on individuals in response to biotic and abiotic environments and from random, nonselective fixation of genes.
New developments in the study of molecular evolution and modern laboratory techniques make it possible to determine the degree or closeness of relationships within and between populations (Avise 1994, 1995; Hillis and others 1996). Molecular data and traditional anatomical information permit us to deduce phylogenies—the branching patterns of genealogical lineages and ancestry of sets of species (Hillis and others 1996).
Much genetic variation is detectable only biochemically, but some is evident as variation in anatomy, physiology, behavior, and life-history characteristics—phenotypes—of individuals in a population. Genetic variation is the basis of local adaptations and of common phenomenon of gradual change in phenotype along a geographic transect where the environment changes. Genetic variation is also the basis of coevolution, whereby species evolve adaptations in response to each other's adaptations.
There are many examples of adaptive evolution within species. Across the extensive continuous range of the common mussel off the eastern coast of North America, despite its enormous reproductive output and high rates of genetic exchange, populations are genetically differentiated over surprisingly small distances—from a few meters to several kilometers (Koehn and Hilbish 1995). The common yarrow, a composite plant from California, is able to live over a great range of habitats, from the high Sierra Nevada to the Pacific Coast, and shows distinctive, genetically determined forms in different habitats (Clausen and others 1958). Drosophila flies show extensive variation in genome organization according to habitat, elevation, regional geography, and seasonality (Dobzhansky 1970).
Effective environmental management includes considerations of genetic variation. For example, salmon stocks in different rivers in the same region exhibit differences in genetic makeup. These are the result of independent evolution of distinct stocks, each of which has adapted to local conditions. The differences seen reflect the histories of the stocks, some resulting from local selection pressures and others from the accumulation of random changes associated with the degree of isolation and population size.
Genetic diversity provides an economic basis for protecting and conserving biodiversity (McNeely and others 1990; Oldfield 1984; Potter and others 1993; Reid and Miller 1989; Reid and others 1993; WRI/IUCN/UNEP 1992). For example, Douglas fir trees grow abundantly across the western United States. Their success is due to their diversity despite their similar appearance (Rudolph 1990). Coastal and interior populations show genetic differences in cold hardi-