Periods Of Allopatry Research Articles

The limits of the metapopulation: Lineage fragmentation in a widespread terrestrial salamander (Plethodon cinereus)

Abstract In vicariant species formation, divergence results primarily from periods of allopatry and restricted gene flow. Widespread species harboring differentiated, geographically distinct sublineages offer a window into what may be a common mode of species formation, whereby a species originates, spreads across the landscape, then fragments into multiple units. However, incipient lineages usually lack reproductive barriers that prevent their fusion upon secondary contact, blurring the boundaries between a single, large metapopulation-level lineage and multiple independent species. Here we explore this model of species formation in the Eastern Red-backed Salamander (Plethodon cinereus), a widespread terrestrial vertebrate with at least six divergent mitochondrial clades throughout its range. Using anchored hybrid enrichment data, we applied phylogenomic and population genomic approaches to investigate patterns of divergence, gene flow, and secondary contact. Genomic data broadly match most mitochondrial groups but reveal mitochondrial introgression and extensive admixture at several contact zones. While species delimitation analyses in BPP supported five lineages of P. cinereus, genealogical divergence indices (gdi) were highly sensitive to the inclusion of admixed samples and the geographic representation of candidate species, with increasing support for multiple species when removing admixed samples or limiting sampling to a single locality per group. An analysis of morphometric data revealed differences in body size and limb proportions among groups, with a reduction of forelimb length among warmer and drier localities consistent with increased fossoriality. We conclude that P. cinereus is a single species, but one with highly structured component lineages of various degrees of independence.

Can demographic histories explain long-term isolation and recent pulses of asymmetric gene flow between highly divergent grey fox lineages?

Secondary contact zones between deeply divergent, yet interfertile, lineages provide windows into the speciation process. North American grey foxes (Urocyon cinereoargenteus) are divided into western and eastern lineages that diverged approximately 1 million years ago. These ancient lineages currently hybridize in a relatively narrow zone of contact in the southern Great Plains, a pattern more commonly observed in smaller-bodied taxa, which suggests relatively recent contact after a long period of allopatry. Based on local ancestry inference with whole-genome sequencing (n = 43), we identified two distinct Holocene pulses of admixture. The older pulse (500-3500 YBP) reflected unidirectional gene flow from east to west, whereas the more recent pulse (70-200 YBP) of admixture was bi-directional. Augmented with genotyping-by-sequencing data from 216 additional foxes, demographic analyses indicated that the eastern lineage declined precipitously after divergence, remaining small throughout most of the late Pleistocene, and expanding only during the Holocene. Genetic diversity in the eastern lineage was highest in the southeast and lowest near the contact zone, consistent with a westward expansion. Concordantly, distribution modelling indicated that during their isolation, the most suitable habitat occurred far east of today's contact zone or west of the Great Plains. Thus, long-term isolation was likely caused by the small, distant location of the eastern refugium, with recent contact reflecting a large increase in suitable habitat and corresponding demographic expansion from the eastern refugium. Ultimately, long-term isolation in grey foxes may reflect their specialized bio-climatic niche. This system presents an opportunity for future investigation of potential pre- and post-zygotic isolating mechanisms.

Simulated evolution of mating signal diversification in a primate radiation.

Divergence in allopatry and subsequent diversification of mating signals on secondary contact (reinforcement) is a major driver of phenotypic diversity. Observing this evolutionary process directly is often impossible, but simulated evolution can pinpoint key drivers of phenotypic variation. We developed evolutionary simulations in which mating signals, modelled as points in phenotype space, evolve across time under varying evolutionary scenarios. We model mate recognition signals in guenons, a primate radiation exhibiting colourful and diverse face patterns hypothesized to maintain reproductive isolation via mate choice. We simulate face pattern evolution across periods of allopatry and sympatry, identifying the role of key parameters in driving evolutionary endpoints. Results show that diversification in allopatry and assortative mate choice on secondary contact can induce rapid phenotypic diversification, resulting in distinctive (between species) and stereotyped (within species) face patterns, similar to extant guenons. Strong selection against hybrids is key to diversification, with even low levels of hybrid fitness often resulting in merged populations on secondary contact. Our results support a key role for reinforcement by assortative mating in the maintenance of species diversity and support the long-proposed prehistorical scenario for how such striking diversity was produced and maintained in perhaps the most colourful of all mammalian clades.

Response to Hill and Powers: It is irrelevant that the mode and tempo of Cassia crossbill speciation is not typical for birds

Species delineation has a long and contentious history, yet most agree that sympatric populations exhibiting high levels of reproductive isolation and evolving independently are species. In an opinion piece, Hill and Powers (2021; hereafter H&P) claim that several recognized species of crossbills (Loxia spp.) do not represent species because by no measure are they discrete, the vocalizations used to categorize crossbills are learned, modified and can switch to that of a different species, and reproductive isolation is incomplete and weak. We argue that the behavioral and genetic evidence indicate that Cassia crossbills L. sinesciuris, which we focus on because the data relevant to species status are more diverse and extensive, are genetically discrete; call modification rarely leads to crossbill misclassification and overwhelmingly results in call divergence and enhanced discrimination; and are nearly completely reproductively isolated with little evidence of introgression from sympatric red crossbills. The differences in our conclusions result in part from H&P mischaracterizing and misconstruing the ecology of Cassia crossbills, geographic context of their divergence, and evidence for reproductive isolation. H&P seemingly require that crossbills must adhere to the typical model of bird speciation–protracted divergence in allopatry, followed by a gradual increase in sympatry if reproductive isolation and ecological divergence allow–and require evidence such as initial long periods of allopatry, FST values > 0.2, divergent mtDNA and intrinsic postzygotic isolation. Although such evidence commonly distinguishes bird species, an increasing number of studies show that such criteria are not necessary to indicate sympatric, evolutionarily independent lineages.

Genome-wide patterns of divergence and introgression after secondary contact between Pungitius sticklebacks.

Speciation is a continuous process. Although it is known that differential adaptation can initiate divergence even in the face of gene flow, we know relatively little about the mechanisms driving complete reproductive isolation and the genomic patterns of divergence and introgression at the later stages of speciation. Sticklebacks contain many pairs of sympatric species differing in levels of reproductive isolation and divergence history. Nevertheless, most previous studies have focused on young species pairs. Here, we investigated two sympatric stickleback species, Pungitius pungitius and P. sinensis, whose habitats overlap in eastern Hokkaido; these species show hybrid male sterility, suggesting that they may be at a late stage of speciation. Our demographic analysis using whole-genome sequence data showed that these species split 1.73 Ma and came into secondary contact 37 200 years ago after a period of allopatry. This long period of allopatry might have promoted the evolution of intrinsic incompatibility. Although we detected on-going gene flow and signatures of introgression, overall genomic divergence was high, with considerable heterogeneity across the genome. The heterogeneity was significantly associated with variation in recombination rate. This sympatric pair provides new avenues to investigate the late stages of the stickleback speciation continuum. This article is part of the theme issue 'Towards the completion of speciation: the evolution of reproductive isolation beyond the first barriers'.

Peripatric speciation associated with genome expansion and female-biased sex ratios in the moss genus Ceratodon.

A period of allopatry is widely believed to be essential for the evolution of reproductive isolation. However, strict allopatry may be difficult to achieve in some cosmopolitan, spore-dispersed groups, like mosses. We examined the genetic and genome size diversity in Mediterranean populations of the moss Ceratodon purpureus s.l. to evaluate the role of allopatry and ploidy change in population divergence. We sampled populations of the genus Ceratodon from mountainous areas and lowlands of the Mediterranean region, and from Western and Central Europe. We performed phylogenetic and coalescent analyses on sequences from five nuclear introns and a chloroplast locus to reconstruct their evolutionary history. We also estimated genome size using flow cytometry (employing propidium iodide) and determined the sex of samples using a sex-linked PCR marker. Two well-differentiated clades were resolved, discriminating two homogeneous groups: the widespread C. purpureus and a local group mostly restricted to the mountains in Southern Spain. The latter also possessed a genome size 25% larger than the widespread C. purpureus, and the samples of this group consist entirely of females. We also found hybrids, and some of them had a genome size equivalent to the sum of the C. purpureus and Spanish genome, suggesting that they arose by allopolyploidy. These data suggest that a new species of Ceratodon arose via peripatric speciation, potentially involving a genome size change and a strong female-biased sex ratio. The new species has hybridized in the past with C. purpureus.

The geography of divergence with gene flow facilitates multitrait adaptation and the evolution of pollinator isolation in Mimulus aurantiacus.

Ecological adaptation is the driving force during divergence with gene flow and generates reproductive isolation early in speciation. Although gene flow opposes divergence, local adaptation can be facilitated by factors that prevent the breakup of favorable allelic combinations. We investigated how selection, genetic architecture, and geography have contributed to the maintenance of floral trait divergence and pollinator isolation between parapatric ecotypes of Mimulus aurantiacus. Combining greenhouse, field, and genomic studies, we show that sharp clines in floral traits are maintained by spatially varying selection. Although adaptation breaks down where the ecotypes co-occur, leading to the formation of a hybrid zone, the largely non-overlapping distributions of the ecotypes shield them from immigrant genes, facilitating divergence across most of the range. In contrast to the sharp genetic discontinuities observed across most hybrid zones, we observed a gradual cline in genome-wide divergence and a pattern of isolation by distance across the landscape. Thus, contrary to a long period of allopatry followed by recent re-contact, our data suggest that floral trait divergence in M. aurantiacus may have evolved with locally restricted, but ongoing gene flow. Therefore, our study reveals how the geographic distribution of an organism can contribute to the evolution of premating isolation in the early stages of divergence with gene flow.

Can female preference drive intraspecific diversity dynamics in brown trout (Salmo trutta, L.)?

AbstractIntraspecific diversity can be strongly impacted by the dynamics of reproductive isolation during secondary contacts. The high levels of hybridisation and introgression between Atlantic and Mediterranean lineages in contemporary populations of brown trout in the Northern part of the French Alps are a good case in point. After a long period of allopatry, which one assumes has facilitated their divergent phenotypic and genetic evolution, man has removed a geographical reproductive barrier via stocking the Atlantic lineage in the Mediterranean area, thereby potentially enabling gene flow between native and non native populations. We investigate how much a prezygotic reproductive behaviour, that is female preference for male phenotype, can influence hybridisation in natural environments, using subaquatic video recording of reproduction. Our statistical model indicates that female preference appears to be largely heterogamous: females tend to select dissimilar males with respect to their own phenotype; thus, rather than acting as a barrier to gene flow, female preference is favouring gene flow between lineages that have been artificially placed in sympatry. This finding based on observational data is in agreement with previously reported genetic data, high levels of hybridisation between the lineages. We suggest that a knowledge of reproductive behaviour in natural environments is an essential tool for biodiversity managers to assess the potential risks associated with the introduction into recipient populations of non‐native lineage fish at the intraspecific level.

Theoretical models of the influence of genomic architecture on the dynamics of speciation.

A long-standing problem in evolutionary biology has been determining whether and how gradual, incremental changes at the gene level can account for rapid speciation and bursts of adaptive radiation. Using genome-scale computer simulations, we extend previous theory showing how gradual adaptive change can generate nonlinear population transitions, resulting in the rapid formation of new, reproductively isolated species. We show that these transitions occur via a mechanism rooted in a basic property of biological heredity: the organization of genes in genomes. Genomic organization of genes facilitates two processes: (i) the build-up of statistical associations among large numbers of genes and (ii) the action of divergent selection on persistent combinations of alleles. When a population has accumulated a critical amount of standing, divergently selected variation, the combination of these two processes allows many mutations of small effect to act synergistically and precipitously split one population into two discontinuous, reproductively isolated groups. Periods of allopatry, chromosomal linkage among loci, and large-effect alleles can facilitate this process under some conditions, but are not required for it. Our results complement and extend existing theory on alternative stable states during population divergence, distinct phases of speciation and the rapid emergence of multilocus barriers to gene flow. The results are thus a step towards aligning population genomic theory with modern empirical studies.

Diversification of the Alpine Chipmunk, Tamias alpinus, an alpine endemic of the Sierra Nevada, California

BackgroundThe glaciation cycles that occurred throughout the Pleistocene in western North America caused frequent shifts in species’ ranges with important implications for models of species divergence. For example, long periods of allopatry during species’ range contractions allowed for the accumulation of differences between separated populations promoting lineage divergence. In contrast, range expansions during interglacial periods may have had homogenizing effects via increased gene flow following secondary contact. These range dynamics are particularly pronounced in the Sierra Nevada, California, given the complex topography and climatic history of the area, thus providing a natural laboratory to examine evolutionary processes that have led to the diversity patterns observed today.ResultsHere we examined the role of late Pleistocene climate fluctuations on the divergence of the Sierra Nevada endemic Alpine Chipmunk (Tamias alpinus) from its sister taxon, western populations of the Least Chipmunk (T. minimus) from the Great Basin. We used one mitochondrial gene (cytochrome b) and 14 microsatellite loci to examine the evolutionary relationship between these species. Mitochondrial sequence data revealed that T. alpinus and T. minimus populations share mitochondrial haplotypes with no overall geneaological separation, and that diversity at this locus is better explained by geography than by species’ boundaries. In contrast, the microsatellite analysis showed that populations of the same species are more similar to each other than they are to members of the other species. Similarly, a morphological analysis of voucher specimens confirmed known differences in morphological characters between species providing no evidence of recent hybridization. Coalescent analysis of the divergence history indicated a late Pleistocene splitting time (~450 ka) and subsequent, though limited, gene flow between the two lineages.ConclusionsOur results suggest that the two species are distinct and there is no contemporary introgression along their geographic boundary. The divergence of T. alpinus during this time period provides additional evidence that Pleistocene glacial cycles played an important role in diversification of species in Sierra Nevada and North America in general.

Evidence for asymmetrical divergence-gene flow of nuclear loci, but not mitochondrial loci, between seabird sister species: blue-footed (Sula nebouxii) and Peruvian (S. variegata) boobies.

Understanding the process of speciation requires understanding how gene flow influences divergence. Recent analyses indicate that divergence can take place despite gene flow and that the sex chromosomes can exhibit different levels of gene flow than autosomes and mitochondrial DNA. Using an eight marker dataset including autosomal, z-linked, and mitochondrial loci we tested the hypothesis that blue-footed (Sula nebouxii) and Peruvian (S. variegata) boobies diverged from their common ancestor with gene flow, paying specific attention to the differences in gene flow estimates from nuclear and mitochondrial markers. We found no gene flow at mitochondrial markers, but found evidence from the combined autosomal and z-linked dataset that blue-footed and Peruvian boobies experienced asymmetrical gene flow during or after their initial divergence, predominantly from Peruvian boobies into blue-footed boobies. This gene exchange may have occurred either sporadically between periods of allopatry, or regularly throughout the divergence process. Our results add to growing evidence that diverging species can remain distinct but exchange genes.

Biased sex ratios and aposematic polymorphism in African butterflies: A hypothesis

In East and Central Africa, the nymphalid butterflies Danaus chrysippus, Acraea encedon and Acraea encedana are involved in a Müllerian mimicry complex. Unusually for aposematic mimetic organisms, the three species show extensive colour pattern polymorphism. Within the same geographic zone, the three species show female-biased sex ratios as a consequence of infection by maternally-inherited, male-killing bacteria. The co-occurrence of biased sex ratios and aposematic polymorphism within these three sympatric, mimetic butterfly species has led to the speculation that invasion by the male-killing bacteria and the subsequent female-biased sex ratios are the underlying reason for the maintenance of colour polymorphism in these species, following its establishment by periods of allopatry and monomorphism and hybridisation. In this paper, we present a novel hypothesis that describes a mechanism by which such causal link might have taken place; in our view, positive frequency-dependent selection favouring the most common colour form in the species is disrupted as a consequence of the recurrent extinction-recolonization cycles undergone at the level of the species populations following the spread of the male-killer. Likewise, extinctions and recolonizations taking place in the other mimics randomly change the direction of selection on each species, potentially leading to multiple selective pressures favouring different colour patterns in the mimicry complex. Thus, selection for monomorphism within each species as well as between the mimetic species will be continuously randomized by the enhanced metapopulation dynamics and the state of polymorphism will be maintained consequently.

Genetic analysis of a contact zone between two lineages of the ocellated lizard (Lacerta lepidaDaudin 1802) in south-eastern Iberia reveal a steep and narrow hybrid zone

Measuring the diffusion of genes between diverging taxa through zones of secondary contact is an essential step to understand the extent and nature of the reproductive isolation that has been achieved. Previous studies have shown that the ocellated lizard (Lacerta lepida Daudin, 1802) has endured repeated range fragmentation associated with the climatic oscillations of the Plio-Pleistocene that promoted diversification of many different evolutionary units within the species. However, the oldest divergence within the group is estimated to have occurred much earlier, during the Miocene, around 9 Ma and corresponds to the split between the subspecies Lacerta lepida nevadensis Buchholz (1963) and Lacerta lepida lepida Daudin (1802). Although these two evolutionary units have documented genetic and morphological differentiation, most probably accumulated during periods of allopatry, little is known about patterns of gene flow between them. In this study, we performed a population genetic analysis of a putative area of secondary contact between these two taxa, using mtDNA and microsatellite data. We assessed levels of gene flow across the contact zone to clarify to what extent gene flow may be occurring. Hybridization between the subspecies was observed by the presence of genetically introgressed individuals. However, the overall coincidence of mitochondrial and multilocus nuclear clines and generally steep clines support the idea that this contact zone is acting as a barrier to gene flow. Taken together, these results suggest that L. l. lepida and L. l. nevadensis are in independent evolutionary trajectories and should be considered as two different species.

Phylogeographic history of the swallowtail Papilio bianor Cramer (Lepidoptera: Papilionidae) from China

The mitochondrial DNA (mtDNA) variation in Papilio bianor Cramer from its extant range was evaluated to provide a phylogeographic history of its populations from China. Molecular variation in a 655 bp section of COII from 128 individuals (22 populations) was analysed, and 17 haplotypes were detected. Phylogenetic analysis revealed the presence of three major lineages, northeastern, southwestern and central lineages. The sequence divergence suggests that these regions had been isolated since the early Pleistocene. Nested clade, coalescent and genetic structure analyses indicate that allopatric fragmentation might be the cause of the deep divergence within the species. This period of allopatry was followed by contiguous range expansion, presumably involving recolonisation of formerly glaciated or uninhabitable areas from at least three refuges.

Population Genomics of Secondary Contact

One common form of reticulate evolution arises as a consequence of secondary contact between previously allopatric populations. Using extensive coalescent simulations, we describe the conditions for, and extent of, the introgression of genetic material into the genome of a colonizing population from an endemic population. The simulated coalescent histories are sampled from models that describe the evolution of entire chromosomes, thereby allowing the expected length of introgressed haplotypes to be estimated. The results indicate that our ability to identify reticulate evolution from genetic data is highly variable and depends critically upon the duration of the period of allopatry, the timing of the secondary contact event, as well as the sizes of the populations at the time of contact. One particularly interesting result arises when secondary contact occurs close to the time of a severe founder event, in this case, genetic introgression can be substantially more difficult to detect. However, if secondary contact occurs after such a founding event, when the range of the colonizing population increases, introgression is more readily detectable across the genome. This result may have important implications for our ability to detect introgression between ancestrally bottlenecked modern human populations and archaic hominin species, such as Neanderthals.

The roles of allopatric divergence and natural selection in quantitative trait variation across a secondary contact zone in the lizard Anolis roquet

Populations of the Caribbean lizard, Anolis roquet, are thought to have experienced long periods of allopatry before recent secondary contact. To elucidate the effects of past allopatry on population divergence in A. roquet, we surveyed parallel transects across a secondary contact zone in northeastern Martinique. We used diagnostic molecular mitochondrial DNA markers to test fine-scale association of mitochondrial DNA lineage and geological region, multivariate statistical techniques to explore quantitative trait pattern, and cline fitting techniques to model trait variation across the zone of secondary contact. We found that lineages were strongly associated with geological regions along both transects, but quantitative trait patterns were remarkably different. Patterns of morphological and mitochondrial DNA variation were consistent with a strong barrier to gene flow on the coast, whereas there were no indications of barriers to gene flow in the transitional forest. Hence, the coastal populations behaved as would be predicted by an allopatric model of divergence in this complex, while those in the transitional forest did not, despite the close proximity of the transects and their shared geological history. Patterns of geographical variation in this species complex, together with environmental data, suggest that on balance, selection regimes on either side of the secondary contact zone in the transitional forest may be more convergent, while those either side of the secondary contact zone on the coast are more divergent. Hence, the evolutionary consequences of allopatry may be strongly influenced by local natural selection regimes.

Conservation and variation in the agonistic repertoire of the brook stickleback, Culaea inconstans (Kirtland)

Interpopulation variability in behaviour is thought to play a role in the production of bio- logical diversity on this planet. The brook stickle- back, Culaea inconstans, is a prime candidate for investigations of such variability because mtDNA sequences indicate that it diverged into two distinct lineages 3.6-4.8 mya; more than enough time for differences to evolve. In this paper we show that, despite long periods of allopatry, the brook stickle- back's agonistic repertoire has remained quite conser- vative. Existing variability represents shifts in the mean population frequencies (performance parameter) of some displays, rather than changes in the compo- sition of the agonistic repertoire itself (presence or absence of behavioural elements).

The What and Why of Research on Reinforcement

Reinforcement, like sympatric speciation (see Box 1), has charisma. Evolutionary biologists are still deeply uncertain about how often these processes take place, and hence how important they are in explaining the biological diversity we see today. Empirical and theoretical support for both ideas has waxed and waned over recent decades. Yet both ideas have consistently garnered an unusual amount of attention. Box 1. Glossary Sympatry—Area(s) of overlap in the ranges of populations, enabling potential interbreeding. Allopatry—Area(s) of population ranges that do not overlap with one another, preventing interbreeding. Sympatric speciation—Speciation that occurs within a range of sympatry. Reinforcement—The evolution of mechanisms that prevent interbreeding between newly interacting incipient species, as a result of selection against hybrids (narrow definition) or interspecific matings (broad definition) (See Figure 1). Figure 1 Schematic Diagram of Reinforcement Conspecific sperm precedence—Disproportional fertilization of a female by sperm of a conspecific male, when that female has mated with both conspecific and heterospecific males. Much of the appeal of both reinforcement and sympatric speciation lies in the way they unite micro- and macroevolution. Reinforcement, a concept popularized by Dobzhansky (1937), is a process by which speciation, a macroevolutionary process, can be driven directly by natural selection, one of the primary microevolutionary forces. Sympatric speciation can make the same claim. Because of this close linkage between the concepts, the study of one can tell us a great deal about the other (see Kirkpatrick and Ravigne 2002). Such studies can also reveal a lot about the general role of microevolution in species divergence. Reinforcement provides a pathway toward the completion of the speciation process. Imagine that two divergent populations (potentially even classified as separate species) come into contact after a period of allopatry (Figure 1). If the populations have been apart for a long time, evolved differences between them will cause a certain degree of incompatibility when the populations come together. Often, this incompatibility comes in the form of low hybrid fitness (postzygotic isolation) or mismatched mating characteristics (premating isolation). The degree of the development of these isolating mechanisms is roughly proportional to the genetic distance between the populations, reflecting the fact that incompatibilities accumulate over time (Coyne and Orr 1989). If the isolating mechanisms between these populations are only partially complete, extensive hybridization may occur. This can result in fusion back into a single population, or in the swamping of one population's gene pool by the genes of the other (extinction). But there is another possibility, one that can cause the speciation between the two populations to proceed. Remember that if the populations have been separated for long enough, it is likely that hybrids between them will have relatively low fitness. Individuals who mate with members of the opposing population will therefore produce offspring of poor quality, and hence have lower fitness than individuals that mate within their own population. This favors the evolution (or further divergence) of characteristics that cause mating within, rather than between, populations (Figure 1C). Speciation between the populations is driven further towards completion through this increase in premating isolation. This process, the evolution of premating isolation after secondary contact due to selection against hybrids, is reinforcement sensu Dobzhansky (1937). Recent authors have broadened the definition of reinforcement to include as a driving force any form of selection against mating between populations (e.g., Servedio and Noor 2003). This could include, for example, lower fertility, or higher mortality of females that mate with members of other populations. In all definitions, however, the microevolutionary process of selection is essential for reinforcement. In fact, in reinforcement, speciation itself can be thought of as an adaptive response to selection. It is little wonder that this causal linking of micro- and macroevolution has appeal for many evolutionary biologists.

The origins of postmating reproductive isolation: testing hypotheses in the grasshopper Chorthippus parallelus

AbstractAlthough there are several well‐established hypotheses for the origins of postmating isolation during allopatric divergence, there have been very few attempts to determine their relative importance in nature. We have developed an approach based on knowledge of the differing evolutionary histories of populations within species that allows systematic comparison of the predictions of these hypotheses. In previous work, we have applied this methodology to mating signal variation and premating reproductive isolation between populations of the meadow grasshopper Chorthippus parallelus. Here we review the principles behind our approach and report a study measuring postmating isolation in the same set of populations. The populations have known and differing evolutionary histories and relationships resulting from the colonization of northern Europe following the last glaciation. We use a maximum‐likelihood analysis to compare the observed pattern of postmating isolation with the predictions of the hypotheses that isolation primarily evolves either as a result of gradual accumulation of mutations in allopatry, or through processes associated with colonization, such as founder events. We also quantify the extent to which degree of postmating isolation can be predicted by genetic distance. Our results suggest that although there is only a weak correlation between genetic distance and postmating isolation, long periods of allopatry do lead to postmating isolation. In contrast to the pattern of premating isolation described in our previous study, colonization does not seem to be associated with increased postmating isolation.

Behavioural Divergence in Allopatric and Sympatric Gerbil Species (Rodentia: Gerbillinae)

AbstractSouthern African gerbils comprise 3 genera, each genus represented by morphologically‐ and ecologically‐similar species. All species are monomorphic, with social systems varying from asocial (Desmodillus auricularis) to colonial (Tatera species).This study compares the behaviour patterns of an allopatric species, T. afra, 2 sympatric species, T. leucogaster and T. brantsii, and a species of a different genus, D. auricularis which is sympatric with T. leucogaster and T. brantsii.Interactions between males and females of the same species were staged and quantified in terms of the frequencies of each of 23 behavioural acts. D. auricularis performed 2 acts, namely “sidle” and “fight” which have not been identified in Gerbillurus or Tatera interactions. Sequences of behaviour were similar in all species, and involved females moving away from males, which followed, sniffed the anogenital region and attempted to mount the females. Females responded by presenting, if receptive, or agonistic behaviour such as aggressive or upright postures. Exploration of the terrarium accounted for a large proportion of the behaviour during the 20 min encounter, and watching the opponent also occurred frequently.Discriminant function analysis successfully separated all 4 species on the basis of behaviour frequencies. Individuals were assigned to the correct species with an accuracy of 80%, indicating that each species exhibits species‐specific patterns of behaviour. D. auricularis displayed more agonistic and sexual behaviour and less huddling behaviour than any of the Tatera species.The sympatric species‐pair, T. leucogaster and T. brantsii, exhibited more significant difference in behaviour than either of the allopatric species‐pairs, T. afra and T. brantsii, and T. afra and T. leucogaster. Divergence in behaviour patterns has occurred in allopatric species, but divergence is more marked in species which are sympatric. It is not known whether divergence between T. leucogaster and T. brantsii occurred during a period of allopatry, or whether divergence has occurred due to selection against hybrids in sympatry.