MORPHOLOGICAL EVOLUTION OF THE DROSOPHILA-VIRILIS SPECIES GROUP AS ASSESSED BY RATE TESTS FOR NATURAL-SELECTION ON QUANTITATIVE CHARACTERS

Two rate tests for assessing natural selection on quantitative traits are discussed for their usefulness in macroevolutionary and adaptation al studies. The underlying assumptions and parameter estimation for th e constant-heritability (CH) and mutation-drift-equilibrium (MDE) mode ls, which are the bases for these tests, are discussed. The purpose of these rate tests is to determine whether morphological change has occ urred either too fast to be explained by neutral drift, which suggests directional selection, or too slow, which suggests stabilizing select ion. Previous formulations of these rate tests have ignored the phylog enetic component. Several models of evolution are considered to help a ccount for phylogeny in the context of rate tests. The MDE rate test f or stabilizing selection was performed on nine morphological character s among several species of the Drosophila virilis species group. These tests can be interpreted to suggest that stabilizing selection has pr obably been a major factor in producing the observed similarity among the Drosophila species examined.