An organism's life history is its lifetime pattern of growth, differentiation, storage and reproduction; and we have seen something in the preceding sections about the variety of patterns life histories may take and what the consequences may be in terms of population rates of increase. But can we understand how different species' life histories have evolved? In fact, there are at least three different types of question that are commonly asked about the evolution of life histories.
The first is concerned with individual life history traits. Why is it that swifts, for example, usually produce clutches of three eggs, when other birds produce larger clutches, and the swifts themselves are physiologically capable of doing so? Can we establish that this clutch size is ultimately the most productive, i.e. the fittest in evolutionary terms, and what is it about this particular clutch size that makes it so?
The second question is concerned with links between life history traits. Why is it, for example, that the ratio between age at maturity and average lifespan is often roughly constant within a group of organisms but markedly different between groups (e.g. mammals 1.3, fish 0.45)? What is the basis for the link between these two traits within a group of related organisms? What is the basis for differences amongst groups?
The third question, then, is concerned with links between life histories and habitats. How does it come about that orchids, for example, produce vast numbers of tiny seeds when tropical Mora trees produce just a few enormous ones? Can the difference be related directly to differences in the habitats that they occupy, or to any other differences between them?
In short, the study of the evolution of life histories is a search for patterns - and for explanations for those patterns. We must remember, however, that every life history, and every habitat, is unique. In order to find ways in which life histories might be grouped, classified and compared, we must find ways of describing them that apply to all life histories and all habitats. Only then can we search for associations between one life history trait and another or between life history traits and features of the habitats in which the life histories are found. It is also important to realize that the possession of one life history trait may limit the possible range of some other trait, and the morphology and physiology of an organism may limit the possible range of all its life history traits. The most that natural selection can do is to favor, in a particular environment with its many, often conflicting demands, the life history that has been most (not 'perfectly') successful, overall, at leaving descendants in the past.
None the less, most of the successes in the search for an understanding of life history evolution have been based on the idea of optimization: establishing that observed combinations of life history traits are those with the highest fitness (Stearns, 2000). It is also important to note, however, that there are alternative approaches - one long-established, two others more recent - that certainly have much to recommend them in theory, even if their explanatory powers to date have been limited compared to the optimization approach (Stearns, 2000). The first is 'bet-hedging': the idea that when fitness fluctuates, it may be most important to minimize the setbacks from periods of low fitness rather than evolving to a single optimum (Gillespie, 1977). The second acknowledges that the fitness of any life history cannot be seen in isolation: it depends on the life histories of other individuals in the population, such that fitness of a life history is 'frequency dependent' - dependent on the proportions of that and other life histories in the population (e.g. Sinervo et al., 2000). The third, then, includes an explicit consideration of the dynamics of the population concerned, rather than making the usual simplifying assumption of population stability (e.g. Ranta et al., 2000). Here, though, we focus on the optimization approach.
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