On fitness and adaptedness and their role in evolutionary explanation.

Abstract

This paper deals with the structure of evolutionary explanations that are based on natural selection. Scientific explanations have two components: the explanandum, or phenomenon to be explained, and the explanans, or that which does the explaining.' In evolutionary explanations derived from natural selection, the explanandum is the presence of some trait in a certain frequency or its rate of increase in frequency over time; in other words, the explanandum is usually fitness in the operational sense of Fisher. The explanans of natural selection explanations invariably make reference to design characteristics of the trait, or its adaptedness. Indeed, many evolutionary explanations, such as those that are commonplace in evolutionary ecology, make reference only to adaptedness. In this essay I discuss situations in which fitness and adaptedness are only indirectly related and are sometimes totally decoupled, in the sense that factors other than adaptedness dominate in the determination of fitness. I argue that one of the main processes contributing to this masking of adaptedness is sexual reproduction. Thus a portion of my argument is concerned with the evolution of sexual reproduction. In my discussion of fitness and adaptedness I distinguish between the intrinsic or genetic aspects of a fitness component and its extrinsic or environmental aspects, and argue that the concept of adaptedeness should apply only to the former. I use the term "fitness" to mean "per capita rate of increase," a usage that was originally Fisher's.2 Thus the fitness of any type -