Convergent (or parallel) evolution provides strong evidence for a deterministic role of natural selection: similar phenotypes evolve when independent populations colonize similar environments. In reality, however, independent populations in similar environments always show some differences: some non-convergent evolution is present. It is therefore important to explicitly quantify the convergent and non-convergent aspects of trait variation, and to investigate the ecological and genetic explanations for each. We performed such an analysis for threespine stickleback (Gasterosteus aculeatus) populations inhabiting lake and stream habitats in independent watersheds. Morphological traits differed in the degree to which lake-stream divergence was convergent across watersheds. Some aspects of this variation were correlated with ecological variables related to diet, presumably reflecting the strength and specifics of divergent selection. Furthermore, a genetic scan revealed some markers that diverged between lakes and streams in many of the watersheds and some that diverged in only a few watersheds. Moreover, some of the lake-stream divergence in genetic markers was associated within some of the lake-stream divergence in morphological traits. Our results suggest that convergent evolution, and deviations from it, are primarily the result of natural selection, which corresponds in only some respect to the dichotomous habitat classifications frequently used in such studies.

STRUCTURE is the most widely used clustering software to detect population genetic structure. The last version of this software (STRUCTURE 2.1) has been enhanced recently to take into account the occurrence of linkage disequilibrium (LD) caused by admixture between populations. This last version, however, still does not consider the effects of strong background LD caused by genetic drift, and which may cause spurious results. STRUCTURE authors have, therefore, suggested a rough threshold value of the distance (1.0 cM) between two loci below which the pair of loci should not be used. Because of the sensitiveness of LD to demographic events, the distance between loci is not always a good indicator of the strength of LD. In this study, we examine the link between genomic distance and the strength of the correlation between loci (r LD ) in a free-ranging population of mouflon (Ovis aries), and we present an empirical test of effect of r LD on the clustering results provided by the linkage model in STRUCTURE. We showed that a high r LD value increases the probability of detecting spurious clustering. We propose to use r LD as an index to base a decision on whether or not to use a pair of loci in a clustering analysis. Keywords: STRUCTURE software; linkage disequilibrium; population structure; clustering Introduction STRUCTURE (Pritchard et al., 2000) is the most widely used clustering software applied to detect population genetic structure, with more than 1000 citations for its first version (Pritchard et al., 2000) and more than 170 citations for its recent enhanced version (Falush et al., 2003) (source: ISI Web of Science database). STRUCTURE generates clusters based on both transient HardyWeinberg disequilibrium (HWD) and linkage disequilibrium (LD) caused by admixture between populations. The program works by clustering individuals in groups, where both linkage and HWD are minimized, and therefore, the presence of LD in the data improves clustering results (Falush et al., 2003). On the other hand, 'strong' LD or departure from Hardy-Weinberg equilibrium could lead to an overestimation of the number of clusters detected (Falush et al., 2003).STRUCTURE deals with two kinds of LD: the first is mixture LD, which occurs across loci even if they are unlinked due to the correlation of allele frequencies 'because individuals with a large component of ancestry in population k have an excess of alleles that are common in k' (Falush et al., 2003). The second is admixture LD, which is 'the correlation that arises between linked markers in recently admixed populations ' (Pritchard and Wen, 2004). This LD occurs because markers are on the same 'chunk' of chromosome that derives from an ancestral population. The 'admixture model' implemented in the latest version of STRUCTURE (STRUCTURE 2.1;Falush et al., 2003) combines admixture LD with map distances between markers to improve clustering results. Falush et al. (2003) defined a third kind of LD: the background LD measured between syntenous loci separated by few cM. Background...

Parallel (and convergent) phenotypic variation is most often studied in the wild, where it is difficult to disentangle genetic vs. environmentally induced effects. As a result, the potential contributions of phenotypic plasticity to parallelism (and nonparallelism) are rarely evaluated in a formal sense. Phenotypic parallelism could be enhanced by plasticity that causes stronger parallelism across populations in the wild than would be expected from genetic differences alone. Phenotypic parallelism could be dampened if site-specific plasticity induced differences between otherwise genetically parallel populations. We used a common-garden study of three independent lake-stream stickleback population pairs to evaluate the extent to which adaptive divergence has a genetic or plastic basis, and to investigate the enhancing vs. dampening effects of plasticity on phenotypic parallelism. We found that lake-stream differences in most traits had a genetic basis, but that several traits also showed contributions from plasticity. Moreover, plasticity was much more prevalent in one watershed than in the other two. In most cases, plasticity enhanced phenotypic parallelism, whereas in a few cases, plasticity had a dampening effect. Genetic and plastic contributions to divergence seem to play a complimentary, likely adaptive, role in phenotypic parallelism of lake-stream stickleback. These findings highlight the value of formally comparing wild-caught and laboratory-reared individuals in the study of phenotypic parallelism.

In population and conservation genetics, there is an overwhelming body of evidence that genetic diversity is lost over time in small populations. This idea has been supported by comparative studies showing that small populations have lower diversity than large populations. However, longitudinal studies reporting a decline in genetic diversity throughout the whole history of a given wild population are much less common. Here, we analysed changes in heterozygosity over time in an insular mouflon (Ovis aries) population founded by two individuals in 1957 and located on one of the most isolated locations in the world: the Kerguelen Sub-Antarctic archipelago. Heterozygosity measured using 25 microsatellite markers has actually increased over 46 years since the introduction, and exceeds the range predicted by neutral genetic models and stochastic simulations. Given the complete isolation of the population and the short period of time since the introduction, changes in genetic variation cannot be attributed to mutation or migration. Several lines of evidence suggest that the increase in heterozygosity with time may be attributable to selection. This study shows the importance of longitudinal genetic surveys for understanding the mechanisms that regulate genetic diversity in wild populations.