Rates of Evolution in Developmental Processes

The tempo and mode of morphological evolution are influencedby several factors, among which evolutionary transformationsin developmental processes are likely to be important. Comparingthe embryos of extant species in an explicit phylogenetic framwork allows the estimation of minimum average rates of evolutionin quantitative developmental parameters. It also allows delineationof the maximum time that complex qualitative transformationsin developmental mechanism take to evolve. This paper analyzesrates of quantitative and qualitative developmental evolutionusing examples drawn primarily from echinoderms. The resultsdemonstrate that rates of developmental evolution can be comparableto rates of morphological evolution. There is no indicationthat rates of evolution in development are lower for earlierstages, contrary to the prediction of "tree" models of epigeneticinteractions. In particular, rates of evolution in oogenesiscan exceed rates of evolution in adult body size. Rates of developmentalevolution can vary by up to two orders of magnitude within aclade. Whether such large scale variation in evolutionary ratesof developmental processes is a general phenomenon can onlybe answered by further study.