Separating the wheat from the chaff: patterns of genetic differentiation in high gene flow species

In many marine species, high levels of gene flow ensure that the genetic signal from population differentiation is weak. As a consequence, various errors associated with estimating population genetic parameters that might normally be safely ignored assume a relatively greater importance. This fact has important implications for the use of genetic data to address two common questions in fishery conservation and management: (1) How many stocks of a given species are there? and (2) How much gene flow occurs among stocks? This article discusses strategies to maximize the signal:noise ratio in genetic studies of marine species and suggests a quantitative method to correct for bias due to a common sampling problem. For many marine species, however, genetic methods alone cannot fully resolve these key management questions because the amount of migration necessary to eliminate most genetic evidence of stock structure (only a handful of individuals per generation) will generally be inconsequential as a force for rebuilding depleted populations on a time scale of interest to humans. These limitations emphasize the importance of understanding the biology and life history of the target species-first, to guide design of the sampling program, and second, so that additional information can be used to supplement indirect estimates of migration rates based on genetic data.