Local adaptation despite high gene flow in the waterfall-climbing Hawaiian goby, Sicyopterus stimpsoni.

Environmental heterogeneity can promote the emergence of locally adapted phenotypes among subpopulations of a species, whereas gene flow can result in phenotypic and genotypic homogenization. For organisms like amphidromous fishes that change habitats during their life history, the balance between selection and migration can shift through ontogeny, making the likelihood of local adaptation difficult to predict. In Hawaiian waterfall-climbing gobies, it has been hypothesized that larval mixing during oceanic dispersal counters local adaptation to contrasting topographic features of streams, like slope gradient, that can select for predator avoidance or climbing ability in juvenile recruits. To test this hypothesis, we used morphological traits and neutral genetic markers to compare phenotypic and genotypic distributions in recruiting juveniles and adult subpopulations of the waterfall-climbing amphidromous goby, Sicyopterus stimpsoni, from the islands of Hawai'i and Kaua'i. We found that body shape is significantly different between adult subpopulations from streams with contrasting slopes and that trait divergence in recruiting juveniles tracked stream topography more so than morphological measures of adult subpopulation differentiation. Although no evidence of population genetic differentiation was observed among adult subpopulations, we observed low but significant levels of spatially and temporally variable genetic differentiation among juvenile cohorts, which correlated with morphological divergence. Such a pattern of genetic differentiation is consistent with chaotic genetic patchiness arising from variable sources of recruits to different streams. Thus, at least in S. stimpsoni, the combination of variation in settlement cohorts in space and time coupled with strong postsettlement selection on juveniles as they migrate upstream to adult habitats provides the opportunity for morphological adaptation to local stream environments despite high gene flow.

Genetics of species differences in sailfin and shortfin mollies.

Premating reproductive isolation is a strong barrier to hybridization in natural populations, but little is known about the genetic mechanisms that allow changes in mating signals to develop and whether different components of a mating signal can evolve in concert when sexual selection favors phenotypic associations between them. In this study, we report results suggesting that changes in a behavioural trait (courtship display) and multiple phenotypically associated morphological traits (dorsal fin characters and length of the gonopodium) have contributed to divergence in mating signals used by sailfin mollies. Through the use of reciprocal F1 and backcross hybrids, we show that morphological traits important in separating sailfin from shortfin molly species have a genetic basis and are inherited in an autosomal, additive manner. We also report significant associations between the size of certain morphological traits (length of the dorsal fin and length of the gonopodium) and the tendency of males to perform courtship displays or gonopodial thrusts. In particular, higher courtship display rates were associated with increased dorsal fin length but decreased gonopodium length, characteristics most similar to sailfin species. Such phenotypic associations between different components of a mating signal suggest that selective forces can act in concert on multiple aspects of the signal, hence, promoting divergence and speciation in sailfin mollies.

Geographical variation of genetic and phenotypic traits in the Mexican sailfin mollies, Poecilia velifera and P. petenensis.

Comparing the patterns of population divergence using both neutral genetic and phenotypic traits provides an opportunity to examine the relative importance of evolutionary mechanisms in shaping population differences. We used microsatellite markers to examine population genetic structure in the Mexican sailfin mollies Poecilia velifera and P. petenensis. We compared patterns of genetic structure and divergence to that in two types of phenotypic traits: morphological characters and mating behaviours. Populations within each species were genetically distinct, and conformed to a model of isolation by distance, with populations within different geographical regions being more genetically similar to one another than were populations from different regions. Bayesian clustering and barrier analyses provided additional support for population separation, especially between geographical regions. In contrast, none of the phenotypic traits showed any type of geographical pattern, and population divergence in these traits was uncorrelated with that found in neutral markers. There was also a weaker pattern of regional differences among geographical regions compared to neutral genetic divergence. These results suggest that while divergence in neutral traits is likely a product of population history and genetic drift, phenotypic divergence is governed by different mechanisms, such as natural and sexual selection, and arises at spatial scales independent from those of neutral markers.

Molecular phylogeny of the live-bearing fish genus Poecilia (Cyprinodontiformes: Poeciliidae).

Members of the genus Poecilia exhibit extensive morphological, behavioral, and life history variation within and between species. This natural variation, coupled with short generation times and the ease with which members of this genus can be cultured in the lab, have made several species model systems for studying the effects of sexual and natural selection on the evolution of natural populations. Given that there is no clear understanding of the phylogenetic relationships within the genus, these studies have not been put into a historical context, and between-species comparisons have been limited. We sequenced the complete NADH Dehydrogenase Subunit 2 (ND2) mitochondrial gene (1047 bp) in representatives of the major divisions of the genus in order to examine these relationships. The subgeneric groups of Rosen and Bailey (1963) are, for the most part, supported, with some adjustment within the subgenera Poecilia and Pamphorichthys. The morphological distinctness of the groups within Poecilia suggest that the original generic designations be reinstated, but this awaits a more thorough analysis. Two implications from the phylogeny are particularly relevant to sexual selection studies: within the North and Central American mollies, the three species of sailfin mollies form a monophyletic group, and within the subgenus Lebistes, the sister taxon to the guppy, P. reticulata, is most likely the group of species previously designated as Micropoecilia.