Natural Selection and the Evolution of Adaptive Genetic Variation in Northern Freshwater Fishes

<jats:title>Abstract</jats:title> <jats:p>The fishes inhabiting depauperate northern postglacial lakes and rivers have lately experienced high rates of divergence and species formation. The study of these fishes provides insight into how genetic variation in populations is molded by divergent environmentally based selection and how genetic variation and selection interact to cause rapid diversification. We summarize patterns of variation and divergence in the trophic traits of these fishes and show that (1) populations in depauperate environments often have elevated levels of niche-based phenotypic variation, which regularly takes the form of trophic or resource polymorphisms, and which appear to represent cases of character release; (2) much of this variation is genetically based and is the product of disruptive selection; (3) parallel patterns of diversification occur within and among populations; (4) in some instances this process has led to stable species coexistence despite gene flow and incipient to full-blown sympatric speciation; and (5) a reserve of genetic variation persists in most postglacial fishes that may allow a response to certain kinds of environmental change. Contemporary patterns of variation reflect a recent history of natural selection that has operated since the last glaciation some 15,000 years ago, but they may also reflect species-level selection that has occurred over the much longer time scale of millions of years representing many glacial periods.</jats:p>