RATES OF EVOLUTIONARY CHANGE IN CHROMOSOME NUMBERS IN SNAILS AND VERTEBRATES

Mollusks and most non-mammalian vertebrates have been characterized as evolving an order of magnitude more slowly in morphology and karyotype compared with most groups of placental mammals. New calculations of the previously used measures of chromosomal rates of evolution for different groups of gastropods, using a larger and taxonomically broader sample, indicate that these rates had been previously underestimated, although they are still lower than those of the most rapidly evolving placental groups. When genera of approximately the same geological age are compared, little difference (less than an order of magnitude) in fossil-based measures of average rate of karyotypic evolution are found among placental mammals, frogs, lizards, and snails. Variation in rates within major groups obtained from the limited available data does not allow clear generalizations on among-group differences in chromosomal rates of evolution.