Character displacement and the evolution of mate choice: an artificial neural network approach

Interactions with heterospecifics can promote the evolution of divergent mating behaviours between populations that do and do not occur with heterospecifics. This process--reproductive character displacement--potentially results from selection to minimize the risk of mating with heterospecifics. We sought to determine whether heterospecific interactions lead to divergence of female preferences for aspects of conspecific male signals. We used artificial neural network models to simulate a mate recognition system in which females co-occur with different heterospecifics in different populations. Populations that evolved conspecific recognition in the presence of different heterospecifics varied in their preferences for aspects of conspecific male signals. When we tested networks for their preferences of conspecific versus heterospecific signals, however, we found that networks from allopatric populations were usually able to select against heterospecifics. We suggest that female preferences for aspects of conspecific male signals can result in a concomitant reduction in the likelihood that females will mate with heterospecifics. Consequently, even females in allopatry may discriminate against heterospecific mates depending on the nature of their preferences for conspecifics. Such a pattern could potentially explain cases where reproductive character displacement is expected, but not observed.     

REINFORCEMENT OF STICKLEBACK MATE PREFERENCES: SYMPATRY BREEDS CONTEMPT

Detailed studies of reproductive isolation and how it varies among populations can provide valuable insight into the mechanisms of speciation. Here we investigate how the strength of premating isolation varies between sympatric and allopatric populations of threespine sticklebacks to test a prediction of the hypothesis of reinforcement: that interspecific mate discrimination should be stronger in sympatry than in allopatry. In conducting such tests, it is important to control for ecological character displacement between sympatric species because ecological character divergence may strengthen prezygotic isolation as a by-product. We control for ecological character displacement by comparing mate preferences of females from a sympatric population (benthics) with mate preferences of females from two allopatric populations that most closely resemble the sympatric benthic females in ecology and morphology. No-choice mating trials indicate that sympatric benthic females mate less readily with heterospecific (limnetic) than conspecific (benthic) males, whereas two different populations of allopatric females resembling benthics show no such discrimination. These differences demonstrate reproductive character displacement of benthic female mate choice. Previous studies have established that hybridization between sympatric species occurred in the past in the wild and that hybrid offspring have lower fitness than either parental species, thus providing conditions under which natural selection would favor individuals that do not hybridize. Results are therefore consistent with the hypothesis that female mate preferences have evolved as a response to reduced hybrid fitness (reinforcement), although direct effects of sympatry or a biased extinction process could also produce the pattern. Males of the other sympatric species (limnetics) showed a preference for smaller females, in contrast to the inferred ancestral preference for larger females, suggesting reproductive character displacement of limnetic male mate preferences as well.     

DIVERSIFICATION OF CONSPECIFIC SIGNALS IN SYMPATRY: GEOGRAPHIC OVERLAP DRIVES MULTIDIMENSIONAL REPRODUCTIVE CHARACTER DISPLACEMENT IN FROGS

Theoretical models suggest that geographic overlap with different heterospecific assemblages can promote divergence of mate recognition systems among conspecific populations. Divergence occurs when different traits undergo reproductive character displacement across populations within a contact zone. Here, I tested this hypothesis by assessing patterns of acoustic signal divergence in two- and three-species assemblages of chorus frogs ( Pseudacris ), focusing in particular on P. feriarum and P. nigrita . In addition, I tested one criterion for reinforcement, by examining the evolution of female P. feriarum preferences in the contact zone. Patterns of signal evolution indicated that in each of the four sympatric populations studied, only the rarer species displaced substantially ( P. feriarum in three cases and P. nigrita in one instance). Moreover, the three displaced P. feriarum populations diverged in different signal traits across the contact zone, evolving in directions that increased the energetic cost of calling relative to the allopatric call, and in ways that maximized differences from the particular heterospecific assemblage present. Consistent with reinforcement, divergence of female preferences in sympatry was estimated to reduce their propensity to hybridize by 60%. Together, signal and preference data suggest that interactions between species can promote diversification within species, potentially contributing to reproductive isolation among conspecific populations.     

Reproductive character displacement generates reproductive isolation among conspecific populations: an artificial neural network study

When interactions with heterospecifics prevent females from identifying conspecific mates, natural selection can promote the evolution of mating behaviours that minimize such interactions. Consequently, mating behaviours may diverge among conspecific populations in sympatry and in allopatry with heterospecifics. This divergence in conspecific mating behaviours-reproductive character displacement-can initiate speciation if mating behaviours become so divergent as to generate reproductive isolation between sympatric and allopatric conspecifics. We tested these ideas by using artificial neural networks to simulate the evolution of conspecific mate recognition in populations sympatric and allopatric with different heterospecifics. We found that advertisement calls diverged among the different conspecific populations. Consequently, networks strongly preferred calls from their own population to those from foreign conspecific populations. Thus, reproductive character displacement may promote reproductive isolation and, ultimately, speciation among conspecific populations.     

Asymmetric reproductive character displacement in male aggregation behaviour

Reproductive character displacement--the evolution of traits that minimize reproductive interactions between species--can promote striking divergence in male signals or female mate preferences between populations that do and do not occur with heterospecifics. However, reproductive character displacement can affect other aspects of mating behaviour. Indeed, avoidance of heterospecific interactions might contribute to spatial (or temporal) aggregation of conspecifics. We examined this possibility in two species of hybridizing spadefoot toad (genus Spea). We found that in Spea bombifrons sympatric males were more likely than allopatric males to associate with calling males. Moreover, contrary to allopatric males, sympatric S. bombifrons males preferentially associated with conspecific male calls. By contrast, Spea multiplicata showed no differences between sympatry and allopatry in likelihood to associate with calling males. Further, sympatric and allopatric males did not differ in preference for conspecifics. However, allopatric S. multiplicata were more variable than sympatric males in their responses. Thus, in S. multiplicata, character displacement may have refined pre-existing aggregation behaviour. Our results suggest that heterospecific interactions can foster aggregative behaviour that might ultimately contribute to clustering of conspecifics. Such clustering can generate spatial or temporal segregation of reproductive activities among species and ultimately promote reproductive isolation.     

Does character displacement initiate speciation? Evidence of reduced gene flow between populations experiencing divergent selection

Character displacement – trait evolution stemming from selection to lessen resource competition or reproductive interactions between species – has long been regarded as important in finalizing speciation. By contrast, its role in initiating speciation has received less attention. Yet because selection for character displacement should act only where species co‐occur, individuals in sympatry will experience a different pattern of selection than conspecifics in allopatry. Such divergent selection might favour reduced gene flow between conspecific populations that have undergone character displacement and those that have not, thereby potentially triggering speciation. Here, we explore these ideas empirically by focusing on spadefoot toads, Spea multiplicata, which have undergone character displacement, and for which character displacement appears to cause post‐mating isolation between populations that are in sympatry with a heterospecific and those that are in allopatry. Using mitochondrial sequence data and nuclear microsatellite genotypes, we specifically asked whether gene flow is reduced between populations in different selective environments relative to that between populations in the same selective environment. We found a slight, but statistically significant, reduction in gene flow between selective environments, suggesting that reproductive isolation, and potentially ecological speciation, might indeed evolve as an indirect consequence of character displacement. Generally, character displacement may play a largely underappreciated role in instigating speciation.     

Asymmetric interspecific competition drives shifts in signalling traits in fan-throated lizards

Interspecific competition can occur when species are unable to distinguish between conspecific and heterospecific mates or competitors when they occur in sympatry. Selection in response to interspecific competition can lead to shifts in signalling traits—a process called agonistic character displacement. In two fan-throated lizard species—Sitana laticeps and Sarada darwini—females are morphologically indistinguishable and male agonistic signalling behaviour is similar. Consequently, in areas where these species overlap, males engage in interspecific aggressive interactions. To test whether interspecific male aggression between Si. laticeps and Sa. darwini results in agonistic character displacement, we quantified species recognition and signalling behaviour using staged encounter assays with both conspecifics and heterospecifics across sympatric and allopatric populations of both species. We found an asymmetric pattern, wherein males of Si. laticeps but not Sa. darwini showed differences in competitor recognition and agonistic signalling traits (morphology and behaviour) in sympatry compared with allopatry. This asymmetric shift in traits is probably due to differences in competitive abilities between species and can minimize competitive interactions in zones of sympatry. Overall, our results support agonistic character displacement, and highlight the role of asymmetric interspecific competition in driving shifts in social signals.     

Remating responses are consistent with male postcopulatory manipulation but not reinforcement in D. pseudoobscura

Reinforcement occurs when hybridization between closely related lineages produces low‐fitness offspring, prompting selection for elevated reproductive isolation specifically in areas of sympatry. Both premating and postmating prezygotic behaviors have been shown to be the target of reinforcing selection, but it remains unclear whether remating behaviors experience reinforcement, although they can also influence offspring identity and limit formation of hybrids. Here, we evaluated evidence for reinforcing selection on remating behaviors in Drosophila pseudoobscura, by comparing remating traits in females from populations historically allopatric and sympatric with Drosophila persimilis. We found that the propensity to remate was not higher in sympatric females, compared to allopatric females, regardless of whether the first mated male was heterospecific or conspecific. Moreover, remating behavior did not contribute to interspecific reproductive isolation among any population; that is, females showed no higher propensity to remate following a heterospecific first mating than following a conspecific first mating. Instead, we found that females are less likely to remate after initial matings with unfamiliar males, regardless of species identity. This is consistent with one scenario of postmating sexual conflict in which females are poorly defended against postcopulatory manipulation by males with whom they have not coevolved. Our results are generally inconsistent with reinforcement on remating traits and suggest that this behavior might be more strongly shaped by the consequences of local antagonistic male–female interactions than interactions with heterospecifics.     

Males,but not females,contribute to sexual isolation between two sympatric species of <Emphasis Type="Italic">Gambusia</Emphasis>

Reproductive isolation restricts genetic exchange between species. Various pre- and post-mating barriers, such as behavior, physiology and gametic incompatibility, have been shown to evolve in sympatry. In certain scenarios, isolation can be asymmetrical, where species differentially prefer conspecifics. We examined sexual isolation via conspecific mate preference between Gambusia affinis and G. geiseri in both sexes. To investigate male contribution to sexual isolation, we compared the number of mating attempts (gonopodial thrusts) directed at either a conspecific or a heterospecific female, in both species. We also examined sperm priming and expenditure in males in the presence of conspecific or heterospecific females. We then measured female preference for either a conspecific or heterospecific male, in both species. We found that males of both species preferred to mate with conspecific females, but showed no difference in sperm production or expenditure between conspecific and heterospecific females. Females of both species did not prefer conspecific over heterospecific males. Our results suggest that sexual isolation might be mediated by male mate choice in this system and not female choice, suggesting that there is asymmetrical reproductive isolation between the sexes in G. affinis and G. geiseri, but symmetrical species isolation.     

REPRODUCTIVE CHARACTER DISPLACEMENT OF MALE STICKLEBACK MATE PREFERENCE: REINFORCEMENT OR DIRECT SELECTION?

Reproductive isolation can evolve between species as a byproduct of adaptation to different niches, through reinforcement, and by direct selection on mating preferences. We investigated the potential role of direct selection in the reproductive isolation between sympatric species of threespine sticklebacks. Each sympatric pair consists of a small-bodied limnetic species and large-bodied benthic species. We compared the mate preferences and courtship behavior of males from one sympatric limnetic population and two allopatric populations. We used limnetic-like allopatric populations to control for the effects of ecological character displacement and adaptation to different niches on mate preferences. The sympatric limnetic males preferred the small limnetic females, whereas the allopatric limnetic-like males preferred the large benthic females, suggesting that adaptation to the limnetic niche does not automatically confer a preference for small limnetic females. This reproductive character displacement of male preference is consistent with the predictions of both reinforcement and direct selection on mate preferences. To test for direct selection, we assessed a prediction of one proposed mechanism: predation by benthic females on eggs guarded by limnetic males. The allopatric males come from populations in which there is no egg predation. Sympatric limnetic males were more aggressive toward benthic females than toward limnetic females, whereas the allopatric limnetic-like males did not treat the two types of females differently. The contrast in male behavior suggests that egg predation has shaped male preferences. Direct selection is potentially more effective than indirect selection via reinforcement, and it is likely that it has been important in building up reproductive isolation between limnetic and benthic sticklebacks.     

The evolution of female mating preferences: differentiation from species with promiscuous males can promote speciation

Females of many species are frequently courted by promiscuous males of their own and other closely related species. Such mating interactions may impose strong selection on female mating preferences to favor trait values in conspecific males that allow females to discriminate them from their heterospecific rivals. We explore the consequences of such selection in models of the evolution of female mating preferences when females must interact with heterospecific males from which they are completely postreproductively isolated. Specifically, we allow the values of both the most preferred male trait and the tolerance of females for males that deviate from this most preferred trait to evolve. Also, we consider situations in which females base their mating decisions on multiple male traits and must interact with males of multiple species. Females will rapidly differentiate in preference when they sometimes mistake heterospecific males for suitable mates, and the differentiation of female preference will select for conspecific male traits to differentiate as well. In most circumstances, this differentiation continues indefinitely, but slows substantially once females are differentiated enough to make mistakes rare. Populations of females with broader preference functions (i.e., broader tolerance for males with trait values that deviate from females' most preferred values) will evolve further to differentiate if the shape of the function cannot evolve. Also, the magnitude of separation that evolves is larger and achieved faster when conspecific males have lower relative abundance. The direction of differentiation is also very sensitive to initial conditions if females base their mate choices on multiple male traits. We discuss how these selection pressures on female mate choice may lead to speciation by generating differentiation among populations of a progenitor species that experiences different assemblages of heterospecifics. Opportunities for differentiation increase as the number of traits involved in mate choice increase and as the number of species involved increases. We suggest that this mode of speciation may have been particularly prevalent in response to the cycles of climatic change throughout the Quaternary that forced the assembly and disassembly of entire communities on a continentwide basis.     

An experimental test of character displacement's role in promoting postmating isolation between conspecific populations in contrasting competitive environments

Ecological character displacement takes place when two closely related species co-occur in only part of their geographical range, and selection to minimize competition between them promotes divergence in resource-use traits in sympatry but not in allopatry. Because populations sympatric with the heterospecific competitor will experience a different competitive environment than conspecific populations in allopatry, conspecific populations from these two competitive environments will also diverge in resource traits as an indirect consequence of interspecific ecological character displacement. Ultimately, ecologically dependent postmating isolation may arise between conspecific populations from these divergent competitive environments if offspring produced by matings between them are competitively inferior in either type of competitive environment. Yet, there are no direct tests of character displacement's role in initiating such postmating isolation. Here, we present a test by comparing the phenotypes and performances of spadefoot toad tadpoles produced from between-competitive-environment (BCE) matings versus those produced from within-competitive-environment (WCE) matings. When raised with naturally occurring competitors, BCE offspring grew significantly less and were significantly smaller than WCE offspring. BCE offspring generally performed worse even when raised alone, suggesting that they may have harbored intrinsic genetic incompatibilities. Moreover, the difference in growth and body size of BCE versus WCE offspring was significantly greater when each was raised with competitors than when each was raised alone, suggesting that BCE tadpoles were competitively inferior to WCE tadpoles. Presumably, this enhanced difference arose because BCE tadpoles produced an intermediate resource-use phenotype that is less well adapted to either competitive environment. Because larval size is under strong, positive, directional selection, reduced growth and size of BCE offspring may diminish gene flow between populations in divergent competitive environments, thereby generating postmating isolation. Thus, postmating isolation between conspecific populations, and possibly even speciation, may arise as a by-product of interactions between species.     

Parallel evolution and ecological selection: replicated character displacement in spadefoot toads

Ecological character displacement—trait evolution stemming from selection to lessen resource competition between species—is most often inferred from a pattern in which species differ in resource-use traits in sympatry but not in allopatry, and in which sympatric populations within each species differ from conspecific allopatric populations. Yet, without information on population history, the presence of a divergent phenotype in multiple sympatric populations does not necessarily imply that there has been repeated evolution of character displacement. Instead, such a pattern may arise if there has been character displacement in a single ancestral population, followed by gene flow carrying the divergent phenotype into multiple, derived, sympatric populations. Here, we evaluate the likelihood of such historical events versus ongoing ecological selection in generating divergence in trophic morphology between multiple populations of spadefoot toad (Spea multiplicata) tadpoles that are in sympatry with a heterospecific and those that are in allopatry. We present both phylogenetic and population genetic evidence indicating that the same divergent trait, which minimizes resource competition with the heterospecific, has arisen independently in multiple sympatric populations. These data, therefore, provide strong indirect support for competition''s role in divergent trait evolution.     

Reproductive interference between animal species

Although sexual interactions between species (reproductive interference) have been reported from a wide range of animal taxa, their potential for determining species coexistence is often disregarded. Here, we review evidence from laboratory and field studies illustrating that heterospecific sexual interactions are frequently associated with fitness loss and can have severe ecological and evolutionary consequences. We define reproductive interference as any kind of interspecific interaction during the process of mate acquisition that adversely affects the fitness of at least one of the species involved and that is caused by incomplete species recognition. We distinguish seven types of reproductive interference: signal jamming, heterospecific rivalry, misdirected courtship, heterospecific mating attempts, erroneous female choice, heterospecific mating, and hybridization. We then discuss the sex-specific costs of these types and highlight two typical features of reproductive interference: density-dependence and asymmetry. Similar to competition, reproductive interference can lead to displacement of one species (sexual exclusion), spatial, temporal, or habitat segregation, changes in life history parameters, and reproductive character displacement. In many cases, patterns of coexistence might be shaped by reproductive interference rather than by resource competition, as the presence of a few heterospecifics might substantially decrease reproductive success. Therefore, interspecific sexual interactions should receive more attention in ecological research. Reproductive interference has mainly been discussed in the context of invasive species or hybrid zones, whereas its influence on naturally-occurring sympatric species pairs has rarely been addressed. To improve our knowledge of the ecological significance of reproductive interference, findings from laboratory experiments should be validated in the field. Future studies should also focus on ecological mechanisms, such as temporal spatial, or habitat partitioning, that might enable sexually interacting species to coexist. Reproductive interference also has implications for the management of endangered species, which can be threatened by sexual interactions with invasive or common species. Studies of reproductive interference might even provide new insights for biological pest control.     

Reproductive character displacement and the genetics of gamete recognition in tropical sea urchins

Reproductive character displacement occurs when sympatric and allopatric populations of a species differ in traits crucial to reproduction, and it is commonly thought of as a signal of selection acting to limit hybridization. Most documented cases of reproductive character displacement involve characters that are poorly understood at the genetic level, and rejecting alternative hypotheses for biogeographic shifts in reproductive traits is often very difficult. In sea urchins, the gamete recognition protein bindin evolves under positive selection when species are broadly sympatric, suggesting character displacement may be operating in this system. We sampled sympatric and allopatric populations of two species in the sea urchin genus Echinometra for variation in bindin and for the mitochondrial cytochrome oxidase I to examine patterns of population differentiation and molecular evolution at a reproductive gene. We found a major shift in bindin alleles between central Pacific (allopatric) and western Pacific (sympatric) populations of E. oblonga. Allopatric populations of E. oblonga are polyphyletic with E. sp. C at bindin, whereas sympatric populations of the two species are reciprocally monophyletic. There is a strong signal of positive selection (P(N)/P(S) = 4.5) in the variable region of the first exon of bindin, which is associated with alleles found in sympatric populations of E. oblonga. These results indicate that there is a strong pattern of reproductive character displacement between E. oblonga and E. sp. C and that the divergence is driven by selection. There is much higher population structure in sympatric populations at the bindin locus than at the neutral mitochondrial locus, but this difference is not seen in allopatric populations. These data suggest a pattern of speciation driven by selection for local gamete coevolution as a result of interactions between sympatric species. Although this pattern is highly suggestive of speciation by reinforcement, further research into hybrid fitness and egg-sperm interactions is required to address this potential mechanism for character displacement.     

Conflicting intersexual mate choices maintain interspecific sexual interactions

Reproductive interference, interspecific sexual interactions that affect reproductive success, is found in various taxa and has been considered as a fundamental source of reproductive character displacement (RCD). Once RCD has occurred, persistent interspecific sexual interactions between species pairs are expected to diminish. However, reproductive interference has been reported from some species pairs that sympatrically coexist. Thus, the question arises, can reproductive interference persist even after RCD? We modeled the evolutionary dynamics of signal traits and mate recognition that determine whether interspecific sexual interactions occur. Our models incorporate male decision making based on the recognition of signal traits, whereas most previous models incorporate only female decision making in mate selection. Our models predict the following: (1) even when male decision making is incorporated, males remain promiscuous; (2) nevertheless, the frequency of interspecific mating is maintained at a low level after trait divergence; (3) the rarity of interspecific mating is due to strict female mate recognition and the consequent refusal of interspecific courtship by females; and (4) the frequency of interspecific mating becomes higher as the cost to females of refusing interspecific courtship increases. These predictions are consistent with empirical observations that males of some species engage in infrequent heterospecific mating. Thus, our models predict that reproductive interference can persist even after RCD occurred.     

Female spadefoot toads compromise on mate quality to ensure conspecific matings

When high-quality conspecifics resemble heterospecifics, femalesmay be unable to engage effectively in both species recognition(identification of conspecifics) and mate-quality recognition(identification of high-quality mates). Consequently, femalesthat engage primarily in mate-quality recognition may riskheterospecific matings, and females that engage primarily inspecies recognition may risk mating with low-quality mates.I examined the evolutionary consequences of this conflict betweenspecies and mate-quality recognition in spadefoot toads, Speamultiplicata. I compared mate preferences and the fitness consequencesof these preferences in spadefoot toad populations that didand did not overlap with congeners. In non-overlapping populations,S. multiplicata females preferred an extreme call characterresembling that of heterospecifics, and they had more eggsfertilized. In overlapping populations, S. multiplicata females preferred those call characteristics that were closest to thenorm for their population, and they did not receive the benefitof enhanced fertilization success. Thus, S. multiplicata femalesappear to trade off species and mate-quality recognition, suchthat those co-occurring with heterospecifics forgo the benefitsof high-quality matings to ensure conspecific matings. Theseresults suggest that the interaction between species and mate-qualityrecognition may influence mate choice decisions in importantand nonintuitive ways.     

Do allopatric male Calopteryx virgo damselflies learn species recognition?

There is a growing amount of empirical evidence that premating reproductive isolation of two closely related species can be reinforced by natural selection arising from avoidance of maladaptive hybridization. However, as an alternative for this popular reinforcement theory, it has been suggested that learning to prefer conspecifics or to discriminate heterospecifics could cause a similar pattern of reinforced premating isolation, but this possibility is much less studied. Here, we report results of a field experiment in which we examined (i) whether allopatric Calopteryx virgo damselfly males that have not encountered heterospecific females of the congener C. splendens initially show discrimination, and (ii) whether C. virgo males learn to discriminate heterospecifics or learn to associate with conspecifics during repeated experimental presentation of females. Our experiment revealed that there was a statistically nonsignificant tendency for C. virgo males to show initial discrimination against heterospecific females but because we did not use sexually naïve individuals in our experiment, we were not able to separate the effect of innate or associative learning. More importantly, however, our study revealed that species discrimination might be further strengthened by learning, especially so that C. virgo males increase their association with conspecific females during repeated presentation trials. The role of learning to discriminate C. splendens females was less clear. We conclude that learning might play a role in species recognition also when individuals are not naïve but have already encountered potential conspecific mates.     

Extreme genetic differences between queens and workers in hybridizing Pogonomyrmex harvester ants

The process of reproductive caste determination in eusocial insect colonies is generally understood to be mediated by environmental, rather than genetic factors. We present data demonstrating unexpected genetic differences between reproductive castes in a variant of the rough harvester ant, Pogonomyrmex rugosus var. fuscatus. Across multiple loci, queens were consistently more homozygous than expected, while workers were more heterozygous. Adult colony queens were divided into two highly divergent genetic groups, indicating the presence of two cryptic species, rather than a single population. The observed genetic differences between castes reflect differential representation of heterospecific and conspecific patrilines in these offspring groups. All workers were hybrids; by contrast, winged queens were nearly all pure-species. The complete lack of pure-species workers indicates a loss of worker potential in pure-species female offspring. Hybrids appear to be bipotential, but do not normally develop into reproductives because they are displaced by pure-species females in the reproductive pool. Genetic differences between reproductive castes are expected to be rare in non-hybridizing populations, but within hybrid zones they may be evolutionarily stable and thus much more likely to occur.     

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.