Evolutionary aspects of fish distribution: genetic variability and adaptation

Abstract

Many species of freshwater and anadromous fish are fragmented into local populations, some of which display adaptive traits that enhance survival and reproduction in particular environments. This paper focuses on the genetic component of response to heterogeneous environments, with emphasis on how the levels and distribution of genetic diversity may influence the distribution of conspecific populations. It addresses three aspects of genetic variability and adaptation: (1) the significance of genetic variability in relation to adaptation; (2) the adaptive significance of population differentiation; (3) the effects of stocking on local adaptation. In addition to considering local adaptation in salmonids, especially in relation to the genetic consequences of stocking, examples are chosen across fish taxa, with emphasis on recent studies in the guppy, Poecilia reticulata. The significance of within-population genotypic variance in fitness traits, genetic load and outbreeding depression is discussed in relation to adaptation. Evidence for local adaptation is drawn from molecular investigations, genotypic selection experiments, and integrated breeding and field studies. The fragility and uniqueness of locally adapted gene pools is emphasized. Threats arise from direct interactions between native and introduced fish through stocking and escapees of cultured fish. Conservation of genetic resources must not only include practices that maximize levels of genetic diversity, but also measures to preserve the associated genotypic variance in ecologically-significant traits. The integration of descriptive population genetic studies with empirical demonstrations of adaptive traits is encouraged.