Contemporary fisherian life-history evolution in small salmonid populations

The relative importance of natural selection(1) and random drift(2) in phenotypic evolution has been discussed since the introduction of the first population genetic models(3-5). The empirical evidence used to evaluate the evolutionary theories of Fisher(1) and Wright(2) remains obscure because formal tests for neutral divergence(6-8) or sensitive attempts to separate the effects of selection and drift are scarce, subject to error, and have not been interpreted in the light of well-known population demography. We combined quantitative genetic and microsatellite DNA analyses to investigate the determinants of contemporary life-history evolution in isolated populations of grayling (Thymallus thymallus, Salmonidae) that originated from a common source 80-120 years ago. Here we show that natural selection was the dominant diversifying agent in the evolution of the quantitative traits. However, the populations were founded by a small number of individuals, exhibit very low microsatellite-based effective sizes and show genetic imprints of severe 'bottlenecks'; which are conditions often suggested to constrain selection and favour drift(6,8,9). This study demonstrates a very clear case of fisherian evolution in small natural populations across a contemporary timescale.