A century of life-history evolution in grayling

Synchronic and allochronic data sets consisting of phenotypic values of various life-history traits from five grayling Thymallus thymallus populations with common ancestors were analysed for the purpose of estimating evolution and divergence rates. The synchronic data contained both juvenile and adult traits from populations that have been segregated for 44-88 years (9-22 generations). The allochronic time series contained growth- and maturation data spanning 95 years (16 generations). Estimated evolution and divergence rates were high compared with other life-history studies on the same temporal scale (0.002-1.008 haldanes, 10-30, 500 darwins). The divergence of adult traits were most probably caused by differential mortalities induced by variation in fishing intensity. For the population with allochronic data, 48 years (eight generations) of intense and consistent size-selective gill-net fishing resulted in a constant reduction in age (-0.33 years pr 10 year) and length (-18mm pr 10 year) at maturity. Length-at-age for ages one to five also decreased during the same period. When gill-net fishing was relaxed, age and length at maturity and length-at-age increased. Divergence rates for juvenile traits derived from a common-garden experiment were high, and standardized selection differentials (s') were high, especially for yolk-sac volume (s' = 2.6). We also document that low divergence rates for juvenile traits were lower between populations having similar spawning/nursery habitats (running water) than populations having relatively different habitats (running water v.s. still water). We suggest that the major part of the observed phenotypic divergence is mostly due to adaptive evolution, although microsatellite data indicate that genetic drift also has occurred.