Size change in development and evolution

Abstract

Phylogeny is a succession of ontogenies, and the two have been compared by considering them in terms of rates of size change. In development, the larger the organism, the slower its rate of size increase. In evolution, the rates of size change can be put into three distinct categories: fast, medium, and slow. The fast changes occur over short periods of time (1–10 thousand years) and are as likely to show size decrease as increase. The medium changes occur over longer time spans (5–20 million years) and are predominantly or entirely instances of size increase. The slow changes occur over the entire span of organic evolution and represent the maximum size attained in various phyla, which again show an over-all increase.For ontogeny, a decrease in rate of size change is correlated with an increase in complexity, an increase in the number of gene actions. For evolution, it is correlated with an increase in the number of genetic changes required of the genome by natural selection in fluctuating environments.