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<br />-; <br />t <br /> <br />,~,,' <br />,I',:~',' <br />i <br /> <br />Dizon et aI, <br /> <br />phological species representing 9 genera of cl1iclids <br />show almost no differentiation in mtDNA sequences <br />that are 803 base pairs in length (Meyer et al, 1990). In <br />this case mtDNA would be a poor proxy for demonstrat- <br />ing differences in adaptive variation between popula- <br />tions. <br />In fact, most past genetic studies of marine mammals <br />and other pelagic animals were not very useful for man- <br />agement purposes. When significant gene frequency dif- <br />ferences between two putative populations were not <br />found, it was easy to pass off the result as a negative one <br />(i,e., the test was insufficiently sensitive because not <br />enough samples or polymorphic loci were tested), And <br />when significant gene frequency differences were de- <br />tected among populations, the resource biologists have <br />probably known from a variety of other criteria that <br />gene flow among the groups was highly restricted, This <br />is because considerable time and isolation were re- <br />quired to generate detectable differences, that is, no <br />more than a few individuals per generation, Resource <br />managers, however, divide populations into separate <br />units when interchange rates are as much as sev~ral <br />percent per year. <br />However, sequencing progressively longer segments <br />of the mtDNA and nuclear genome now offers means of <br />detecting genetic differentiation in the face of exchange <br />rates that may be considerably higher than a few ge- <br />nomes per generation moving between the populations, <br />After all, at some level, all genomes are different. So now <br />one must ask what level of differentiation or gene flow <br />indicates that a population has become "pan-mixed"? <br />Deciding this requires an understanding of how much <br />gene flow and consequent genetic diversity is "signifi- <br />cant" or not <br /> <br />Phylogeographic Taxa <br /> <br />Clearly, the complex interactions among movement, in- <br />trogression, and selection of nuclear and cytoplasmic <br />genomes (and the variability of expression of those ge- <br />names between local populations of a species) result in <br />situations that are not conducive to a simplistic defini- <br />tion of stock or a binomial decision of stock or not- <br />stock. Deciding whether a putative local population <br />should be managed as a separate unit requires consid- <br />erable expert biological judgment. As long as there are <br />no means of routinely measuring the total genetic vari- <br />ability of local adaptation, rather than its proxies (dis- <br />tribution, population responses, phenotypic variation, <br />or neutral genotypic variation), quasisubjective deci- <br />sions are necessary. <br />For other purposes, Avise et aL (1987; Avise 1989) <br />devised a framework for organizing hypothetical popu- <br />lation structures and relating the two critical concepts <br /> <br />Rethinking the Stock Concepl <br /> <br />Z9 <br /> <br />of selection and movement or. their products- <br />phylogeny and distribution. They ffrst proposed five cat- <br />egories but later reduced them to four (Avise 1989), <br />two with discontinuous genetic p:atterns and two with <br />continuous ones. The approach classified assemblages <br />into one of four categories based on two criteria: <br />mtDNA genetic distance and spatial distribution. In a <br />recent publication, A vise and Ball (1990) characterize <br />the interaction of both extrinsic and intrinsic reproduc- <br />tive barriers in the formation of subspecific divisions <br />and emphasize the surrogate nature of most genetic and <br />other measures (which we call proxies) of so-called <br />stock distinctiveness. . <br />We altered their original catagory criteria slightly to <br />be more general and to emphasize that meaningful dis- <br />tinctiveness must come from the expression of multiple, <br />independent genetically based traits (Avise & Ball, <br />1990). In our alteration, the horizontal axis indicates <br />the degree of response to differential selection likely to <br />have occurred in one population relative to another. It <br />represents differences in characters that are the expres- <br />sion of the locally adapted genome, rather than simply <br />the mtDNA genetic dis'tance (Fig. 2). Under this scheme, <br />differences found in demographic, morphological, <br />isozyme, or mtDNA measures are taken to be proxies <br />indicating that selection may be operating differentially <br /> <br />Theoretical classification: <br /> <br />PHYLOGEOGRAPHIC TYPES <br /> <br />GENE FLOW <br /> <br />little or none <br />high <br /> <br /> <br />Operational classification: <br /> <br />GEOGRAPHIC <br />LOCALIZATION <br /> <br />PHYLOGEOGRAPHIC TYPES <br /> <br /> <br />Figure 2. Four phylogeographic classification catego- <br />ries. The horizontal axes in the theoretical classifica- <br />tion refer to differences in the characters expressed <br />by the genes that make up the locally adapted ge- <br />nome The horizontal axis in the operational classi- <br />fication refers to differences in characters that may <br />or may not be expressed by the genes that make up <br />the locally adapted genome If they are not ex- <br />pressed, the differences represent proxy measure- <br />ments indicating the probability that sufficient time <br />has elapsed and selection pressure bas been applied <br />so that a locally adapted genome bas evolved. Gene <br />flow is defined as production of locally fit offspring. <br />Adapted from Avise (1989). <br /> <br />ConscrvaUon Biology <br />Volume 6, No. I, March 1992 <br />