We have already seen numerous examples of the increase in ecological versatility associated with differentiation within populations. Such differentiation conies in many forms, including gender-based, age- and stage-specific, and polymorphic differences in resource use. There also are instances in which phenotypically indistinguishable individuals develop particular patterns of resource use due to an environmental interaction early in their lifetimes. Thus, the algal species to which an individual of the sea slug Placida dendritica is exposed first largely determines the diet of that individual, and this appears to be unrelated to underlying genotypes (Trowbridge 1991). Similarly, individuals of the Cocos finch, Pinaroloxias inornata, develop idiosyncratic foraging behaviour apparently by watching older, experienced birds (i.e., cultural transmission, Werner and Sherry 1987; see also Wcislo 1989).

However, ecological differentiation within populations can also arise from the occurrence of morphologically distinguishable ‘types’ within populations, which increases the range of resources used (e.g., Decho and Fleeger 1988, Malmquist 1992). For example, in many species of anuran amphibians, juveniles are obligatorily herbivorous or detritivorous while adults are strictly carnivorous (Toft 1985), while the reverse occurs in some fishes (Braband 1985). Gender-based (e.g., Freeman et al. 1976, Dawson and Ehleringer 1993, Le V. dit Durell et al. 1993) or morphological (e.g., Ehlinger 1990) differentiation of resource utilization is common. Even parthenogenetic clones have different ecological characteristics from one-another (e.g., Jaenike et al. 1980, Weider and Hebert 1987), as have strains of some parasites (e.g., the parasitic nematode Howardula aoronymphium of Drosophila, Jaenike 1993). Stages of many species of parasites utilize very distinct sets of hosts, often displaying a large difference in specificity (Jones 1967).