ABSTRACT
Rates of molecular evolution were assessed by two approaches, allozyme electrophoresis and immunology (microcomplement fixation). The former gives a measure of the proportion of genes that differ between taxa and the latter gives a measure of the proportion of DNA triplets that differ between taxa for a single gene. We have both electrophoretic and immunological data for dasyurid marsupials, Australian rodents and Australian Rattus snakes. The estimates of evolutionary rates for each group, based on the two data sets, are comparable.
Rates of chromosome evolution were assessed from G-banding analyses. Comparisons between rates of chromosome evolution and rates of molecular evolution were available for the family Dasyuridae (marsupial mice), Australian Rattus (rodents), and Australian Hydromyinae (rodents). The data show virtually no correlation between chromosomal and molecular evolution. Moreover, cladistic analysis of the data shows that rates of chromosome evolution vary enormously between different lineages not only between groups but also within groups.
Although rates of electrophoretic, immunological and chromosomal evolution can be measured objectively, the same cannot be said of morphological evolution. We have attempted to overcome this problem by choosing extremes of morphological divergence, from sibling species (which by definition are morphologically very similar) to species that are clearly very different morphologically even to the casual observer. Examples of sibling species that nevertheless differ greatly at the molecular level include bats of the Eptesicus pumilis complex in Australia and marsupial mice of the Sminthopsis murina complex. The other extreme is represented by 176Australian cockatoos of the genus Cacatua and Australian elapid snakes, where species that are very similar at the molecular level are nevertheless very different at the morphological level.
The data show that molecules, chromosomes and morphology evolve at very different rates. A large body of evidence suggests that, of the three, molecular evolution is the closest to being constant with time. If this is true, then rates of morphological and chromosomal evolution vary enormously with time.
