Evolution Of Phenology In A Salmonid Population

Accumulating evidence has indicated that many fish populations are responding to climate change through shifts in migration time, but genetic data identifying the role of evolution in these shifts are rare. One of the first demonstrations of evolution of migration time was produced by monitoring allozyme alleles that were experimentally manipulated to genetically mark late-migrating pink salmon. Here, we extend that research by using observations of the marker alleles in fry to demonstrate that these changes in migration time were caused by directional selection against the late-migrating phenotype during the oceanic phase of the salmonid life cycle. The selective event, which appeared to be driven by early vernal warming of the nearshore marine environment and consequent decreased survival of late-migrating fry relative to early migrating fry, decreased the late-migrating phenotype from more than 50% to approximately 10% of the total fry abundance in only one generation. These demographic changes have persisted over the subsequent 13 generations and suggest that a larger trend toward earlier migration time in this population may reflect adaptation to warming sea-surface temperatures.

A primary mechanism by which animal populations can respond to environmental change is by shifting the timing of life history events, known as phenology, to track optimal environmental conditions in time. Phenology is of critical importance for migratory fish, such as Pacific salmon, which must initiate an array of physiological, morphological, and behavioral changes at precise times during their life cycle. The precision with which salmonids time migration can constrain gene flow between seasonally distinct spawning segments, thereby enabling local adaptation of phenology. Evidence of local adaptation of phenology in salmonids has been provided by comparisons of seasonally distinct groups of Chinook salmon that spawn in the same river, which demonstrated significant genetic divergence at three circadian clock genes, but not at neutral markers. The tight coupling of salmonid ecology and phenology, along with the generally high heritability of phenological traits in salmonids suggests that salmonid populations may respond to periodic environmental fluctuations or persistent climatic trends through contemporary evolution of phenology.