ABSTRACT
Development is defined as the set of processes occurring at all organismic levels (i.e., genetic, cellular, tissue, organ, system, behavior), in a particular environment, that result in the formation of a sexually mature animal. Although previous chapters emphasized the idea that natural selection acts at the trait level, in reality, natural selection usually works by affecting collections of traits, as in a life-history strategy-a set of instructions that results in a particular sequence of developmental steps. Thus, natural selection ultimately selects among alternative developmental programs. This notion was first suggested by Walter Garstand (1868-1949). Figure 4.1 presents a sequence of ancestors and descendants; each fertilized egg, called a zygote and symbolized with Z letters, gives rise to an adult form (A letters) via some developmental process (the thick arrows). In the course of development, organisms reproduce (thin arrow) and the cycle repeats itself. However, zygotes might be changed by mutations or other processes, thus leading to evolutionary change (represented by the subscript number).
