Evolution of temporal isolation in the wild: Genetic divergence in timing of migration and breeding by introduced chinook salmon populations

The timing of migration and breeding are key life-history traits; they are not only adaptations of populations to their environments, but can serve to increase reproductive isolation, facilitating further divergence among pop ulations. As part of a study of divergence of chinook salmon, Oncorhynckus tshawytscha, populations, established in New Zealand from a common source i n the early 1900s, we tested the hypotheses that the timing of migration an d breeding are under genetic control and that the populations genetically d iffer in these traits despite phenotypic overlap in timing in the wild. Rep resentatives of families from two populations were collected within a day o r two of each other, reared in a common environment, and then released to s ea from each of two different rivers, while other family representatives we re retained in fresh water to maturity. The date of maturation of fish held in fresh water and the dates of return from the ocean and maturation of fi sh released to sea all showed significant differences between the two popul ations and among families within populations. The very high heritabilities and genetic correlations estimated for migration and maturation date indica ted that these traits would respond rapidly to selection. Combined with the results of related studies on these chinook salmon populations, it appears that spawning time may not only evolve during the initial phases of diverg ence, but it may play an important role in accelerating divergence in other traits.