Evolution and numbers

So far we have covered how ecology causes anagenetic change, the place of ecology being largely subsumed under the term 'selection'. The present chapter, like Chapter 3, explores more the effect of evolution on ecology, thus closing a causation loop. Specifically, we will investigate the effects of anagenetic change on population dynamics, an ecological characteristic, and ask whether we can better understand population level phenomena by incorporating evolutionary assumptions. Further, we will explore an additional theoretical step in the evolutionary process by using assumptions about population dynamics to help estimate the path of evolutionary change. Thus, evolution influences ecology, which influences evolution again.

There are essentially two situations in which knowledge of adaptation can help predict the dynamics of populations. The first is when organisms display plastic phenotypic strategies (such as behaviours) that are adapted to suit particular environments, and these behaviours affect population ecology in some way. In this scenario no evolution occurs per se during the timescale of the study, but the plastic responses are assumed to be the outcome of past selection. The second way is, if evolution itself occurs on ecological timescales, by which I mean roughly the length of a human lifespan. This is more common than it might at first appear. It is not a great leap of understanding to realize that changes to a trait can affect a population's dynamics. That then requires ecologists to think about evolution. However, a further logical step is possible. The fitness of phenotypes may be affected by the ecological interactions between individuals, for example if fitness is density- or frequency-dependent. This may then decide which genotypes spread or disappear. Thus, there may be a case for incorporating assumptions about the effect of a trait on population dynamics merely to work out what the final evolutionary state itself is. The theoretical approach that incorporates these assumptions is known as adaptive dynamics, it is relatively new, and we will examine how it works later in the chapter.

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