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245 GENETIC VARIATION AND POPULATION STRUCTURE STEPHEN R. PALUMBI Stanford University Measurement of genetic variation within and between species living on rocky shores shows a wide variety of different patterns that depends on population size, dispersal prowess, recent climate history, and other features of the physical and biological environment. In addition, DNA comparisons have been very useful in helping distinguish species that are extremely similar morphologically . For example, the Northeast Pacific barnacles Chthamalus dalli and C. fissus are similar enough visually to require identification by an expert (Fig. ) but are easily distinguished by a short sequence from a mitochondrial gene. POPULATION STRUCTURE: DIVERSITY WITHIN AND BETWEEN POPULATIONS Rocky-shore species can be very abundant and have large geographic ranges, leading to enormous population sizes—in the hundreds of millions or billions. Overall genetic diversity within any species varies with population structure but is determined by two fundamental forces: the mutation rate of genes from generation to generation and the rate at which genetic variation is lost through genetic drift. Drift occurs when alleles in a population are lost because individuals by chance leave different numbers of offspring. Generally speaking, small populations have high amounts of drift and subsequently low amounts of genetic diversity. A consequence is that rocky-shore species with large populations have low rates of drift and very large amounts of genetic diversity. In fact, genetic studies of rocky shore species often show very large numbers of alleles within populations, and a high degree of DNA sequence divergence among alleles. Exceptions occur in species with low dispersal and low population size, in species that have experienced recent population bottlenecks, or for genetic loci under strong natural selection (see also Measurement of Genetic Variation). Much of the work on the genetics of rocky shores involves comparison of different populations within a species from one place to another. The major motivation for this kind of work is to understand the spatial scale of population differentiation. Populations of a species diverge FIGURE 1 The rocky shore barnacle Chthamalus fissus or C. dalli. These species are visually so similar that it is often difficult to tell them apart. However, genetic differences between the two species are significant, and they can be easily distinguished using this approach. Photograph by Chris Patton. G from one another through the action of natural selection and through random genetic drift from one generation to the next. Populations also experience immigration from other populations; this individual movement (of spores, eggs, larvae, juveniles, or adults) can result in movement of alleles from one population to another and causes populations to remain genetically similar. By examining the alleles in a population at a particular genetic locus and comparing the frequencies of these alleles among populations, we can ask which populations have so little immigration that they have become genetically distinct, and which populations are genetically homogenized by dispersal. MODES OF DISPERSAL: CRAWLERS AND FLOATERS Although rocky-shore species are in many different animal and algal phyla, many of them are firmly attached to the rock surface as adults and have poor powers of adult dispersal. Some of them, such as many algae, are permanently attached to one place. Others, such as starfish and snails, move slowly along rock surfaces. Other species, such as many crabs, amphipods, and worms, are mobile as adults but do not move large distances. For these species , migration among populations occurs primarily during the larval stage of development. Differences in the dispersal potential of larvae of different species can be a prime determinant of differences in genetic differentiation from place to place. For example, some species of intertidal snails in the genus Nucella lay egg capsules on rock surfaces, from which hatch out tiny juvenile snails that crawl away from the egg capsule. These species have poor powers of dispersal at both adult and larval stages, and populations tend to be genetically distinct from one another over small spatial scales. In such cases, DNA sequences from mitochondrial genes of individuals are much more similar among individuals collected from the same population than among individuals collected in different populations. The ratio of genetic variation within populations (within), compared to average genetic variation between populations (between), is used to estimate Wright’s fixation index, FST = – within/between, a measure of the fraction of genetic variation that is distributed geographically in a population. For snails with crawl-away larvae, FST is high even over short spatial scales of a few kilometers, showing that dispersal...
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