Does sympatry predict life history and morphological diversification in the Mexican livebearing fish Poeciliopsis baenschi?

Understanding why some species coexist and others do not remains one of the fundamental challenges of ecology. Although there is evidence to suggest that closely‐related species are unlikely to occupy the same habitat because of competitive exclusion, there are many cases where closely‐related species do co‐occur. Research comparing sympatric and allopatric populations of co‐occurring species provides a framework for understanding the role of phenotypic diversification in species coexistence. In the present study, we compare phenotypic divergence between sympatric and allopatric populations of the livebearing fish, Poeciliopsis baenschi. We focus on life‐history traits and body shape, comprising two sets of integrated traits likely to diverge in response to varying selective pressures. Given that males and females can express different phenotypic traits, we also test for patterns of divergence among sexes by comparing size at maturity and sexual dimorphism in body shape between males and females in each population type. We take advantage of a natural experiment in western Mexico where, in some locations, P. baenschi co‐occur with a closely‐related species, Poeciliopsis turneri (sympatric populations) and, in other locations, they occur in isolation (allopatric populations). The results obtained in the present study show that sympatric populations of P. baenschi differed significantly in life‐history traits and in body shape compared to their allopatric counterparts. Additionally, males and females showed different responses for size at maturity in sympatric conditions versus allopatric conditions. However, the amount of sexual dimorphism did not differ between sympatric and allopatric populations of P. baenschi. Hence, we conclude that not all traits show similar levels of phenotypic divergence in response to sympatric conditions. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100 , 608–618.     

Patterns and processes in the evolutionary history of parrotfishes (Family Labridae)

Phylogenetic reconstruction of the evolutionary relationships among 61 of the 70 species of the parrotfish genera Chlorurus and Scarus (Family Labridae) based on mitochondrial and nuclear gene sequences retrieved 15 well‐supported clades with mid Pliocene/Pleistocene diversification. Twenty‐two reciprocally monophyletic sister‐species pairs were identified: 64% were allopatric, and the remainder were sympatric. Age of divergence was similar for allopatric and sympatric species pairs. Sympatric sister pairs displayed greater divergence in morphology, ecology, and sexually dimorphic colour patterns than did allopatric pairs, suggesting that both genetic drift in allopatric species pairs and ecologically adaptive divergence between members of sympatric pairs have played a role in diversification. Basal species typically have small geographical ranges and are restricted to geographically and ecologically peripheral reef habitats. We found little evidence that a single dominant process has driven diversification, nor did we detect a pattern of discrete, sequential stages of diversification in relation to habitat, ecology, and reproductive biology. The evolution of Chlorurus and Scarus has been complex, involving a number of speciation processes. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 107 , 529–557.     

A Comparative Study of the Behaviour of Six Taxa of Male and Female Gerbils (Rodentia) in Intra- and Interspecific Encounters

Frequencies and sequences of visually-identifiable acts were investigated in male-female combinations of gerbils of the taxa Tatera brantsii, Gerbillurus paeba paeba, G. p. exilis, G. tytonis, G. setzeri, and G. vallinus. Sexual dimorphism of behaviour was most apparent in T. brantsii, G. setzeri, and G. vallinus, in which following, anogenital sniffing, and mounting were significantly more frequently performed by males than females. Females of all taxa presented and darted, while males did not perform these acts. Analysis of first-order transitions of acts in intraspecific encounters revealed remarkable similarity among species in the sequences identified as most significantly deviating from expected frequency. Species-specific patterns of behaviour were detected by discriminant analysis. G. p. paeba, G. p. exilis and G. vallinus were not clearly distinguished by discriminant analysis, and presented a behaviour pattern characterized by low levels of aggression and high levels of huddling behaviour. G. tytonis, G. setzeri and T. brantsii were characterized by low levels of huddling, G. tytonis performed the largest number of upright postures of all taxa, G. setzeri watched more than other taxa, and T. brantsii nasal sniffed less frequently than Gerbillurus taxa. Differences among taxa were more apparent in males than in females. Interspecific encounters were staged between G. p. paeba and G. p. exilis, G. p. paeba and G. tytonis, G. setzeri and G. vallinus. Males showed more evidence of changed behaviour in interspecific encounters than females. More sexual and less agonistic behaviour was detected in intraspecific encounters than in interspecific encounters, but this was statistically significant in only 5 of 12 combinations. Visual, tactile and olfactory communication during physical contact with another individual may complement auditory communication in promoting conspecific recognition in southern African gerbils.     

Reinforcement shapes clines in female mate discrimination in Drosophila subquinaria

Reinforcement of species boundaries may alter mate recognition in a way that also affects patterns of mate preference among conspecific populations. In the fly Drosophila subquinaria, females sympatric with the closely related species D. recens reject mating with heterospecific males as well as with conspecific males from allopatric populations. Here, we assess geographic variation in behavioral isolation within and among populations of D. subquinaria and use cline theory to understand patterns of selection on reinforced discrimination and its consequences for sexual isolation within species. We find that selection has fixed rejection of D. recens males in sympatry, while significant genetic variation in this behavior occurs within allopatric populations. In conspecific matings sexual isolation is also asymmetric and stronger in populations that are sympatric with D. recens. The clines in behavioral discrimination within and between species are similar in shape and are maintained by strong selection in the face of gene flow, and we show that some of their genetic basis may be either shared or linked. Thus, while reinforcement can drive extremely strong phenotypic divergence, the long‐term consequences for incipient speciation depend on gene flow, genetic linkage of discrimination traits, and the cost of these behaviors in allopatry.     

Temperature Effects on the Temporal Properties of Calling Songs in the Crickets <Emphasis Type="Italic">Gryllus fultoni</Emphasis> and <Emphasis Type="Italic">G. vernalis</Emphasis>: Implications for Reproductive Isolation in Sympatric Populations

Two closely related wood-cricket species, Gryllus fultoni (Orthoptera: Gryllidae) and G. vernalis, produce similar calling songs, consisting of 3-pulse chirps. Analysis of field and laboratory recordings of calling songs showed that, after correction to a common temperature, there was a divergence in chirp and pulse rates between far allopatric populations of G. fultoni and populations sympatric with G. vernalis. To determine whether the divergence in calling songs potentially provides reproductive isolation between G. fultoni and G. vernalis throughout the temperature range over which these insects sing, we recorded calling songs of lab-reared G. fultoni and G. vernalis populations between 18 and 28°C. Mean chirp rate significantly differed between sympatric and far allopatric G. fultoni populations as well as between sympatric G. fultoni and G. vernalis populations. Although there was a significant difference in mean pulse rate between sympatric G. fultoni and G. vernalis populations, pulse rate did not differ between sympatric and far allopatric G. fultoni populations in the laboratory study. Considering the magnitudes of differences in calling song characters discriminated by females of G. fultoni and the mean differences and the variability in calling song characters between the two species, the joint difference in chirp and pulse rates may be adequate for species discrimination over most of the range at which these crickets breed.     

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

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

A Comparative Study of Social Behaviour of Two Sympatric Ground Squirrels (Spermophilus spilosoma,S. mexicanus)

During the reproductive period, intraspecific dyadic encounters were staged within and between S. spilosoma and S. mexicanus individuals under controlled conditions. Video recordings were used to describe and quantify behaviour of males and females of these two sympatric ground squirrels. To compare dyads within and between species two indices (an Agonistic index and a Tolerance index) were designed from measures of behavioural frequencies. During intrasexual encounters, S. mexicanns males and females appear to be more aggressive than male and female S. spilosoma. In both species, encounters between males were more aggressive than between females, which were very tolerant to each other in S. spilosoma. In most cases encounters between male and female conspecifics exhibit an intermediate level of agonistic behaviour and tolerance. The null hypothesis of similarity of the social behaviour of S. spilosoma and S. mexicanus is falsified. Some alternative hypotheses are proposed. First, the tolerance of S. spilosoma is an adaptation to the difficult environmental conditions. Thus differences between the two species reflect differences in behavioural evolution in allopatric populations. Second, S. spilosoma tolerance is a behavioural response to S. mexicanus' recent arrival.     

Testing the role of climate in speciation: New methods and applications to squamate reptiles (lizards and snakes)

Climate may play important roles in speciation, such as causing the range fragmentation that underlies allopatric speciation (through niche conservatism) or driving divergence of parapatric populations along climatic gradients (through niche divergence). Here, we developed new methods to test the frequency of climate niche conservatism and divergence in speciation, and applied it to species pairs of squamate reptiles (lizards and snakes). We used a large‐scale phylogeny to identify 242 sister species pairs for analysis. From these, we selected all terrestrial allopatric pairs with sufficient occurrence records (n = 49 pairs) and inferred whether each originated via climatic niche conservatism or climatic niche divergence. Among the 242 pairs, allopatric pairs were most common (41.3%), rather than parapatric (19.4%), partially sympatric (17.7%), or fully sympatric species pairs (21.5%). Among the 49 selected allopatric pairs, most appeared to have originated via climatic niche divergence (61–76%, depending on the details of the methods). Surprisingly, we found greater climatic niche divergence between allopatric sister species than between parapatric pairs, even after correcting for geographic distance. We also found that niche divergence did not increase with time, further implicating niche divergence in driving lineage splitting. Overall, our results suggest that climatic niche divergence may often play an important role in allopatric speciation, and the methodology developed here can be used to address the generality of these findings in other organisms.     

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.     

A ROLE FOR LEARNING IN POPULATION DIVERGENCE OF MATE PREFERENCES

Learning and other forms of phenotypic plasticity have been suggested to enhance population divergence. Mate preferences can develop by learning, and species recognition might not be entirely genetic. We present data on female mate preferences of the banded demoiselle (Calopteryx splendens) that suggest a role for learning in population divergence and species recognition. Populations of this species are either allopatric or sympatric with a phenotypically similar congener (C. virgo). These two species differ mainly in the amount of wing melanization in males, and wing patches thus mediate sexual isolation. In sympatry, sexually experienced females discriminate against large melanin wing patches in heterospecific males. In contrast, in allopatric populations within the same geographic region, females show positive (“open‐ended”) preferences for such large wing patches. Virgin C. splendens females do not discriminate against heterospecific males. Moreover, physical exposure experiments of such virgin females to con‐ or hetero‐specific males significantly influences their subsequent mate preferences. Species recognition is thus not entirely genetic and it is partly influenced by interactions with mates. Learning causes pronounced population divergence in mate preferences between these weakly genetically differentiated populations, and results in a highly divergent pattern of species recognition at a small geographic scale.     

Hairstreak butterflies (Lepidoptera,Lycaenidae) and evolution of their male secondary sexual organs

Hairstreak butterflies in the Atlides Section of the Eumaeini are biologically notable for a diverse array of male secondary sexual organs. A “species recognition” hypothesis postulates that females use these organs to choose between conspecific and non-conspecific males, thereby promoting reproductive isolation. Alternately, a “sexual selection” hypothesis posits that females use these organs to choose among conspecific males. These hypotheses need not be mutually exclusive but make different predictions about the evolutionary gain and loss of male secondary sexual organs. We analysed most of the Atlides Section (Theclinae, Eumaeini) phylogenetically. Sister lineages were sympatric at 22 of 37 nodes. Nine evolutionary gains occurred in lineages that were sympatric with their phylogenetic sister, and one occurred in a lineage that was allopatric/parapatric with its sister. By contrast, seven of ten evolutionary losses occurred in lineages that were allopatric/parapatric with their sisters. These results are significantly different from those predicted by a sexual selection hypothesis. We conclude that male secondary sexual organs in the Atlides Section function primarily for species recognition and thereby promote sympatric diversification.     

Does character displacement demonstrate density-dependent expression in females? A test on the wing shape of two species of European damselflies

Character displacement (CD) is the evolutionary process which leads to the divergence in trait expression of closely related species in regions where species co-occur, compared to allopatric populations. In Europe CD has been investigated in males of Calopteryx splendens and C. virgo and has been related to species recognition. If species recognition is relevant for males, also females should benefit from CD. The most obvious differences between females of these two species are wing profile and colour. We sampled females from allopatric and from sympatric populations with different relative abundances of these species. Wing shape and pigmentation were evaluated for each damselfly. CD was found in wing profile but not in wing transparency. The relative abundance of species significantly affected CD, but with a different pattern in each species. The prediction that wing shape become more different from the allopatric state when the species was relatively rare, but more similar to the allopatric state when the species was common was evident only for C. splendens. Wing shape changes might increase differences in flying patterns making males more effective to discriminate between heterospecific females. So, CD we observed may be the result of a selection directed to reduce interspecific reproductive interference.     

Inferring the mode of speciation in Indo-West Pacific Conus (Gastropoda: Conidae)

Aim This study aims to initially explore the mode of speciation in Indo‐West Pacific Conus. Location The Indo‐West Pacific island arc, Indian and Pacific Oceans. Methods Relating evolutionary divergence in a molecular phylogeny [T.F. Duda & S.R. Palumbi (1999) Proceedings of the National Academy of Science USA, 96 , 10272] using node height with modern range extents as a possible measure of allopatric or sympatric speciation following that of T.G. Barraclough, A.P. Vogler & P.H. Harvey [(1999) Evolution of Biological Diversity. Oxford University Press, Oxford] models of sympatric and allopatric speciation. Results The analysis seems to indicate that the relationship of sympatry with node height is not informative. Species that have diverged quite recently show 100% sympatry with the sister species. A clearer signal of recent allopatric speciation is observed in species whose distribution is at the edge of the Indian and Pacific Ocean basins. In the widely distributed Conus ebraeus clade, the relationships of node heights and range extents of the member species support a key prediction of sympatric speciation. In highly ecologically specialized species, there is a smaller degree of sympatry than those species that are less specialized. Main conclusions The modes of speciation models presented in this study are not informative. This suggests that there had been large and possibly rapid changes in range size after speciation in the various clades. This could have been due to the fact that the wide dispersal life‐history strategy in the genus had been largely conserved in Conus evolution. There is evidence of sympatric and parapatric speciation in one Conus clade. Overall, the patterns of phylogeny and range distribution when related to the timing of speciation lend circumstantial support to a Neogene centre of origin hypothesis but not to speciation on the Pacific Plate. Speciation is likely to have been associated with the Tethys Sea closure event, with rapid speciation occurring after closure.     

Spatial and host related genomic variation in partially sympatric cactophagous moth species

Surveys of patterns of genetic variation in natural sympatric and allopatric populations of recently diverged species are necessary to understand the processes driving intra- and interspecific diversification. The South American moths Cactoblastis cactorum, Cactoblastis doddi and Cactoblastis bucyrus are specialized in the use of cacti as host plants. These species have partially different geographic ranges and differ in patterns of host plant use. However, there are areas that overlap, particularly, in northwestern Argentina, where they are sympatric. Using a combination of genome-wide SNPs and mitochondrial data we assessed intra and interspecific genetic variation and investigated the relative roles of geography and host plants on genetic divergence. We also searched for genetic footprints of hybridization between species. We identified three well delimited species and detected signs of hybridization in the area of sympatry. Our results supported a hypothetical scenario of allopatric speciation in the generalist C. cactorum and genetic interchange during secondary geographic contact with the pair of specialists C. bucyrus and C. doddi that probably speciated sympatrically. In both cases, adaptation to new host plants probably played an important role in speciation. The results also suggested the interplay of geography and host plant use as drivers of divergence and limiting gene flow at intra and interspecific levels.     

The relative importance of geography and ecology in species diversification: evidence from a tropical marine intertidal snail (Nerita)

Aim In this study, I examined the relative contributions of geography and ecology to species diversification within the genus Nerita, a prominent clade of marine snails that is widely distributed across the tropics and intertidal habitats. Specifically, I tested whether geographical patterns of speciation correspond primarily to allopatric or sympatric models, and whether habitat transitions have played a major role in species diversification. Location Indo‐West Pacific, eastern Pacific, Atlantic, tropical marine intertidal. Methods I used a previously reconstructed molecular phylogeny of Nerita as a framework to assess the relative importance of geographical and ecological factors in species diversification. To evaluate whether recently diverged clades exhibit patterns consistent with allopatric or sympatric speciation, I mapped the geo‐graphical distribution of each species onto the species‐level phylogeny, and examined the relationship between range overlap and time since divergence using age–range correlation analyses. To determine the relative contribution of habitat transitions to divergence, I traced shifts in intertidal substrate affinity and vertical zonation across the phylogeny using parsimony, and implemented randomization tests to evaluate the resulting patterns of ecological change. Results Within the majority of Nerita clades examined, age–range correlation analysis yielded a low intercept and a positive slope, similar to that expected under allopatric speciation. Approximately 75% of sister species pairs have maintained allopatric distributions; whereas more distantly related sister taxa often exhibited complete or nearly complete geographical overlap. In contrast, only 19% of sister species occupy distinct habitats. For both substrate and zonation, habitat transitions failed to concentrate towards either the tips or the root of the phylogeny. Instead, habitat shifts have occurred throughout the history of Nerita, with a general transition from the lower and mid‐littoral towards the upper and supra‐littoral zones, and multiple independent shifts from hard (rock) to softer substrates (mangrove, mud and sand). Main conclusions Both geography and ecology appear to have influenced diversification in Nerita, but to different extents. Geography seems to play a principal role, with allopatric speciation driving the majority of Nerita divergences. Habitat transitions appear insignificant in shaping the early and recent history of speciation, and promoting successive diversification in Nerita; however, shifts may have been important for respective divergences (i.e. those that correspond to the transitions) and enhancing diversity throughout the clade.     

Lack of congruence of genetic and niche divergence in Podarcis hispanicus complex

Niche divergence among closely related lineages can be informative on the ecological and evolutionary processes involved in differentiation, particularly in the case of cryptic species complexes. Here we compared phylogenetic relationships and niche similarity between pairs of lineages included in the Podarcis hispanicus complex to examine patterns of niche divergence and its role in the organization of current diversity patterns, as allopatric, parapatric, and sympatric lineages occur in the Western Mediterranean Basin. First, we used ecological niche models to characterize the realized climatic niche of each Podarcis hispanicus complex lineage based on topographic and climatic variables, to identify important variables, and to test for niche conservatism or divergence between pairs of lineages. Variables related to precipitation generally exhibited the highest contribution to niche models, highlighting the importance of rainfall levels in shaping distributions of Podarcis wall lizards. We found that most forms have significant differences in realized climatic niches that do not follow the pattern of mitochondrial divergence. These results lend support to the hypothesis that genetic divergence across Podarcis hispanicus complex most likely occurred in allopatric conditions, mostly with significant niche divergence. Competition after secondary contact is also suggested by the common occurrence of niche overlap between lineages that exhibit strictly parapatric distribution. The almost continuous distribution of Podarcis lizards in the study area appears to be a result of a combination of complementary suitable niches and competition, which seem two important mechanisms limiting geographic distributions and restricting the existence of extensive contact zones.     

REPRODUCTIVE CHARACTER DISPLACEMENT OF EPICUTICULAR COMPOUNDS AND THEIR CONTRIBUTION TO MATE CHOICE IN DROSOPHILA SUBQUINARIA AND DROSOPHILA RECENS

Interactions between species can alter selection on sexual displays used in mate choice within species. Here we study the epicuticular pheromones of two Drosophila species that overlap partially in geographic range and are incompletely reproductively isolated. Drosophila subquinaria shows a pattern of reproductive character displacement against Drosophila recens, and partial behavioral isolation between conspecific sympatric versus allopatric populations, whereas D. recens shows no such variation in mate choice. First, using manipulative perfuming experiments, we show that females use pheromones as signals for mate discrimination both between species and among populations of D. subquinaria. Second, we show that patterns of variation in epicuticular compounds, both across populations and between species, are consistent with those previously shown for mating probabilities: pheromone compositions differ between populations of D. subquinaria that are allopatric versus sympatric with D. recens, but are similar across populations of D. recens regardless of overlap with D. subquinaria. We also identify differences in pheromone composition among allopatric regions of D. subquinaria. In sum, our results suggest that epicuticular compounds are key signals used by females during mate recognition, and that these traits have diverged among D. subquinaria populations in response to reinforcing selection generated by the presence of D. recens.     

REINFORCEMENT AND REPRODUCTIVE CHARACTER DISPLACEMENT IN GASTROPHRYNE CAROLINENSIS AND G. OLIVACEA (ANURA: MICROHYLIDAE): A REEXAMINATION

Tape-recorded advertisement calls of Gastrophryne carolinensis and G. olivacea, obtained in Texas and southern Louisiana, were analyzed by means of an analogue audiospectrograph. Samples were grouped into four areas: allopatric and sympatric for G. carolinensis, and combined adjacent allopatric/shallow sympatric, and sympatric for G. olivacea. Three attributes of the advertisement call (call duration, pulse rate, and dominant frequency) were investigated, with water temperature at the calling site as the independent variable. Values for dominant frequency do not overlap between species, across the full range of recording temperatures, and those of sympatric G. carolinensis are displaced away from those of both groups of G. olivacea (which are very similar)—thus indicating a pattern of geographic variation consistent with reproductive character displacement. There is considerable overlap in the values for duration and for pulse rate of each species when considered alone, but there is only slight overlap of the scatters of points for the pairs of values. For both species, no consistent patterns of correlation were detected between the three attributes of the call and the snout–vent length of the emitter, thus reducing the likelihood that the divergence in calls is due to pleiotropic effects of body size.     

Incomplete behavioural isolation and asymmetric female preference in darter sister species (Percidae: Etheostoma)

In this study, female preferences and behavioural isolation were estimated in a pair of allopatric sister species, Etheostoma duryi and Etheostoma flavum. Dichotomous mate preference trials were conducted to determine whether females prefer to associate with conspecific over heterospecific males and free‐spawning assays were conducted to determine whether those preferences translated into behavioural isolation. Dichotomous mate choice trials revealed asymmetric female preference, as female E. flavum preferred conspecific males, whereas female E. duryi showed no preference. Free‐spawning assays indicated that behavioural isolation remains incomplete between E. duryi and E. flavum (I_B = 0·19). In addition to female mating preferences, male behaviour also appeared to influence mating outcomes as male E. flavum consistently courted conspecific females more often in free‐spawning assays whereas male E. duryi did not. The data therefore suggest that despite marked divergence in male nuptial colour, divergence in female preferences between these species may not be sufficient to maintain species boundaries upon secondary contact. These results contrast with similar work in a sympatric darter species pair and may be explained by considering the contributions of reinforcement and differences in colour pattern as well as colour value.     

Is the spinule pattern on the leg 4 coxopodite a tactile signal in the specific mate recognition system of Mesocyclops (Copepoda, Cyclopidae)?

Several recent observations on e.g. birds and fruit flies support the hypothesis that when previously geographically separated populations, as neospecies, come into contact, sexual isolation mechanisms become stronger as a result of direct selection against inferior hybrids (reinforcement hypothesis). Many aspects of the reproductive biology of cyclopid copepods are still very poorly known. However, all reports on mating processes in the genus Mesocyclops confirm that males attach to the fourth legs of the females by their geniculate antennulae before transferring spermatophores. Accordingly, it is reasonable to assume that the signals helping recognition of the conspecific mates are located on the leg 4 of the females. The posterior surface of the leg 4 coxopodite in Cyclopidae is ornamented with a complex and diverse spinule pattern which, despite some infraspecific variability, is a reliable species-specific trait in species of Mesocyclops. Depending upon whether the spinule ornamentation has any role in the recognition system, and whether it evolved in geographically separated populations or later when they had begun to overlap, different predictions can be made about the zoogeographical pattern of differences among these character states. In the following study, comparisons are made for 10 allopatric, four sympatric and two partially sympatric species pairs. These comparisons reveal that the degree of divergence in the spinule ornamentation is significantly greater between sympatric than between allopatric species. These results argue in favor of the spinule ornamentation of the leg 4 coxopodite being a signal in specific mate recognition system (SMRS), and that the divergence of this state is initiated when previously separated populations, as neospecies, begin secondary contact.