility and self-incompatibility and the evolution of reproductive isolation in the large and diverse tropical genus Dendrobium (Orchidaceae)

Background and Aims The evolution of interspecific reproductive barriers is crucial to understanding species evolution. This study examines the contribution of transitions between self-compatibility (SC) and self-incompatibility (SI) and genetic divergence in the evolution of reproductive barriers in Dendrobium, one of the largest orchid genera. Specifically, it investigates the evolution of pre- and postzygotic isolation and the effects of transitions between compatibility states on interspecific reproductive isolation within the genus.Methods The role of SC and SI changes in reproductive compatibility among species was examined using fruit set and seed viability data available in the literature from 86 species and ∼2500 hand pollinations. The evolution of SC and SI in Dendrobium species was investigated within a phylogenetic framework using internal transcribed spacer sequences available in GenBank.Key Results Based on data from crossing experiments, estimations of genetic distance and the results of a literature survey, it was found that changes in SC and SI significantly influenced the compatibility between species in interspecific crosses. The number of fruits produced was significantly higher in crosses in which self-incompatible species acted as pollen donor for self-compatible species, following the SI × SC rule. Maximum likelihood and Bayesian tests did not reject transitions from SI to SC and from SC to SI across the Dendrobium phylogeny. In addition, postzygotic isolation (embryo mortality) was found to evolve gradually with genetic divergence, in agreement with previous results observed for other plant species, including orchids.Conclusions Transitions between SC and SI and the gradual accumulation of genetic incompatibilities affecting postzygotic isolation are important mechanisms preventing gene flow among Dendrobium species, and may constitute important evolutionary processes contributing to the high levels of species diversity in this tropical orchid group.     

Behavioural reproductive isolation and speciation in Drosophila

The origin of premating reproductive isolation continues to help elucidate the process of speciation and is the central event in the evolution of biological species. Therefore, during the process of species formation the diverging populations must acquire some means of reproductive isolation so that the genes from one gene pool are prevented from dispersing freely into a foreign gene pool. In the genus Drosophila, the phenomenon of behavioural reproductive isolation, which is an important type of premating (prezygotic) reproductive isolating mechanisms, has been extensively studied and interesting data have been documented. In many cases incomplete sexual isolation has been observed and the pattern and degree of isolation within and between the species have often been used to elucidate the phylogenetic relationships. The present review documents an overview of speciation mediated through behavioural incompatibility in different species groups of Drosophila with particular reference to the models proposed on the basis of one-sided ethological isolation to predict the direction of evolution. This study is crucial for understanding the mechanism of speciation through behavioural incompatibility and also for an understanding of speciation genetics in future prospects.     

Models of Evolution of Reproductive Isolation

Mathematical models are presented for the evolution of postmating and premating reproductive isolation. In the case of postmating isolation it is assumed that hybrid sterility or inviability is caused by incompatibility of alleles at one or two loci, and evolution of reproductive isolation occurs by random fixation of different incompatibility alleles in different populations. Mutations are assumed to occur following either the stepwise mutation model or the infinite-allele model. Computer simulations by using It?'s stochastic differential equations have shown that in the model used the reproductive isolation mechanism evolves faster in small populations than in large populations when the mutation rate remains the same. In populations of a given size it evolves faster when the number of loci involved is large than when this is small. In general, however, evolution of isolation mechanisms is a very slow process, and it would take thousands to millions of generations if the mutation rate is of the order of 10(-5) per generation. Since gene substitution occurs as a stochastic process, the time required for the establishment of reproductive isolation has a large variance. Although the average time of evolution of isolation mechanisms is very long, substitution of incompatibility genes in a population occurs rather quickly once it starts. The intrapopulational fertility or viability is always very high. In the model of premating isolation it is assumed that mating preference or compatibility is determined by male- and female-limited characters, each of which is controlled by a single locus with multiple alleles, and mating occurs only when the male and female characters are compatible with each other. Computer simulations have shown that the dynamics of evolution of premating isolation mechanism is very similar to that of postmating isolation mechanism, and the mean and variance of the time required for establishment of premating isolation are very large. Theoretical predictions obtained from the present study about the speed of evolution of reproductive isolation are consistent with empirical data available from vertebrate organisms.     

Variation in reproductive isolation across a species range

Reproductive isolation is often variable within species, a phenomenon that while largely ignored by speciation studies, can be leveraged to gain insight into the potential mechanisms driving the evolution of genetic incompatibilities. We used experimental greenhouse crosses to characterize patterns of reproductive isolation among three divergent genetic lineages of Campanulastrum americanum that occur in close geographic proximity in the Appalachian Mountains. Substantial, asymmetrical reproductive isolation for survival due to cytonuclear incompatibility was found among the lineages (up to 94% reduction). Moderate reductions in pollen viability, as well as cytoplasmic male sterility, were also found between some Mountain populations. We then compared these results to previously established patterns of reproductive isolation between these Mountain lineages and a fourth, widespread Western lineage to fully characterize reproductive isolation across the complete geographic and genetic range of C. americanum. Reproductive isolation for survival and pollen viability was consistent across studies, indicating the evolution of the underlying genetic incompatibilities is primarily determined by intrinsic factors. In contrast, reproductive isolation for germination was only found when crossing Mountain populations with the Western lineage, suggesting the underlying genetic incompatibility is likely influenced by environmental or demographic differences between the two lineages. Cytoplasmic male sterility was also limited in occurrence, being restricted to a handful of Mountain populations in a narrow geographic range. These findings illustrate the complexity of speciation by demonstrating multiple, independent genetic incompatibilities that lead to a mosaic of genetic divergence and reproductive isolation across a species range.     

The genetics of species differences

Species are separated by reproductive isolation as well as by more 'ordinary' differences in morphology and behavior that play no necessary role in blocking gene flow. Although a great deal is now known about the genetics of reproductive isolation, we are only beginning to understand the genetic basis of ordinary phenotypic differences between species. I review what is known about the number of genes involved in such differences, as well as about the role of major genes and epistasis in the evolution of these differences. I also compare and contrast these findings with those on the genetics of reproductive isolation.     

Persistent postmating,prezygotic reproductive isolation between populations

Studying reproductive barriers between populations of the same species is critical to understand how speciation may proceed. Growing evidence suggests postmating, prezygotic (PMPZ) reproductive barriers play an important role in the evolution of early taxonomic divergence. However, the contribution of PMPZ isolation to speciation is typically studied between species in which barriers that maintain isolation may not be those that contributed to reduced gene flow between populations. Moreover, in internally fertilizing animals, PMPZ isolation is related to male ejaculate—female reproductive tract incompatibilities but few studies have examined how mating history of the sexes can affect the strength of PMPZ isolation and the extent to which PMPZ isolation is repeatable or restricted to particular interacting genotypes. We addressed these outstanding questions using multiple populations of Drosophila montana. We show a recurrent pattern of PMPZ isolation, with flies from one population exhibiting reproductive incompatibility in crosses with all three other populations, while those three populations were fully fertile with each other. Reproductive incompatibility is due to lack of fertilization and is asymmetrical, affecting female fitness more than males. There was no effect of male or female mating history on reproductive incompatibility, indicating that PMPZ isolation persists between populations. We found no evidence of variability in fertilization outcomes attributable to different female × male genotype interactions, and in combination with our other results, suggests that PMPZ isolation is not driven by idiosyncratic genotype × genotype interactions. Our results show PMPZ isolation as a strong, consistent barrier to gene flow early during speciation and suggest several targets of selection known to affect ejaculate‐female reproductive tract interactions within species that may cause this PMPZ isolation.     

High divergence of reproductive tract proteins and their association with postzygotic reproductive isolation in Drosophila melanogaster and Drosophila virilis group species

The possible association between gonadal protein divergence and postzygotic reproductive isolation was investigated among species of the Drosophila melanogaster and D. virilis groups. Protein divergence was scored by high-resolution two-dimensional electrophoresis (2DE). Close to 500 protein spots from gonadal tissues (testis and ovary) and nongonadal tissues (malpighian tubules and brain) were analyzed and protein divergence was calculated based on presence vs absence. Both testis and ovary proteins showed higher divergence than nongonadal proteins, and also a highly significant positive correlation with postzygotic reproductive isolation but a weaker correlation with prezygotic reproductive isolation. Particularly, a positive and significant correlation was found between proteins expressed in the testis and postzygotic reproductive isolation among closely related species such as the within-phylad species in the D. virilis group. The high levels of male-reproductive-tract protein divergence between species might be associated with F₁ hybrid male sterility among closely related species. If so, a lower level of ovary protein divergence should be expected on the basis that F₁ female hybrids are fully fertile. However, this is not necessarily true if relatively few genes are responsible for the reproductive isolation observed between closely related species, as recent studies seem to suggest. We suggest that the faster rate of evolution of gonadal proteins in comparison to nongonadal proteins and the association of that rate with postzygotic reproductive isolation may be the result of episodic and/or sexual selection on male and female molecular traits. Correspondence to: A. Civetta     

Post-mating reproductive barriers in two unidirectionally hybridizing sunfish (Centrarchidae: Lepomis)

The evolutionary sequence of events in the evolution of reproductive barriers between species is at the core of speciation biology. Where premating barriers fail, post-mating barriers, such as conspecific sperm precedence (CSP), gamete incompatibility (GI) and hybrid inviability (HI) may evolve to prevent the production of (often) costly hybrid offspring with reduced fitness. We tested the role of post-mating mechanisms for the reproductive isolation between two sunfish species [bluegill (BG) Lepomis macrochirus and pumpkinseed (PS) Lepomis gibbosus] and their first-generation hybrids. Performing in vitro sperm competition experiments, we observed asymmetric CSP as main post-mating isolation mechanism when BG and PS sperm were competing for PS eggs, whereas when sperm from both species were competing for BG eggs it was HI. Furthermore, hybrid sperm--although fertile in the absence of competition--were outcompeted by sperm of either parental species. This result may at least partly explain previous observations that natural hybridization in the study system is unidirectional.     

On the evolution of premating isolation after a founder event

We present a new simple model for the evolution of premating reproductive isolation. Using this model we first analyze the level of genetic variability maintained by mutation in a large stable population. Then we consider the plausibility of the evolution of strong premating reproductive isolation after a founder event. We demonstrate that after a founder event a new adaptive combination of genes may rise to high frequencies in the presence of an old combination of genes. We compare the probabilities of speciation after a founder event with those in a stable population and with those when reproductive isolation is due to viability selection against hybrids. We argue that premating reproductive isolation is more efficient than postmating reproductive isolation in maintaining the integrity of sympatric species. This might have contributed to the pattern of stronger premating isolation than postmating isolation between closely related pairs of sympatric species.     

Species delimitation under the general lineage concept: an empirical example using wild North American hops (Cannabaceae: Humulus lupulus)

There is an emerging consensus that the intent of most species concepts is to identify evolutionarily distinct lineages. However, the criteria used to identify lineages differ among concepts depending on the perceived importance of various attributes of evolving populations. We have examined five different species criteria to ask whether the three taxonomic varieties of Humulus lupulus (hops) native to North America are distinct lineages. Three criteria (monophyly, absence of genetic intermediates, and diagnosability) focus on evolutionary patterns and two (intrinsic reproductive isolation and niche specialization) consider evolutionary processes. Phylogenetic analysis of amplified fragment length polymorphism (AFLP) data under a relaxed molecular clock, a stochastic Dollo substitution model, and parsimony identified all varieties as monophyletic, thus they satisfy the monophyly criterion for species delimitation. Principal coordinate analysis and a Bayesian assignment procedure revealed deep genetic subdivisions and little admixture between varieties, indicating an absence of genetic intermediates and compliance with the genotypic cluster species criterion. Diagnostic morphological and AFLP characters were found for all varieties, thus they meet the diagnosability criterion. Natural history information suggests that reproductive isolating barriers may have evolved in var. pubescens, potentially qualifying it as a species under a criterion of intrinsic reproductive isolation. Environmental niche modeling showed that the preferred habitat of var. neomexicanus is climatically unique, suggesting niche specialization and thus compliance with an ecological species criterion. Isolation by distance coupled with imperfect sampling can lead to erroneous lineage identification using some species criteria. Compliance with complementary pattern- and process-oriented criteria provides powerful corroboration for a species hypothesis and mitigates the necessity for comprehensive sampling of the entire species range, a practical impossibility in many systems. We hypothesize that var. pubescens maintains its genetic identity, despite substantial niche overlap with var. lupuloides, via the evolution of partial reproductive isolating mechanisms. Variety neomexicanus, conversely, will likely persist as a distinct lineage, regardless of limited gene flow with vars. lupuloides and pubescens because of ecological isolation--adaptation to the unique conditions of the Rocky Mountain cordillera. Thus, we support recognition of vars. neomexicanus and pubescens as species, but delay making a recommendation for var. lupuloides until sampling of genetic variation is complete or a stable biological process can be identified to explain its observed genetic divergence.     

The evolution of sex pheromones in an ecologically diverse genus of flies

In theory, pheromones important in specific mate recognition should evolve via large shifts in composition (saltational changes) at speciation events. However, where other mechanisms exist to ensure reproductive isolation, no such selection for rapid divergence is expected. In Bactrocera fruit flies (Diptera: Tephritidae), males produce volatile chemicals to attract females for mating. Bactrocera species exhibit great ecological diversity, with a wide range of geographical locations and host plants used. They also have other mechanisms, including temporal and behavioural differences, which ensure reproductive isolation. Therefore, we predicted that their sex pheromones would not exhibit rapid divergence at speciation events. In the present study, we tested this idea by combining data on male sex pheromone composition for 19 species of Bactrocera with a phylogeny constructed from DNA sequence data. Analyses of the combined data revealed positive correlations between pheromone differences and nucleotide divergence between species, and between the number of pheromone changes along the phylogeny and the branch lengths associated with these changes. These results suggest a gradual rather than saltational mode of evolution. However, remarkable differences in sex pheromones composition exist, even between closely-related species. It appears therefore that the mode of evolution of sex pheromones in Bactrocera is best described by rapid saltational changes associated with speciation, followed by gradual divergence thereafter. Furthermore, species that do not overlap ecologically are just as different pheromonally as species that do. Thus, large changes in pheromone composition appear to be achieved, even in cases where other mechanisms to ensure reproductive isolation exist. We suggest that these differences are closely associated with rapid changes in host plant use, which is a characteristic feature of Bactrocera speciation. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 97 , 594–603.     

Reproductive isolation in Saccharomyces

Although speciation is one of the most interesting processes in evolution, the underlying causes of reproductive isolation are only partially understood in a few species. This review summarizes the results of many experiments on the reproductive isolation between yeast species of the Saccharomyces sensu stricto group. Hybrids between these species form quite readily in the laboratory, but, if given a choice of species to mate with, some are able to avoid hybridization. F1 hybrids are viable but sterile: the gametes they produce are inviable. For one pair of species, hybrid sterility is probably caused by chromosomal rearrangements, but for all the other species, the major cause of hybrid sterility is antirecombination-the inability of diverged chromosomes to form crossovers during F1 hybrid meiosis. Surprisingly, incompatibility between the genes expressed from different species' genomes is not a major cause of F1 hybrid sterility, although it may contribute to reproductive isolation at other stages of the yeast life cycle.     

Sexual isolation evolves faster than hybrid inviability in a diverse and sexually dimorphic genus of fish (Percidae: Etheostoma)

Abstract .Theory predicts that sexual (or behavioral) isolation will be the first form of reproductive isolation to evolve in lineages characterized by sexual selection. Here I directly compare the rate of evolution of sexual isolation with that of hybrid inviability in a diverse and sexually dimorphic genus of freshwater fish. The magnitude of both sexual isolation and hybrid inviability were quantified for multiple pairs of allopatric species. Rates of evolution were inferred by comparing genetic distances of these species pairs with the magnitude of each form of reproductive isolation: the slope of the regression of genetic distance on the magnitude of reproductive isolation represents the rate of evolution. Of the two forms of isolation, the magnitude of sexual isolation exhibited the steeper slope of regression, indicating that sexual isolation will tend to evolve to completion earlier than hybrid inviability, strictly as a by-product of evolution in geographically isolated populations. Additional evidence from the literature is used to qualitatively compare rates of evolution of sexual isolation with that of other forms of reproductive isolation. Preliminary comparisons support the prediction that sexual isolation will evolve more rapidly than other forms. Because Etheostoma is characterized by striking sexual dimorphism, these results are consistent with the hypothesis that sexual selection for exaggerated mate-recognition characters causes the relatively rapid evolution of sexual isolation.     

Evolution of postzygotic reproductive isolation in galliform birds: analysis of first and second hybrid generations and backcrosses

The process of speciation is a crucial aspect of evolutionary biology. In this study, we analysed the patterns of evolution of postzygotic reproductive isolation in Galliformes using information on hybridization and genetic distance among species. Four main patterns arose: (1) hybrid inviability and sterility in F₁ hybrids increase as species diverge; (2) the presence of geographical overlap does not affect the evolution of postzygotic isolation; (3) the galliforms follow Haldane's rule; (4) hybrid inviability is higher in F₂ than in F₁ hybrids, but does not appear to be increased in the backcrosses. This study contributes to the growing evidence suggesting that the patterns of evolution of postzygotic isolation and the process of speciation are shared among avian groups (and animals in general). In particular, our results support the notion of F₂ hybrid inviability as being key for the maintenance of species genetic integrity when prezygotic isolation barriers are overcome in closely related species, in which postzygotic isolation in the F₁ hybrid might still not be fully developed. To the contrary, hybrids from backcrosses did not show serious inviability problems (at least not more than F₁ hybrids), demonstrating that they could generate gene flow among bird species. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 528–542.     

A pragmatic approach to the species problem from a paleontological perspective

The ideal scenario for paleontologists would be for the species they designate to be equivalent to the species recognized for modern animals, in the sense that they were formed as a result of the same evolutionary processes. This would mean, for example, that we could be confident that in combining extant and extinct taxa in phylogenetic analyses we would be dealing with equivalent operational taxonomic units. Notwithstanding the many thousands of pages that have been spent arguing over species concepts, the only concept that has won widespread acceptance for the designation of modern species is Mayr's Biological Species Concept (BSC).¹ In fact, whenever we complete a cladistic analysis, we assume reproductive isolation of our terminal taxa because otherwise their similarities could be the product of interbreeding rather than common ancestry. Fundamentally, we all behave as though the BSC is true.     

Speciation by postzygotic isolation: forces, genes and molecules

New species arise as reproductive isolation evolves between diverging populations. Here we review recent work in the genetics of postzygotic reproductive isolation-the sterility and inviability of species hybrids. Over the last few years, research has taken two new directions. First, we have begun to learn a good deal about the population genetic forces driving the evolution of postzygotic isolation. It has, for instance, become increasingly clear that conflict-driven processes, like sexual selection and meiotic drive, may contribute to the evolution of hybrid sterility. Second, we have begun to learn something about the identity and molecular characteristics of the actual genes causing hybrid problems. Although molecular genetic data are limited, early findings suggest that "speciation genes" correspond to loci having normal functions within species and that these loci sometimes diverge as a consequence of evolution in gene regulation.     

Speciation in the fossil record

It is easy to claim that the fossil record says nothing about speciation because the biological species concept (which relies on interbreeding) cannot be applied to it and genetic studies cannot be carried out on it. However, fossilized organisms are often preserved in sufficient abundance for populations of intergrading morphs to be recognized, which, by analogy with modern populations, are probably biological species. Moreover, the fossil record is our only reliable documentation of the sequence of past events over long time intervals: the processes of speciation are generally too slow to be observed directly, and permanent reproductive isolation can only be verified with hindsight. Recent work has shown that some parts of the fossil record are astonishingly complete and well documented, and patterns of lineage splitting can be examined in detail. Marine plankton appear to show gradual speciation, with subsequent morphological differentiation of lineages taking up to 500000 years to occur. Marine invertebrates and vertebrates more commonly show punctuated patterns, with periods of rapid speciation followed by long-term stasis of species lineages.