Population structure over a broad spatial scale driven by nonanthropogenic factors in a wide-ranging migratory mammal, Alaskan caribou.

Wide-ranging mammals face significant conservation threats, and knowledge of the spatial scale of population structure and its drivers is needed to understand processes that maintain diversity in these species. We analysed DNA from 655 Alaskan caribou (Rangifer tarandus granti) from 20 herds that vary in population size, used 19 microsatellite loci to document genetic diversity and differentiation in Alaskan caribou, and examined the extent to which genetic differentiation was associated with hypothesized drivers of population subdivision including landscape features, population size and ecotype. We found that Alaskan caribou are subdivided into two hierarchically structured clusters: one group on the Alaska Peninsula containing discrete herds and one large group on the Mainland lacking differentiation between many herds. Population size, geographic distance, migratory ecotype and the Kvichak River at the nexus of the Alaska Peninsula were associated with genetic differentiation. Contrary to previous hypotheses, small Mainland herds were often differentiated genetically from large interconnected herds nearby, and genetic drift coupled with reduced gene flow may explain this pattern. Our results raise the possibility that behaviour helps to maintain genetic differentiation between some herds of different ecotypes. Alaskan caribou show remarkably high diversity and low differentiation over a broad geographic scale. These results increase information for the conservation of caribou and other migratory mammals threatened by population reductions and landscape barriers and may be broadly applicable to understanding the spatial scale and ecological drivers of population structure in widespread species.

Reindeer introgression and the population genetics of caribou in southwestern Alaska.

Alaska caribou (Rangifer tarandus granti) in southwestern Alaska are a poorly understood system, with differing descriptions of their regional population structure, population abundance that has varied greatly through time and instances of the release of domestic reindeer (R. t. tarandus) into their range. Here, we use 21 microsatellites and 297 individuals to investigate the genetic population structure of herds and examine for population bottlenecks. Then, using genetic characteristics of existing reindeer populations, we examine introgression into the wild caribou populations. Caribou of the area are genetically diverse (H E between 0.69 and 0.84), with diversity decreasing along the Alaska Peninsula (AP). Using G ST and Jost's D, we find extensive structuring among all herds; Migrate-n finds that AP herds share few effective migrants with other herds, with Southern AP and Unimak Island herds having the least. Bayesian clustering techniques are able to resolve all but Denali and Mulchatna caribou herds. Using a conservative assignment threshold of q reindeer ≥ 0.2, 3% of caribou show signs of domestic introgression. Denali herd has the most introgressed individuals (6.9%); those caribou herds that were historically adjacent to smaller reindeer herds, or were historically without adjacent herding, show no admixture. This domestic introgression persists despite the lack of managed reindeer in the region since the 1940s. Our results suggest that despite previous movement data indicating metapopulation-like dispersal in this region, there may be unknown barriers to reproduction by dispersing individuals. Finally, our results support findings that wild and domestic Rangifer can hybridize and show this introgression may persist dozens of generations after domestics are no longer present.

Rapid adaptation to mammalian sociality via sexually selected traits.

BACKGROUND: Laboratory studies show that the components of sexual selection (e.g., mate choice and intrasexual competition) can profoundly affect the development and fitness of offspring. Less is known, however, about the total effects of sexual selection on offspring in normal social conditions. Complex social networks, such as dominance hierarchies, regulate the opportunity for mating success, and are often missing from laboratory studies. Social selection is an extended view of sexual selection that incorporates competition during sexual and nonsexual interactions, and predicts complex evolutionary dynamics. Whether social selection improves or constrains offspring fitness is controversial. RESULTS: To identify fitness consequences of social selection, wild-derived mice that had bred under laboratory conditions for eight generations were re-introduced to naturalistic competition in enclosures for three consecutive generations (promiscuous line). In parallel, a control lineage bred in cages under random mate assignment (monogamous line). A direct competition experiment using second-generation animals revealed that promiscuous line males had greater reproductive success than monogamous line males (particularly during extra-territorial matings), in spite of higher mortality and equivalent success in social dominance and sperm competition. There were no major female fitness effects (though promiscuous line females had fewer litters than monogamous line females). This result suggested that selection primarily acted upon a sexually attractive male phenotype in the promiscuous line, a hypothesis we confirmed in female odor and mating preference trials. CONCLUSIONS: We present novel evidence for the strength of sexual selection under normal social conditions, and show rapid male adaptation driven largely by sexual trait expression, with tradeoffs in survivorship and female fecundity. Re-introducing wild-derived mice to competition quickly uncovers sexually selected phenotypes otherwise lost in normal colony breeding.