Patterns of contact call differentiation in the panmictic East African Abyssinian White‐eye Zosterops abyssinicus (Aves: Passeriformes)

Species distribution patterns range from highly disjunct to continuous, depending on their ecological demands and the availability of respective habitats. East African savannahs are mostly interconnected and ecologically comparatively homogenous and thus provide a prerequisite for a rather panmictic distribution pattern for species occurring in this habitat. The Abyssinian white‐eye Zosterops abyssinicus is a savannah inhabiting bird species, representing such a continuous distribution. This species occurs in high abundances and is very mobile, and past population genetic studies have suggested that gene flow is high and genetic differentiation is low even across relatively large geographic distances. Further, only little morphological differences were found. In order to test for potential divergence in acoustic traits despite its interconnected geographic distribution, we analyzed 2795 contact calls of Z. abyssinicus, which were recorded at 19 sites across Kenya. Our data indicate weak, but significant differentiation in call characteristics across latitudinal gradients. We found strong changes in call characteristics in populations where Z. abyssinicus occurs in sympatry with its highland congener, Zosterops poliogaster. However, the changes in call characteristics in sympatry were in different directions and lead to strong differentiation of the sympatric populations to other conspecific populations potentially representing a case of cascade reinforcement. The detected spatial gradients likely result from ecological differences and balancing effects of natural and sexual selection.     

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.     

Further acoustic diversity in Hawaiian forests: two new species of Hawaiian cricket (Orthoptera: Gryllidae: Trigonidiinae: Laupala)

The genus Laupala is endemic to the rain forested islands of the Hawaiian archipelago. In his 1994 study Otte described 35 Laupala species, each restricted to a single island, and sometimes a single volcano. All Laupala species produce male calling songs consisting of slow trills. Two new species described here, one from the Koolau Mountains of Oahu and another from Kipahulu Valley of East Maui, produce the fastest and one of the slowest pulse rates, respectively, in the genus. Both species are found with sympatric congeners. The discovery of these species increases the acoustic diversity locally (within their respective communities), within their respective islands, and across the archipelago. This enhances the opportunity for the investigation of signal interference and character displacement in a relatively youthful assemblage of closely related species of crickets.     

Divergence in the calling songs between sympatric and allopatric populations of the southern wood cricket Gryllus fultoni (Orthoptera: Gryllidae)

In the eastern United States the wood cricket Gryllus fultoni (Orthoptera: Gryllidae) occurs in sympatry with G. vernalis in an area between eastern Kansas and west of the Appalachian Mountains. Calling songs were recorded from 13 sympatric and allopatric localities. Both field and laboratory recordings showed that chirp rate (CR) and pulse rate (PR) overlapped extensively between allopatric populations of G. fultoni and sympatric populations of G. vernalis; by contrast, there was little or no overlap in these variables between sympatric populations of these two species. Divergence in PR and CR between the two species was thus greater in areas of sympatry than in areas of allopatry. Our field and laboratory studies of G. fultoni calling songs thus demonstrate the pattern expected of character displacement and support the genetic assumptions of this hypothesis. Other possible explanations for the sympatric divergence such as ecological character displacement and clinal variation are discussed.     

Identification of cryptic species of Miniopterus bats (Chiroptera: Miniopteridae) from Madagascar and the Comoros using bioacoustics overlaid on molecular genetic and morphological characters

The number of Miniopterus bat species on Madagascar and the nearby Comoros islands (Malagasy region) has risen from four to 11. These recently described cryptic taxa have been differentiated primarily based on molecular markers and associated a posteriori morphological characters that corroborate the different clades. Members of this Old World genus are notably conservative in morphology across their range. Several sites on Madagascar hold up to four small‐bodied taxa of this genus that are morphologically similar to one another, although they can be distinguished based on the tragus, an ear structure associated with echolocation. Miniopterus often emit species‐specific calls. In the present study, we analyze the bioacoustics of the 11 species of Miniopterus currently recognized from the Malagasy region, with an initial identification of the 87 recorded and collected individuals based on molecular markers and certain morphological characters. In most cases, bioacoustic parameters differentiate species and have taxonomic utility. Miniopterus griveaudi populations, which occur on three islands (Madagascar, Anjouan, and Grande Comore), showed no significant differences in peak echolocation frequencies. After running a discriminant function analysis based on five bioacoustic parameters, some mismatched assignments of Malagasy species were found, which include allopatric sister‐taxa and sympatric, phylogenetically not closely‐related species of similar body size. Because the peak echolocation frequencies of two species (Miniopterus sororculus and Miniopterus aelleni) were independent of body size, they were acoustically distinguishable from cryptic sympatric congeners. The small variation around the allometric relationship between body size and peak echolocation frequency of Malagasy Miniopterus species suggests that intraspecific communication rather than competition or prey detection may be the driver for the acoustic divergence of these two species. Our well‐defined echolocation data allow detailed ecological work to commence aiming to test predictions about the relative roles of competition, prey availability, and social communication on the evolution of echolocation in Malagasy Miniopterus species. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 284–302.     

Evidence for nonallopatric speciation among closely related sympatric Heliotropium species in the Atacama Desert

The genetic structure of populations of closely related, sympatric species may hold the signature of the geographical mode of the speciation process. In fully allopatric speciation, it is expected that genetic differentiation between species is homogeneously distributed across the genome. In nonallopatric speciation, the genomes may remain undifferentiated to a large extent. In this article, we analyzed the genetic structure of five sympatric species from the plant genus Heliotropium in the Atacama Desert. We used amplified fragment length polymorphisms (AFLPs) to characterize the genetic structure of these species and evaluate their genetic differentiation as well as the number of loci subject to positive selection using divergence outlier analysis (DOA). The five species form distinguishable groups in the genetic space, with zones of overlap, indicating that they are possibly not completely isolated. Among‐species differentiation accounts for 35% of the total genetic differentiation (F_ST = 0.35), and F_ST between species pairs is positively correlated with phylogenetic distance. DOA suggests that few loci are subject to positive selection, which is in line with a scenario of nonallopatric speciation. These results support the idea that sympatric species of Heliotropium sect. Cochranea are under an ongoing speciation process, characterized by a fluctuation of population ranges in response to pulses of arid and humid periods during Quaternary times.     

Similar patterns of local barn owl adaptation in the Middle East and Europe with respect to melanic coloration

The maintenance of phenotypic variation is a central question in evolutionary biology. A commonly suggested mechanism is that of local adaptation, whereby different phenotypes are adapted to alternative environmental conditions. A recent study in the European barn owl (Tyto alba) has shown that natural selection maintains a strong clinal variation in reddish pheomelanin‐based coloration. Studies in the region where phenotypic variation in this owl is the highest in Europe have further demonstrated that dark‐reddish and pale‐reddish owls exploit open and wooded habitats, predate voles and wood mice, and are long‐tailed and short‐tailed, respectively. However, it remains unclear as to whether these traits evolved as a consequence of allopatric evolution of dark colour in northern Europe and white colour in southern Europe, during which owls could have also evolved different morphologies and foraging behaviour. This scenario implies that covariation between coloration and foraging behaviour could be a specificity of the European continent, which is not found in other worldwide‐distributed populations. To investigate this issue, we studied a barn owl population in the Middle East. The results obtained show that, as in Central Europe, dark‐reddish female owls breed more often in the open landscape than their pale‐reddish female conspecifics, their offspring are fed with more voles than Muridae, and they are longer‐winged and longer‐tailed. These findings indicate that, in the barn owl, the association in females between pheomelanin‐based coloration and foraging behaviour and morphology is not restricted to the European continent but may well evolve in sympatry in many barn owl populations worldwide. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 447–454.     

Geographic variation in jungle cat (Felis chaus Schreber, 1777) (Mammalia,Carnivora, Felidae) body size: is competition responsible?

There is a striking difference in body size of jungle cats ( Felis chaus ) in the west and the east of their distribution, with Israeli cats being 43% heavier than Indian cats. We tested the hypothesis that increasing competition from other small felids towards the east is responsible for the difference in body size. We measured jungle cat skulls for eight cranial and dental variables and related these to independent variables such as species richness (local and regional), latitude, longitude, temperature, and precipitation. Data from a narrow band between latitudes 24.0°N and 33.9°N, where Bergmann's rule was largely not observed, showed that the western population (≤ 50.0°E longitude) of jungle cats is larger than the eastern (> 60.0°E longitude) population with the size difference being most evident in the upper carnassials (P⁴L). Species richness at the regional level showed a significant negative relation to P⁴L. An even spacing in condylobasal length for a small-cat guild from India through null model analysis indicated the occurrence of character displacement. The results support the hypothesis that competition is responsible for geographical variation in jungle cat body size in the region where Bergmann's rule does not apply. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 163–172.     

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.     

Host–pathogen coevolution,secondary sympatry and species diversification

The build-up of species locally within a region by allopatric speciation depends on geographically separated (allopatric) sister populations becoming reproductively incompatible followed by secondary sympatry. Among birds, this has happened frequently in remote archipelagos, spectacular cases including the Darwin''s finches (Geospizinae) and Hawaiian honeycreepers (Drepanidinae), but similar examples are lacking in archipelagos nearer to continental landmasses. Of the required steps in the speciation cycle, achievement of secondary sympatry appears to be limiting in near archipelagos and, by extension, in continental regions. Here, I suggest that secondary sympatry might be prevented by apparent competition mediated through pathogens that are locally coevolved with one population of host and are pathogenic in sister populations. The absence of numerous pathogens in remote archipelagos might, therefore, allow sister populations to achieve secondary sympatry more readily and thereby accelerate diversification. By similar reasoning, species should accumulate relatively slowly within continental regions. In this essay, I explore the assumptions and some implications of this model for species diversification.     

Interspecific competition between stream-dwelling brown trout and Alpine bullhead

Density and composition of benthic invertebrates and the diet of brown trout Salmo trutta and Alpine bullhead Cottus poecilopus were studied at two sites in one Norwegian stream. The sites were separated by an impassable waterfall, and brown trout density was five to 10 times higher at the upper, allopatric site than downstream where it lived in sympatry with the Alpine bullhead. Benthic invertebrate communities did not differ between sites; however, the size distribution of chironomids and trichopterans were skewed towards lighter individuals at the sympatric site. Diet composition suggested that sympatric brown trout foraged more on invertebrate drift and from the surface than allopatric brown trout. Alpine bullhead diet did not differ significantly from brown trout diet, except that the Alpine bullhead fed on heavier individual prey within a few taxa and did not consume chironomid pupae or surface insects. The collected data support the hypothesis that brown trout living in sympatry with Alpine bullhead feed at locations with higher predation risk, which is a probable explanation for their lower population density.     

Host-related genetic differentiation in the anther smut fungus Microbotryum violaceum in sympatric, parapatric and allopatric populations of two host species Silene latifolia and S. dioica

We investigated genetic diversity in West European populations of the fungal pathogen Microbotryum violaceum in sympatric, parapatric and allopatric populations of the host species Silene latifolia and S. dioica, using four polymorphic microsatellite loci. In allopatric host populations, the fungus was highly differentiated by host species, exhibiting high values of F(ST) and R(ST), and revealed clear and distinct host races. In sympatric and parapatric populations we found significant population differentiation as well, except for one sympatric population in which the two host species grew truly intermingled. The mean number of alleles per locus for isolates from each of the host species was significantly higher in sympatric/parapatric than in allopatric populations. This suggests that either gene flow between host races in sympatry, or in case of less neutral loci, selection in a more heterogeneous host environment can increase the level of genetic variation in each of the demes. The observed pattern of host-related genetic differentiation among these geographically spread populations suggest a long-term divergence between these host races. In sympatric host populations, both host races presumably come in secondary contact, and host-specific alleles are exchanged depending on the amount of fungal gene flow.     

Interspecific aggression causes negative selection on sexual characters

Interspecific aggression originating from mistaken species recognition may cause selection on secondary sexual characters, but this hypothesis has remained untested. Here we report a field experiment designed to test directly whether interspecific aggression causes selection on secondary sexual characters, wing spots, in wild damselfly populations. Males of Calopteryx virgo are more aggressive toward males of C. splendens with large than with small wing spots. This differential interspecific aggression may cause negative selection on wing spot size. Indeed, our results show that directional survival selection on wing spot size of C. splendens males was changed by experimental removal of C. virgo males. Without removal, directional selection went from positive to negative with increasing relative abundance of C. virgo males. In populations where C. virgo males were removed, this relationship disappeared. These results verify that interspecific aggression can cause negative selection on sexual characters. Thus, interspecific aggression has the potential to cause divergence on these characters between two species offering an alternative explanation for reinforcement for generating character displacement in secondary sexual characters.     

Evidence that bears are induced ovulators

The objective of this study was to determine if black bears are induced ovulators. We conducted a single experiment with two replicates; each replicate was divided into two arms: females exposed to male bears and females without male exposure. We used laparoscopy to examine ovaries for corpora lutea and measured serum progesterone concentrations. Six of the seven isolated females failed to ovulate, while seven of the eight females exposed to males produced one to four corpora lutea. Furthermore, isolated females had significantly lower progesterone concentrations than females exposed to males. Thus, our data suggest that the American black bear is an induced ovulator. These results may aid biologists in their efforts to reproduce ursids in controlled environments.     

Cryptic lineage diversity,body size divergence,and sympatry in a species complex of Australian lizards (Gehyra)

Understanding the joint evolutionary and ecological underpinnings of sympatry among close relatives remains a key challenge in biology. This problem can be addressed through joint phylogenomic and phenotypic analysis of complexes of closely related lineages within, and across, species and hence representing the speciation continuum. For a complex of tropical geckos from northern Australia—Gehyra nana and close relatives—we combine mtDNA phylogeography, exon‐capture sequencing, and morphological data to resolve independently evolving lineages and infer their divergence history and patterns of morphological evolution. Gehyra nana is found to include nine divergent lineages and is paraphyletic with four other species from the Kimberley region of north‐west Australia. Across these 13 taxa, 12 of which are restricted to rocky habitats, several lineages overlap geographically, including on the diverse Kimberley islands. Morphological evolution is dominated by body size shifts, and both body size and shape have evolved gradually across the group. However, larger body size shifts are observed among overlapping taxa than among closely related parapatric lineages of G. nana, and sympatric lineages are more divergent than expected at random. Whether elevated body size differences among sympatric lineages are due to ecological sorting or character displacement remains to be determined.     

Phylogeographic patterns of host-race evolution in Tephritis conura (Diptera: Tephritidae)

Host-race evolution is a prime candidate for sympatric speciation because host shifts must take place in the presence of both hosts. However, the geographic context in which the shift takes place may have strong allopatric or peripatric components if the primary host within a localized area is scarce or even goes extinct. Inference of the relative importance of the geographic mode of speciation may be gained from phylogeographic imprints. Here, we investigate the phylogeography of host races of the tephritid fly Tephritis conura from sympatric, parapatric and allopatric populations of Cirsium heterophyllum and Cirsium oleraceum (Asteraceae) in Europe, for addressing the age and direction, and the geographic context of host-race formation. Haplotype networks of the host races differed significantly in molecular diversity and topology. However, host-race haplotypes were nested within the same network, with a central haplotype H1 being the most common haplotype in both host races. C. heterophyllum flies were genetically more diverse and substructured than flies from C. oleraceum, suggesting that the latter is the derived race. The phylogeographic imprint indicates either that C. heterophyllum flies colonized C. oleraceum during range expansion or that heterophyllum flies went extinct in an area where oleraceum flies persisted (followed by re-immigration). These imprints are in concordance with peripatric diversification, probably in the European Alps and related to the last ice age, where the host-race diversification was largely completed before the postglacial range expansion on C. oleraceum to current areas of sympatry and parapatry with C. heterophyllum.     

Adaptive radiation through phenological shift: the importance of the temporal niche in species diversification

Abstract 1. Phenological shift in oviposition in seed predators may be a key factor for adaptive radiation if temporal differences lead to less intense competition. 2. This hypothesis was tested at two sites in the French Alps in three sympatric species of larch cone flies grouped into two phenological groups (early and late) differing in adult emergence and oviposition timing by approximately 2 weeks. The present study assessed the intensity of competition within and between groups by measuring four larval traits. Cone traits were measured, and the impact of early species parasitism on cone development was assessed. 3. The occupation of the central axis of a developing cone by one early larva has a strong detrimental effect on cone growth and seed production. However, there was almost no correlation between the variables measured on the cones and on the larvae, suggesting that the resources available were not limiting. 4. Inter‐group competition had no significant effect on early larvae. In contrast, both inter‐ and intra‐group competition had a significant negative effect on late larvae length (–11% and –16% respectively), dry mass (–8% and –23%), and lipid mass (–15% and –26%). The intensity of competition was stronger among larvae in the same phenological group, which is consistent with the hypothesis that shifts in oviposition promote adaptive radiation in larch cone flies by reducing competition among larvae.     

What,if anything,is sympatric speciation?

Sympatric speciation has always fascinated evolutionary biologists, and for good reason; it pits diversifying selection directly against the tendency of sexual reproduction to homogenize populations. However, different investigators have used different definitions of sympatric speciation and different criteria for diagnosing cases of sympatric speciation. Here, we explore some of the definitions that have been used in empirical and theoretical studies. Definitions based on biogeography do not always produce the same conclusions as definitions based on population genetics. The most precise definitions make sympatric speciation an infinitesimal end point of a continuum. Because it is virtually impossible to demonstrate the occurrence of such a theoretical extreme, we argue that testing whether a case fits a particular definition is less informative than evaluating the biological processes affecting divergence. We do not deny the importance of geographical context for understanding divergence. Rather, we believe this context can be better understood by modelling and measuring quantities, such as gene flow and selection, rather than assigning cases to discrete categories like sympatric and allopatric speciation.