Ephemeral metapopulations show high genetic diversity at regional scales.

One of the primary questions concerning the long-term preservation of nature and its diversity is the maintenance of genetic diversity. However, despite numerous theoretical investigations, comparative empirical information on how local extinctions influence regional genetic variation does not exist. To our knowledge, this is the first report of an empirical study comparing the genetic variation of permanent vs. ephemeral species at two scales (local variation, regional variation). This approach, utilizing a microsatellite analysis of six midge species of the genus Chaoborus generated intriguing scale-dependent results. Species that experienced repeated local extinctions had reduced genetic variation at the local level, yet the regional genetic variation was greater than in species with permanent populations. Our findings call into question the assumption that species with repeated local extinctions generally contain lower genetic diversity, especially if they experience a "nomadic" pattern of dispersal. We encourage comparative analyses of empirical genetic data at dual scales as molecular tools become more available in ecological studies.

Population structure in Daphnia obtusa: quantitative genetic and allozymic variation.

Quantitative genetic analyses for body size and for life history characters within and among populations of Daphnia obtusa reveal substantial genetic variance at both hierarchical levels for all traits measured. Simultaneous allozymic analysis on the same population samples indicate a moderate degree of differentiation: GST = 0.28. No associations between electrophoretic genotype and phenotypic characters were found, providing support for the null hypothesis that the allozymic variants are effectively neutral. Therefore, GST can be used as the null hypothesis that neutral phenotypic evolution within populations led to the observed differentiation for the quantitative traits, which I call QST. The results of this study provide evidence that natural selection has promoted diversification for body size among populations, and has impeded diversification for relative fitness. Analyses of population differentiation for clutch size, age at reproduction, and growth rate indicate that neutral phenotypic evolution cannot be excluded as the cause.

Patterns of genetic architecture for life-history traits and molecular markers in a subdivided species.

Understanding the utility and limitations of molecular markers for predicting the evolutionary potential of natural populations is important for both evolutionary and conservation genetics. To address this issue, the distribution of genetic variation for quantitative traits and molecular markers is estimated within and among 14 permanent lake populations of Daphnia pulicaria representing two regional groups from Oregon. Estimates of population subdivision for molecular and quantitative traits are concordant, with QST generally exceeding GST. There is no evidence that microsatellites loci are less informative about subdivision for quantitative traits than are allozyme loci. Character-specific comparison of QST and GST support divergent selection pressures among populations for the majority of life-history traits in both coast and mountain regions. The level of within-population variation for molecular markers is uninformative as to the genetic variation maintained for quantitative traits. In D. pulicaria, regional differences in the frequency of sex may contribute to variation in the maintenance of expressed within-population quantitative-genetic variation without substantially impacting diversity at the genic level. These data are compared to an identical dataset for 17 populations of the temporary-pond species, D. pulex.

Multi-locus genetic evidence for rapid ecologically based speciation in Daphnia.

The process of speciation involves the divergence of two or more subpopulations of a parent species into independent evolutionary trajectories. To study this process in natural populations requires a detailed knowledge of the genetic and ecological characteristics of the parent species and an understanding of how its populations can lose evolutionary cohesion. The cosmopolitan and speciose genus Daphnia provides many of these features by existing in multiple freshwater habitat types, particularly permanent lakes and temporary ponds, each of which presents distinct ecological challenges. We assayed the genetic composition of 20 temporary pond populations of members of the Daphnia pulex species complex in north-western Oregon and compared them to published data on related lake and pond populations. We collected molecular genetic data from 13 allozyme loci, from six microsatellite loci, and from the control region of the mitochondrial DNA. By assaying over 400 individual Daphnia for these data, we were able to compile composite genotypes not only of individual Daphnia but of each pond population as a whole. In these ponds, we discovered two distinct genotypic constellations, one which bears resemblance to the lake-dwelling taxon D. pulicaria, and one which bears resemblance to the pond-dwelling taxon, D. pulex. Using published genetic data from these and other species as a frame of reference, we characterized 13 of these ponds as being 'pond-like', three as being 'lake-like', and four as being 'mixed'. Unlike studies performed elsewhere, however, these ponds do not exhibit high probabilities of interspecific hybridization. Over 95% of all individuals have either a lake-like or a pond-like genotype at all three genetic systems, suggesting the two forms do not represent hybridized vs. nonhybridized genotypes. Because both types can be found in the same ponds at the same time in gametic disequilibrium, we also discount the possibility that they are two extremes of a single species that is highly genetically subdivided. With these genetic data, and with supporting life-history and ecological data previously gathered on these pond populations, we conclude that the most likely description of this system is of a taxon caught in the act of speciating, with new pond-adapted populations periodically stemming from lake-adapted sources during river flooding events.

Hierarchical analysis of population genetic variation in mitochondrial and nuclear genes of Daphnia pulex.

The geographic structure of Daphnia pulex populations from the central United States is analyzed with respect to isozyme and mitochondrial DNA variation. The species complex consists of cyclic and obligate parthenogens. A hierarchical analysis of population structure in the cyclic parthenogens by using a fixation-index approach indicates that this is one of the most extremely subdivided species yet studied. This genetic structure, much of which accrues within 100 km, is certainly due in part to the limited dispersal ability of Daphnia. However, previous work has shown that fluctuating selection can account for the spatial heterogeneity in isozyme frequencies in these populations. This may explain why the population subdivision for the mitochondrial genome increases approximately three times as rapidly with distance as does that for nuclear genes, which is slower than the neutral expectation. The obligate parthenogens are shown to be polyphyletic in origin, evolutionarily young, and, in some cases, geographically widespread.