The hope that “biotechnology” will permit the iden- tification of geographic sources of recruitment to most marine animal populations is not well supported either by logic or fact. First, population genetics tells us that dispersal among geographic populations is expected, at equilibrium, to eliminate the very molecular genetic dif- ferences that are supposed to permit identification of geo- graphic provenance. Second, studies of allozynies and mitochondrial DNA have clearly shown that fish and in- vertebrate species with planktotrophic larvae are genetic- ally quite sindar over large regions, though not necessarily throughout their whole ranges. Genetic studies are, nev- ertheless, contributing new insights into the structure and biology of marine animal populations. One new insight is that sharp genetic subdivisions can occur in continuously distributed species, particularly those spanning biogeographic boundaries. An even more widespread observation is of very slight but significant microgeographic genetic heterogeneity embedded within broad regions of genetically very similar populations. Examples of the latter from the California Current are presented for the barnacle Balanirs glandula and the north- ern anchovy Engranlis rnovdax. Microgeographic hetero- geneity holds interest for biological oceanographers and fisheries scientists because it contradicts the logic of pop- ulation genetics as well as commonly held notions about the structure of zooplankton populations. Evidence sug- gests that genetic heterogeneity on nllcrogeographic scales results from temporal variation in the genetic composi- tion of recruits. This temporal variation could be a con- sequence of either selection on larval populations or large variance in the reproductive success of individuals, owing to chance matching of reproductive activity with win- dows of oceanographic conditions conducive to fertil- ization, larval development, retention, and recruitment. In support of the latter hypothesis, effective sizes for natural oyster populations are estimated to be only small fractions of breeding population numbers. The tempo- ral aspect of population genetic structure forges a strong interdisciplinary bridge to oceanographic research aimed at elucidating the temporally and spatially varying factors affecting recruitment.
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