the first time, evolutionary biologists could attempt to assess the amount of neutral genetic variation within species as well as its spatial distribution. Plant species were found to vary widely, both in levels of genetic variation and in the apportionment of this variation within and among populations. These observations spurred researchers to examine the mechanisms underlying the process of genetic differentiation in plants. One of the most common approaches for doing this analysis has been to look for correlations between the life history characteristics of a species (e.g., mechanisms of pollen and seed dispersal, system of mating, and generation length) and patterns of population genetic differentiation (Hamrick and Godt, 1989). Neutral allelic variation from allozymes also can be used to estimate levels of gene flow among populations. The population genetic theory developed by Wright, Fisher, and Malecot (among others) established that, for a group of populations at equilibrium, the level of genetic differentiation is roughly inversely proportional to the level of interpopulation gene flow per generation. This relationship is expressed by Wright's (1951) familiar equation for estimating gene flow under an island model: FST ≈ 1/(4Nem + 1), where FST is the standardized variance in allele frequencies among populations, Ne is the effective population size, and m is the migration rate.
The use of allozymes has led to more than 30 years of insight into how plant populations evolve. However, inferring population structure solely from allele frequencies has its limitations. Allozyme alleles (or their DNA analogs, restriction fragment length polymorphisms, amplified fragment length polymorphisms, and microsatellites) are unordered, meaning that the genealogical pattern of relationships among alleles cannot be easily inferred. As a result, these data cannot be used in directly assessing genetic change over time but rather require indirect approaches based on models that often assume equilibrium conditions. For example, Wright's equation (above) for quantifying gene flow under the island model assumes that populations have reached an equilibrium between gene flow and random genetic drift. This equilibrium perspective can be biologically misleading, particularly for species in which recent history is a major determinant of population structure. We speculate that very few plant species have reached, or will ever reach, a gene flow-drift equilibrium. Many plants, both temperate and tropical, have altered their range subsequent to glaciations in the last 20,000 years, a recent event on an evolutionary time scale. Likewise, many plant species have a metapopulation structure with subpopulations continually being colonized, dispersing migrants, and going extinct. Such metapopulations may reach a system-wide equilibrium in which probabilities of extinction and recolonization are constant given enough time, but such a situation is unlikely considering the relatively short time frame of global climatic changes in the past.