Following the predominantly ecological considerations so far, the aim of this chapter is to look at behavioral variation with particular emphasis on natural populations. Behavior can be regarded as of great importance in evolutionary change, whether considering aspects concerned with mating or improving adaptedness to a new environment. Behaviors relevant to population continuity need to be isolated and studied under a multiplicity of environments. Assuming that measurement is objective, that is, independent of the observer, there are two emphases that are important in the study of behavioral traits. They are the difficulty of environmental control and the importance of learning (Ehrman and Parsons, 1981a). The first emphasis will be discussed in this chapter mainly with Drosophila examples. The second will be considered to some extent in the next chapter, being of greatest importance in organisms higher in the phylogenetic series.

Phototaxis and light intensities

Hirsch and Boudreau (1958) studied phototaxis (movement with respect to light) in D. melanogaster in an apparatus consisting of a Y maze (part of Figure 7.1C), one arm of which was exposed to light during tests. Quite rapid responses to directional selection were found for both positive and negative phototaxis, which is reasonable as a heritability in the broad sense was estimated in excess of 0.5. Similarly, responses to selection have been found in D. melanogaster (Hadler, 1964) and in D. pseudoobscura (Dobzhansky and Spassky, 1969) for phototaxis in more complex mazes (Figure 7.1C).