Lecture 35

Abstract

As we pose questions about development, we must never lose sight of evolution. Many hereditary factors participate in the development of each organ; for example, more than 40 loci are known, at which mutation interferes with the normal development of the Drosophila eye. Many mutations are pleiotropic, another indication that their normal alleles are involved in the formation of numerous organs. The many genetically controlled malformations, including those leading to death, show the danger of mutation for the delicate spatial and temporal interplay of the genetic effects in the course of cell replication, induction, cell movements, and differentiation in structure and function at the cell level and above. And yet, mutations are the building blocks of speciation; we know no others. In every evolutionary change, selection must establish a new harmonic balance; unbalanced ontogeny is eliminated.