Because size is so important to the lives and family prospects of individuals, and is at least partially a heritable characteristic, it is a powerful agent of evolution. In any generation individuals will be of various sizes, but the ones that happen to have developed the best size (and other qualities) for the conditions of the time will be the ones that leave the most offspring, which will also be more or less that size. If the conditions change, the best size may be different, and natural selection will quickly favor another genetic lineage, with different average body size, for preferential survival. Evolution is the result of a conversation between the environment and the lives, particularly the reproductive processes, of the animals.
We may, therefore, expect to find that the average body size of each species of weasel living in a given place is the one that gives individual breeding adults the best chance of success in producing young in the local conditions. The best size to be in a particular environment need not be the same as in any other environment; hence, there is enormous local variation in size across the huge geographic range occupied by weasels (Table 4.1). Moreover, the best size to be is not the same for both male and female weasels, for reasons explained in Chapter 14. Males are always substantially larger, but not by a constant amount.
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