Disentangling Historic and Contemporary Genetic Effects of Stream Structure: a Darter Example

From a management perspective, differentiating between historical processes (e.g., range expansion, population fragmentation) and contemporary factors (e.g., recent habitat modification, dispersal) that affect stream populations is of interest.  Both are important, as historical processes reflect evolutionary divergence and by extension may represent current or future adaptability (i.e., ESUs). However, often of more immediate interest is the impact of contemporary stream management on stream connectivity, fish population size, and other factors. Genetic methods can reveal much about both of these different temporal scales provided the appropriate marker and analytical choices are made.  Addressing these questions in dendritic systems poses some unique challenges. We illustrate one approach to disentangling historical and contemporary genetic signals from the Okaloosa darter (Etheostoma okaloosae) using molecular genetic data (mtDNA and nDNA). Okaloosa darters have been heavily monitored but until recently there was little knowledge of historical population structuring, nor patterns of contemporary stream reach or inter-basin connectivity. For example, Okaloosa darters are highly structured by Bayou drainage and by stream. However, there is some shared polymorphism at lower reaches of streams that may be due to the retention of historical diversity, recent (past 100 years) habitat modification, or more recent dispersal. Hierarchical sampling strategy and model-based analytical approaches allow us to test these specific hypotheses to best explain the distribution of genetic variation range-wide and predict the impact of recent stream restoration efforts.