Reproductive character displacement is not the only possible outcome of reinforcement

Predicting evolutionary responses to interspecific interference in the wild

Many interspecifically territorial species interfere with each other reproductively, and in some cases, aggression towards heterospecifics may be an adaptive response to interspecific mate competition. This hypothesis was recently formalised in an agonistic character displacement (ACD) model which predicts that species should evolve to defend territories against heterospecific rivals above a threshold level of reproductive interference. To test this prediction, we parameterised the model with field estimates of reproductive interference for 32 sympatric damselfly populations and ran evolutionary simulations. Asymmetries in reproductive interference made the outcome inherently unpredictable in some cases, but 80% of the model's stable outcomes matched levels of heterospecific aggression in the field, significantly exceeding chance expectations. In addition to bolstering the evidence for ACD, this paper introduces a new, predictive approach to testing character displacement theory that, if applied to other systems, could help in resolving long-standing questions about the importance of character displacement processes in nature.

Phylo-comparative analyses reveal the dual role of drift and selection in reproductive character displacement

When incipient species meet in secondary contact, natural selection can rapidly reduce costly reproductive interactions by directly targeting reproductive traits. This process, called reproductive character displacement (RCD), leaves a characteristic pattern of geographic variation where divergence of traits between species is greater in sympatry than allopatry. However, because other forces can also cause similar patterns, care must be given in separating pattern from process. Here we show how the phylo-comparative method together with genomic data can be used to evaluate evolutionary processes at the population level in closely related species. Using this framework, we test the role of RCD in speciation of two cricket species endemic to Anatolian mountains by quantifying patterns of character displacement, rates of evolution and adaptive divergence. Our results show differing patterns of character displacement between species for reproductive vs. non-reproductive characters and strong patterns of asymmetric divergence. We demonstrate diversification results from rapid divergence of reproductive traits towards multiple optima under the dual influence of strong drift and selection. These results present the first solid evidence for RCD in Anatolian mountains, quantify the amount of drift and selection necessary for RCD to lead to speciation, and demonstrate the utility of phylo-comparative methods for quantifying evolutionary parameters at the population level.

Fine-scale geographic patterns of gene flow and reproductive character displacement in Drosophila subquinaria and Drosophila recens

When two species are incompletely isolated, strengthening premating isolation barriers in response to the production of low fitness hybrids may complete the speciation process. Here, we use the sister species Drosophila subquinaria and Drosophila recens to study the conditions under which this reinforcement of species boundaries occurs in natural populations. We first extend the region of known sympatry between these species, and then we conduct a fine-scale geographic survey of mate discrimination coupled with estimates of gene flow within and admixture between species. Within D. subquinaria, reinforcement is extremely effective: we find variation in mate discrimination both against D. recens males and against conspecific allopatric males on the scale of a few kilometres and in the face of gene flow both from conspecific populations and introgression from D. recens. In D. recens, we do not find evidence for increased mate discrimination in sympatry, even where D. recens is rare, consistent with substantial gene flow throughout the species' range. Finally, we find that introgression between species is asymmetric, with more from D. recens into D. subquinaria than vice versa. Within each species, admixture is highest in the geographic region where it is rare relative to the other species, suggesting that when hybrids are produced they are of low fitness. In sum, reinforcement within D. subquinaria is effective at maintaining species boundaries, but even when reinforcing selection is strong it may not always result in a pattern of strong reproductive character displacement due to variation in the frequency of hybridization and gene flow from neighbouring populations.

Geographic variation in hybridization across a reinforcement contact zone of chorus frogs (Pseudacris)

Reinforcement contact zones, which are secondary contact zones where species are diverging in reproductive behaviors due to selection against hybridization, represent natural laboratories for studying speciation‐in‐action. Here, we examined replicate localities across the entire reinforcement contact zone between North American chorus frogs Pseudacris feriarum and P. nigrita to investigate geographic variation in hybridization frequencies and to assess whether reinforcement may have contributed to increased genetic divergence within species. Previous work indicated these species have undergone reproductive character displacement (RCD) in male acoustic signals and female preferences due to reinforcement. We also examined acoustic signal variation across the contact zone to assess whether signal characteristics reliably predict hybrid index and to elucidate whether the degree of RCD predicts hybridization rate. Using microsatellites, mitochondrial sequences, and acoustic signal information from >1,000 individuals across >50 localities and ten sympatric focal regions, we demonstrate: (1) hybridization occurs and (2) varies substantially across the geographic range of the contact zone, (3) hybridization is asymmetric and in the direction predicted from observed patterns of asymmetric RCD, (4) in one species, genetic distance is higher between conspecific localities where one or both have been reinforced than between nonreinforced localities, after controlling for geographic distance, (5) acoustic signal characters strongly predict hybrid index, and (6) the degree of RCD does not strongly predict admixture levels. By showing that hybridization occurs in all sympatric localities, this study provides the fifth and final line of evidence that reproductive character displacement is due to reinforcement in the chorus frog contact zone. Furthermore, this work suggests that the dual action of cascade reinforcement and partial geographic isolation is promoting genetic diversification within one of the reinforced species.

Reinforcement and a cline in mating behaviour evolve in response to secondary contact and hybridization in shield-back katydids (Orthoptera: Tettigoniidae)

In a dispersal-limited species that has evolved reproductive character displacement at a contact zone, a cline in mating behaviour may result if gene flow diffuses alleles out of the contact zone into allopatric populations. Prior work has found such a clinal pattern in the shield-back katydid Aglaothorax morsei, in which the male calling songs in a sympatric population have a displaced, short interpulse interval that increases in length with increasing distance from the contact zone. In this study, molecular phylogenetic and female preference data show that (1) sympatric populations result from secondary contact, (2) hybridization in sympatry has resulted in unidirectional mitochondrial introgression and (3) female preferences are consistent with reproductive character displacement and could generate a cline in mating behaviour. These data together suggest a history of reinforcement, generally considered rare in acoustically communicating insects; thus, Aglaothorax represents an important example of a rarely documented evolutionary process.

Reinforcement's incidental effects on reproductive isolation between conspecifics

Reinforcement—the process whereby maladaptive hybridization leads to the strengthening of prezygotic isolation between species—has a long history in the study of speciation. Because reinforcement affects traits involved in mate choice and fertility, it can have indirect effects on reproductive isolation between populations within species. Here we review examples of these “cascading effects of reinforcement” (CER) and discuss different mechanisms through which they can arise. We discuss three factors that are predicted to influence the potential occurrence of CER: rates of gene flow among populations, the strength of selection acting on the traits involved in reinforcement, and the genetic basis of those traits. We suggest that CER is likely if (1) the rate of gene flow between conspecific populations is low; (2) divergent selection acts on phenotypes involved in reinforcement between sympatric and allopatric populations; and (3) the genetic response to reinforcement differs among conspecific populations subject to parallel reinforcing selection. Future work continuing to address gene flow, selection, and the genetic basis of the traits involved in the reinforcement will help develop a better understanding of reinforcement as a process driving the production of species diversity, both directly and incidentally.

Seeking signatures of reinforcement at the genetic level: a hitchhiking mapping and candidate gene approach in the house mouse

Reinforcement is the process by which prezygotic isolation is strengthened as a response to selection against hybridization. Most empirical support for reinforcement comes from the observation of its possible phenotypic signature: an accentuated degree of prezygotic isolation in the hybrid zone as compared to allopatry. Here, we implemented a novel approach to this question by seeking for the signature of reinforcement at the genetic level. In the house mouse, selection against hybrids and enhanced olfactory-based assortative mate preferences are observed in a hybrid zone between the two European subspecies Mus musculus musculus and M. m. domesticus, suggesting a possible recent reinforcement event. To test for the genetic signature of reinforcing selection and identify genes involved in sexual isolation, we adopted a hitchhiking mapping approach targeting genomic regions containing candidate genes for assortative mating in mice. We densely scanned these genomic regions in hybrid zone and allopatric samples using a large number of fast evolving microsatellite loci that allow the detection of recent selection events. We found a handful of loci showing the expected pattern of significant reduction in variability in populations close to the hybrid zone, showing assortative odour preference in mate choice experiments as compared to populations further away and displaying no such preference. These loci lie close to genes that we pinpoint as testable candidates for further investigation.

Noisy neighbors can hamper the evolution of reproductive isolation by reinforcing selection

Reinforcement is the process by which selection against hybridization leads to an increase in reproductive isolation. The influence of reinforcing selection can be detected when sympatric individuals (those from areas of secondary contact) show a higher degree of prezygotic isolation than allopatric individuals (those from areas outside each other's range). In areas of secondary contact with Drosophila santomea, Drosophila yakuba females show reinforcement of gametic isolation but not behavioral isolation, despite the fact that both behavioral and gametic isolation evolve in D. yakuba in experimental sympatry. Using behavioral assays and experimental evolution, I studied how both gametic and behavioral isolation are affected by biotic factors that the two species encounter in their natural environment. I show that if D. yakuba females are in environments where D. yakuba, D. santomea, and males from other species coexist, these females cannot fully discern between conspecific and heterospecific males. In such complex environments, gametic but not behavioral isolation evolves. The presence of nonhybridizing species can constrain the effect of reinforcement on behavioral isolation.

Reinforcement selection acting on the European house mouse hybrid zone

Behavioural isolation may lead to complete speciation when partial postzygotic isolation acts in the presence of divergent-specific mate-recognition systems. These conditions exist where Mus musculus musculus and M. m. domesticus come into contact and hybridize. We studied two mate-recognition signal systems, based on urinary and salivary proteins, across a Central European portion of the mouse hybrid zone. Introgression of the genomic regions responsible for these signals: the major urinary proteins (MUPs) and androgen binding proteins (ABPs), respectively, was compared to introgression at loci assumed to be nearly neutral and those under selection against hybridization. The preference of individuals taken from across the zone regarding these signals was measured in Y mazes, and we develop a model for the analysis of the transition of such traits under reinforcement selection. The strongest assortative preferences were found in males for urine and females for ABP. Clinal analyses confirm nearly neutral introgression of an Abp locus and two loci closely linked to the Abp gene cluster, whereas two markers flanking the Mup gene region reveal unexpected introgression. Geographic change in the preference traits matches our reinforcement selection model significantly better than standard cline models. Our study confirms that behavioural barriers are important components of reproductive isolation between the house mouse subspecies.

Character displacement in the fighting colours of Hetaerina damselflies

Aggression between species is a seldom-considered but potentially widespread mechanism of character displacement in secondary sexual characters. Based on previous research showing that similarity in wing coloration directly influences interspecific territorial aggression in Hetaerina damselflies, we predicted that wing coloration would show a pattern of character displacement (divergence in sympatry). A geographical survey of four Hetaerina damselfly species in Mexico and Texas showed evidence for character displacement in both species pairs that regularly occurs sympatrically. Hetaerina titia, a species that typically has large black wing spots and small red wing spots, shifted to having even larger black spots and smaller red wing spots at sites where a congener with large red wing spots is numerically dominant (Hetaerina americana or Hetaerina occisa). Hetaerina americana showed the reverse pattern, shifting towards larger red wing spots where H. titia is numerically dominant. This pattern is consistent with the process of agonistic character displacement, but the ontogenetic basis of the shift remains to be demonstrated.

Evidence for pre-zygotic reproductive barrier between the B and Q biotypes of Bemisia tabaci (Hemiptera: Aleyrodidae)

The degree of reproductive isolation between the B and Q biotypes of the whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) is currently not clear. Laboratory experiments have shown that the two biotypes are capable of producing viable F1 hybrids but that these females are sterile as their F2 generation failed to develop, indicating, most likely, a post-zygotic reproductive barrier. Here, we confirm, by molecular and ecological tools, that the B and Q biotypes of Israel are genetically isolated and provide two independent lines of evidence that support the existence of a pre-zygotic reproductive barrier between them. Firstly, monitoring of mating behaviors in homogeneous and heterogeneous couples indicated no copulation events in heterogeneous couples compared to approximately 50% in homogeneous B and Q couples. Secondly, we could not detect the presence of sperm in the spermathecae of females from heterogeneous couples, compared to 50% detection in intra-B biotype crosses and 15% detection in intra-Q biotype crosses. The existence of pre-zygotic reproductive barriers in Israeli B and Q colonies may indicate a reinforcement process in which mating discrimination is strengthened between sympatric taxa that were formerly allopatric, to avoid maladaptive hybridization. As the two biotypes continued to perform all courtship stages prior to copulation, we also conducted mixed cultures experiments in order to test the reproductive consequences of inter-biotype courtship attempts. In mixed cultures, a significant reduction in female fecundity was observed for the Q biotype but not for the B biotype, suggesting an asymmetric reproductive interference effect in favour of the B biotype. The long-term outcome of this effect is yet to be determined since additional environmental forces may reduce the probability of demographic displacement of one biotype by the other in overlapping niches.

The genetics and ecology of reinforcement: implications for the evolution of prezygotic isolation in sympatry and beyond

Reinforcement, the evolution of prezygotic reproductive barriers by natural selection in response to maladaptive hybridization, is one of the most debated processes in speciation. Critics point to "fatal" conceptual flaws for sympatric evolution of prezygotic isolation, but recent theoretical and empirical work on genetics and ecology of reinforcement suggests that such criticisms can be overcome. New studies provide evidence for reinforcement in frogs, fish, insects, birds, and plants. While such evidence lays to rest the argument over reinforcement's existence, our understanding remains incomplete. We lack data on (1) the genetic basis of female preferences and the links between genetics of pre- and postzygotic isolation, (2) the ecological basis of reproductive isolation, (3) connections between prezygotic isolation between species and within-species sexual selection (potentially leading to a "cascade" of effects on reproductive isolation), (4) the role of habitat versus mate preference in reinforcement, and (5) additional detailed comparative studies. Here, we review data on these issues and highlight why they are important for understanding speciation.

Review. Sympatric, parapatric or allopatric: the most important way to classify speciation?

The most common classification of modes of speciation begins with the spatial context in which divergence occurs: sympatric, parapatric or allopatric. This classification is unsatisfactory because it divides a continuum into discrete categories, concentrating attention on the extremes, and it subordinates other dimensions on which speciation processes vary, such as the forces driving differentiation and the genetic basis of reproductive isolation. It also ignores the fact that speciation is a prolonged process that commonly has phases in different spatial contexts. We use the example of local adaptation and partial reproductive isolation in the intertidal gastropod Littorina saxatilis to illustrate the inadequacy of the spatial classification of speciation modes. Parallel divergence in shell form in response to similar environmental gradients in England, Spain and Sweden makes this an excellent model system. However, attempts to demonstrate 'incipient' and 'sympatric' speciation involve speculation about the future and the past. We suggest that it is more productive to study the current balance between local adaptation and gene flow, the interaction between components of reproductive isolation and the genetic basis of differentiation.

Evidence of reinforcement of premating isolation between two species of the genus Ochthebius (Coleoptera: Hydraenidae)

The increase in premating reproductive isolation between recently diverged and potentially interbreeding taxa resulting from selection against hybridization (reinforcement) is one of the most contentious issues in evolutionary biology. After many years of debate, its plausibility under various conditions has been shown by theoretical studies and some cases have been documented. At present, interest is arising about the frequency and importance of reinforcement in nature. Ochthebius quadricollis and Ochthebius sp. A are two hydraenid beetles inhabiting marine rock pools in the Mediterranean basin. By molecular analysis of a contact zone between the two species along the Italian Tyrrhenian coast, full reproductive isolation between the two species was evidenced. However, the finding of introgressed specimens at some diagnostic loci suggested that gene flow occurred in the past but then ceased. In this article, by analyzing species composition of mating couples collected in sympatric localities, we show the existence of strong assortative mating between the two species in nature. In laboratory multiple-choice mating trials, sympatric populations showed greater assortative mating than allopatric populations. Reinforcement is suggested as the most parsimonious hypothesis to explain the evolution of discriminative mate recognition systems occurring among O. quadricollis and Ochthebius sp. A under sympatric, but not allopatric, populations.

Reinforcement of mate preference among hybridizing Heliconius butterflies

Recent models of mate preference evolution suggest that direct selection on alleles at preference loci and correlated evolution of preference with locally adapted mating cues are more likely to drive the evolution of assortative mate preference than reinforcement. Mate preference evolution in mimetic Heliconius butterflies has been attributed to all three forms of selection, but here we show that reinforcement has been critical. By examining geographical variation in assortative mating and male mate preference among seven populations of three hybridizing Heliconius species from Costa Rica, we found pronounced character displacement of preference such that sexual isolation was enhanced in areas of interspecific contact. Of the different explanations for the evolution of assortative mate preference, only reinforcement is dependent on interspecific contact in this system. Thus, the observed pattern of reproductive character displacement of mate preference is best explained as a product of indirect selection generated by natural selection against nonmimetic hybrids.

CONTRASTING PATTERNS OF HYBRIDIZATION IN LARGE HOUSE SPIDERS (TEGENARIA ATRICA GROUP, AGELENIDAE)

The integrity of species is not fixed and may vary geographically. Here we investigate the geographic distributions and interactions of species in the Tegenaria atrica group (Araneae: Agelenidae). Detailed mapping of T. saeva and T. gigantea in England and Wales shows them to be broadly allopatric in southern England with a tightly defined, and possibly long-standing, narrow zone of parapatry in central southern England. In the north of England (Yorkshire), by contrast, the species are broadly sympatric as a result of recent range expansions. GIS techniques are used to map the species distributions and to quantify, we believe for the first time, the intimacy of interspecific interactions. The extent and nature of hybridization in these two areas is examined through regression and multivariate analyses of morphology. We show that the relative incidence of hybridization is much greater in Yorkshire than within the parapatric zone in the south. Clear patterns of asymmetric introgression are observed in both northern and southern England, with a greater impact of T. gigantea on T. saeva than vice versa. We find no sign of morphological reproductive character displacement at the zone of parapatry that might indicate reinforcement, although we cannot exclude more subtle effects, for example via cuticular pheromones. The integrity of these two species seems to be breaking down in northern England, a process that might gain momentum as the gene pools become more similar.

Rapid evolution of sexual signals in sympatric Calopteryx damselflies: reinforcement or ?noisy-neighbour? ecological character displacement?

Enhanced prezygotic isolation in sympatry is one of the most intriguing patterns in evolutionary biology and has frequently been interpreted as evidence for reinforcement. However, the frequency with which reinforcement actually completes speciation remains unclear. The Jewelwing damselflies (Calopteryx aequabilis and C. maculata) have served as one of the few classic examples of speciation via reinforcement outside of Drosophila. Although evidence for wing pattern displacement and increased mate discrimination in this system have been demonstrated, the degree of hybridization and gene flow in nature are unknown. Here, we show that sympatric populations of these two species are the result of recent secondary contact, as predicted under a model of speciation via reinforcement. However, we found no phenotypic evidence of hybridization in natural populations and a complete association between species-specific haplotypes at two different loci (mitochondrial CO I and nuclear EF1-alpha), suggesting little or no contemporary gene flow. Moreover, genealogical and coalescent-based estimates of divergence times and migration rates indicate that, speciation occurred in the distant past. The rapid evolution of wing colour in sympatry is recent, therefore, relative to speciation and seems to be better explained by selection against wasting mating effort and/or interspecific aggression resulting from a 'noisy neighbour' signalling environment.

Natural selection and divergence in mate preference during speciation

Sexual isolation can evolve due to natural selection against hybrids (reinforcement). However, many different forms of hybrid dysfunction, and selective processes that do not involve hybrids, can contribute to the evolution of sexual isolation. Here we review how different selective processes affect the evolution of sexual isolation, describe approaches for distinguishing among them, and assess how they contribute to variation in sexual isolation among populations of Timema cristinae stick-insects. Pairs of allopatric populations of T. cristinae living on different host-plant species exhibit greater sexual isolation than those on the same host, indicating that some sexual isolation has evolved due to host adaptation. Sexual isolation is strongest in regions where populations on different hosts are in geographic contact, a pattern of reproductive character displacement that is indicative of reinforcement. Ecological costs to hybridization do occur but traits under ecological selection (predation) do not co-vary strongly with the probability of between-population mating such that selection on ecological traits is not predicted to produce a strong correlated evolutionary response in mate preference. Moreover, F1 hybrid egg inviability is lacking and the factors contributing to reproductive character displacement require further study. Finally, we show that sexual isolation involves, at least in part, olfactory communication. Our results illustrate how understanding of the evolution of sexual isolation can be enhanced by isolating the roles of diverse ecological and evolutionary processes.

Reconstructing asymmetrical reproductive character displacement in a periodical cicada contact zone

Selection against costly reproductive interactions can lead to reproductive character displacement (RCD). We use information from patterns of displacement and inferences about predisplacement character states to investigate causes of RCD in periodical cicadas. The 13-year periodical cicada Magicicada neotredecim exhibits RCD and strong reproductive isolation in sympatry with a closely related 13-year species, Magicicada tredecim. Displacement is asymmetrical, because no corresponding pattern of character displacement exists within M. tredecim. Results from playback and hybridization experiments strongly suggest that sexual interactions between members of these species were possible at initial contact. Given these patterns, we evaluate potential sources of selection for displacement. One possible source is 'acoustical interference', or mate-location inefficiencies caused by the presence of heterospecifics. Acoustical interference combined with the species-specificity of song pitch and preference appears to predict the observed asymmetrical pattern of RCD in Magicicada. However, acoustical interference does not appear to be a complete explanation for displacement in Magicicada, because our experiments suggest a significant potential for direct sexual interactions between these species before displacement. Another possible source of selection for displacement is hybrid failure. We evaluate the attractiveness of inferred hybrid mating signals, and we examine the viability of hybrid eggs. Neither of these shows strong evidence of hybrid inferiority. We conclude by presenting a model of hybrid failure related to life cycle differences in Magicicada.

Asymmetrical reproductive character displacement in the house mouse

Our study addressed reproductive character displacement between two subspecies of the house mouse, Mus musculus musculus and Mus musculus domesticus that hybridize in Europe along a zone where selection against hybridization is known to occur. Based on a multi-population approach, we investigated spatial patterns of divergence of mate preference in the two taxa. Mate preference was significantly higher in the contact zone than in allopatry in both subspecies, suggesting that reproductive character displacement occurs. Moreover, patterns of preference were stronger in M. m. musculus than in M. m. domesticus, indicating an asymmetrical divergence between the two. In the context of selection against hybridization, our results may provide empirical support for the hypothesis of reinforcement in a parapatric hybrid zone. We discuss factors that could explain the asymmetrical pattern of divergence and the possible impact of a unimodal structure on the maintenance of premating divergence between the two subspecies.

The evolution of host preference in allopatric vs. parapatric populations of Timema cristinae walking-sticks

Divergent habitat preferences can contribute to speciation, as has been observed for host-plant preferences in phytophagous insects. Geographic variation in host preference can provide insight into the causes of preference evolution. For example, selection against maladaptive host-switching occurs only when multiple hosts are available in the local environment and can result in greater divergence in regions with multiple vs. a single host. Conversely, costs of finding a suitable host can select for preference even in populations using a single host. Some populations of Timema cristinae occur in regions with only one host-plant species present (in allopatry, surrounded by unsuitable hosts) whereas others occur in regions with two host-plant species adjacent to one another (in parapatry). Here, we use host choice and reciprocal-rearing experiments to document genetic divergence in host preference among 33 populations of T. cristinae. Populations feeding on Ceanothus exhibited a stronger preference for Ceanothus than did populations feeding on Adenostoma. Both allopatric and parapatric pairs of populations using the different hosts exhibited divergent host preferences, but the degree of divergence tended to be greater between allopatric pairs. Thus, gene flow between parapatric populations apparently constrains divergence. Host preferences led to levels of premating isolation between populations using alternate hosts that were comparable in magnitude to previously documented premating isolation caused by natural and sexual selection against migrants between hosts. Our findings demonstrate how gene flow and different forms of selection interact to determine the magnitude of reproductive isolation observed in nature.

Reinforcement and the genetics of hybrid incompatibilities

Recent empirical studies suggest that genes involved in speciation are often sex-linked. We derive a general analytic model of reinforcement to study the effects of sex linkage on reinforcement under three forms of selection against hybrids: one-locus, two-locus, and ecological incompatibilities. We show that the pattern of sex linkage can have a large effect on the amount of reinforcement due to hybrid incompatibility. Sex linkage of genes involved in postzygotic isolation generally increases the strength of reinforcement, but only if genes involved in prezygotic isolation are also sex linked. We use exact simulations to test the accuracy of the approximation and find that qualitative predictions made assuming weak selection can hold when selection is strong. Our simulations also show that incompatibilities that evolve in allopatry by drift or weak selection are likely to be lost by swamping during secondary contact, even when selection against hybrids is strong.