Second, I have suggested that a fruitful way of looking
at variation is in terms of trade-offs of different fitness
benefits and costs (summarized in Table 1 for the Big Five
personality factors). Theories based on trade-offs have
been very successful in providing an understanding of
evolution in other species. Moreover, the idea of trade-offs
can be usefully married to the notion of fluctuating selection
to explain the persistence of diversity. Such accounts
are not speculative. Studies such as those on great tits,
guppies, finches, and sunfish (see the section on Evolution
of Variation) have demonstrated how fluctuations in environmental
context change the fitness outcomes associated
with particular phenotypes, which in turn affects the future
shape of the population through natural selection. Thus,
researchers examining nonhuman variation have been able
to go well beyond post hoc explanations and actually
observe evolution in action