Diversity within and among populations
Individuals in a single population can differ from each other from a variety of causes, some discussed in Chapter 5. The present chapter will use these and a few additional ideas to discuss variation within populations, and then go on to the broader question of differences among populations and phylads at all levels of divergence.
Suppose we got the following results of an experimental test of strains S and s on diets D and d. S produces phenotype Φ on both diets, but s produces Φ on diet D and ϕ on diet d (Fig. 7.1). Simple field observations could yield the same kind of data: One species has the same phenotype in habitats H and h, while another has one phenotype in H and a different one in h. A frequent reaction to this kind of information is to