What Does the Comparative Study of Development Tell Us About Evolution?

Abstract

During the time period when the metazoan phyla were originating, these animals probably had a life cycle which included larval and adult phases with a short metamorphic period separating each phase. Because of adaptive pressures which function to minimize the metamorphic transition by having adult organs begin their development during the larval period, this phase of the life cycle became the period in which most of the changes occurred that were responsible for the origin of new phyla. One consequence of adultation was the juxtaposition of adult organs in new patterns, in different stages of development in larvae, creating the potential for the formation of new cell types, tissues, and organs. Extreme adultation leads to direct development and lecithotrophy. These developmental trends have caused changes in the way in which determinative events occur during early embryogenesis and provided the basis for a radical reorganization of early embryogenesis.