LEVELS OF GENETIC-POLYMORPHISM - MARKER LOCI VERSUS QUANTITATIVE TRAITS

Species are the units used to measure ecological diversity and alleles are the units of genetic diversity. Genetic variation within and amon g species has been documented most extensively using allozyme electrop horesis. This reveals wide differences in genetic variability within, and genetic distances among, species, demonstrating that species are n ot equivalent units of diversity The extent to which the pattern obser ved for allozymes can be used to infer patterns of genetic variation i n quantitative traits depends on the forces generating and maintaining variability. Allozyme variation is probably not strictly neutral but, nevertheless, heterozygosity is expected to be influenced by populati on size and genetic distance will be affected by time since divergence . The same is true for quantitative traits influenced by many genes an d under weak stabilizing selection. However, the limited data availabl e suggest that allozyme variability is a poor predictor of genetic var iation in quantitative traits within populations. It is a better predi ctor of general phenotypic divergence and of postzygotic isolation bet ween populations or species, but is only weakly correlated with prezyg otic isolation. Studies of grasshopper and planthopper mating signal v ariation and assortative mating illustrate how these characters evolve independently of general genetic and morphological variation. The rol e of such traits in prezygotic isolation, and hence speciation, means that they will contribute significantly to the diversity of levels of genetic variation within and among species.