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Most variation in behavior has a genetic basis, but the processes determining the level of diversity at behavioral loci are largely unknown for natural populations. Expression of arginine vasopressin receptor 1a ( Avpr1a ) and oxytocin receptor ( Oxtr ) in specific regions of the brain regulates diverse social and reproductive behaviors in mammals, including humans. That these genes have important fitness consequences and that natural populations contain extensive diversity at these loci implies the action of balancing selection. In Myodes glareolus , Avpr1a and Oxtr each contain a polymorphic microsatellite locus located in their 5' regulatory region (the regulatory region-associated microsatellite, RRAM) that likely regulates gene expression. To test the hypothesis that balancing selection maintains diversity at behavioral loci, we released artificially bred females and males with different RRAM allele lengths into field enclosures that differed in population density. The length of Avpr1a and Oxtr RRAMs was associated with reproductive success, but population density and the sex interacted to determine the optimal genotype. In general, longer Avpr1a RRAMs were more beneficial for males, and shorter RRAMs were more beneficial for females; the opposite was true for Oxtr RRAMs. Moreover, Avpr1a RRAM allele length is correlated with the reproductive success of the sexes during different phases of reproduction; for males, RRAM length correlated with the numbers of newborn offspring, but for females selection was evident on the number of weaned offspring. This report of density-dependence and sexual antagonism acting on loci within the arginine vasopressin-oxytocin pathway explains how genetic diversity at Avpr1a and Oxtr could be maintained in natural populations.

The mechanisms that underpin the varied spatial genetic structures exhibited by free-living marine microorganisms remain controversial, with most studies emphasizing a high dispersal capability that should redistribute genetic diversity in contrast to most macroorganisms whose populations often retain a genetic signature of demographic response to historic climate fluctuations. We quantified the European phylogeographic structure of the marine flagellate Oxyrrhis marina and found a marked difference in spatial genetic structure, population demography, and genetic diversity between the northwest Atlantic and Mediterranean Sea that reflects the persistent separation of these regions as well as context-dependent population responses to contrasting environments. We found similar geographic variation in the level of genetic diversity in the sister species Oxyrrhis maritima. Because the capacity for wide dispersal is not always realized, historic genetic footprints of range expansion and contraction persist in contemporary populations of marine microbes, as they do in larger species. Indeed, the well-described genetic effects of climatic variation on macroorganisms provide clear, testable hypotheses about the processes that drive genetic divergence in marine microbes and thus about the response to future environmental change.

Historical datasets documenting changes to gene frequency clines are extremely rare but provide a powerful means of assessing the strength and relative roles of natural selection and gene flow. In 19th century Britain, blackening of the environment by the coal-fired manufacturing industry gave rise to a steep cline in the frequency of the black (carbonaria) morph of the peppered moth (Biston betularia) across northwest England and north Wales. The carbonaria morph has declined across the region following 1960s legislation to improve air quality, but the cline had not been comprehensively described since the early 1970s. We have quantified changes to the cline as of 2002, equivalent to an interval of 30 generations, and find that a cline still exists but that it is much shallower and shifted eastward. Joint estimation of the dominant fitness cost of carbonaria and dispersal parameters consistent with the observed cline change indicate that selection against carbonaria is very strong across the landscape (s approximately 0.2), and that dispersal is much greater than previously assumed. The high dispersal estimate is further supported by the weak pattern of genetic isolation by distance at microsatellite loci, and it implies that in addition to adult dispersal, wind-dispersed first instar larvae also contribute to lifetime dispersal. The historical perspective afforded by this study of cline reversal provides new insight into the factors contributing to gene frequency change in this species, and it serves to illustrate that, even under conditions of high dispersal and strong reverse selection acting against it, complete erosion of an established cline requires many generations.