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Habitat choice is defined as a nonrandom distribution of genotypes in different microhabitats. Therefore, it could exert a great impact on the genetic variance of natural populations by promoting genetic divergence, local adaptation, and may even lead to sympatric speciation. Despite this potential role in micro- and macro-evolutionary processes, there is little empirical evidence that the various genotypes within a population may differ in habitat choice-related behaviors. Here, we tested whether habitat choice may have contributed to genetic divergence within a local population of the Mediterranean killifish Aphanius fasciatus, which emerged between groups inhabiting microhabitats with different oxygen concentrations during previous field studies. In a first experiment, we studied the distribution of individuals in conditions of hypoxia and normoxia to test whether they had a different ability to shy away from a hypoxic environment; in a second experiment, we analyzed the individual behavior of fish separately in the two conditions, to verify whether they showed peculiar behavioral responses linked to a possible differential distribution. We then analyzed the six allozyme loci, whose allelic and genotypic frequencies were significantly divergent in the previous studies. In the first test, we found that the distribution of the two homozygote genotypes of the glucose-6-phosphate isomerase-1 locus ( GPI-1) was significantly different between the hypoxic and the normoxic conditions. During the second test, all individuals were more active in hypoxic conditions, but the two GPI-1 homozygotes showed a significant difference in time spent performing surface breathing, which was consistent with their distribution observed in the first experiment. These results provide evidence that individual behavioral traits, related to genetic features, may lead to a nonrandom distribution of genotypes in heterogeneous although contiguous microhabitats and, consequently, that habitat choice can play a significant role in driving the micro-evolutionary dynamics of this species. [ABSTRACT FROM AUTHOR]

Environmental stress can promote evolutionary forces and genetic divergence, but at which microgeographic scale divergence may arise, even in the presence of gene flow, remains poorly known. We studied the effects of eutrophication in a saltwork over a period of 6 years on the gene pool of a local population of the Mediterranean killifishAphanius fasciatus, a species resistant to extreme conditions. We hypothesised that during eutrophication, the environmental stress may have acted differently along a gradient of salinity and oxygen concentration in promoting evolutionary forces, generating divergence over a small spatial scale (2 km), despite gene flow. We analysed 24 allozymes in three temporal samples each composed of four spatial sub-samples, collected along the gradient, during eutrophication (2003 and 2005) and after a recovery project (2008). The results suggest that eutrophication promoted natural selection, originating a genetic cline on one locus (adenosine deaminase) significantly linked to salinity and oxygen concentration. Together with selection, both genetic drift and gene flow contributed to shaping the genetic structure under stress by further promoting the genetic heterogeneity and giving rise to deficits of heterozygotes as a secondary effect of the divergence. Environmental stress thus increased diversification, with the effects of selection and drift prevailing on gene flow. When environmental stress was relaxed (2008), allele and genotype frequencies became homogeneous, likely because under less extreme conditions the gene flow prevailed. These results improve our understanding of microgeographic divergence, and highlight the role of environmental stress in moulding microevolutionary dynamics and genetic patterns of animal populations. [ABSTRACT FROM AUTHOR]

How the often highly endemic biodiversity of islands originated has been debated for decades, and it remains a fervid research ground. Here, using mitochondrial and nuclear gene sequence analyses, we investigate the diversity, phylogenetic relationships, and evolutionary history of the mayfly Baetis gr. rhodani on the three largest northwestern Mediterranean islands (Sardinia, Corsica, Elba). We identify three distinct, largely co-distributed, and deeply differentiated lineages, with divergences tentatively dated back to the Eocene-Oligocene transition. Bayesian population structure analyses reveal a lack of gene exchange between them, even at sites where they are syntopic, indicating that these lineages belong to three putative species. Their phylogenetic relationships with continental relatives, together with the dating estimates, support a role for three processes contributing to this diversity: (1) vicariance, primed by microplate disjunction and oceanic transgression; (2) dispersal from the continent; and (3) speciation within the island group. Thus, our results do not point toward a prevailing role for any of the previously invoked processes. Rather, they suggest that a variety of processes equally contributed to shape the diverse and endemic biota of this group of islands. [ABSTRACT FROM AUTHOR]

Mosquitoes belonging to the Aedes mariae complex, including Aedes mariae (Sergent & Sergent), Aedes zammitii (Theobald), and Aedes phoeniciae (Coluzzi & Sabatini), are among the few animals inhabiting the extreme habitat of sea rock pools. Morphological analysis of these species and crossing experiments conducted in laboratory and natural conditions inferred the occurrence of three taxa with a parapatric distribution along the coasts of the Mediterranean Basin. To date, however, genetic characterization of the three species within the Ae. mariae complex and an assessment of their interspecific differentiation are lacking. In this study, we used both nuclear (i.e., 14 allozymic loci) and mitochondrial genetic markers (i.e., sequences of the cytochrome oxidase I and II genes) to genetically characterize the three species belonging to the complex. Then, we used mitochondrial markers to infer phylogenetic relationships between the species and estimate the time to most recent common ancestor (TMRCA). The allozymic and mitochondrial markers showed the occurrence of three distinct gene pools, namely, Ae. mariae, Ae. zammitii, and Ae. phoeniciae. The TMRCAs for the entire in-group were estimated to have occurred during the early Pleistocene (i.e., mean node age of 1.739 million years ago). An important role of Pleistocene climatic changes could be suggested in the origin of the species of the Ae. mariae complex and in shaping their pattern of intraspecific genetic diversity. [ABSTRACT FROM AUTHOR]