Evolution of performance under tradeoffs

The first attempt to model the evolution of the fundamental niche was that of Levins (1968). He devised a model that should seem familiar, for in structure it is very similar to the model of the evolution of co-sexuality by Charnov et al. (1976) (Chapter 5). Imagine an organism faced with two habitats, A and B, of differing frequency. Its fitness in the two habitats is defined by a trade-off, such that when its fitness in A is high, its fitness in B will be lower, and vice versa, a situation known as antagonistic pleiotropy. A severe trade-off is of a concave shape, but a convex trade-off is less severe, such that at best an organism which is very fit in B will also be pretty fit in A (Figure 9.1). Now suppose that the organism wants to maximize its fitness in the environment as a whole: what point on the fitness curve is best for it? If an organism chooses to maximize fitness in a single habitat, at the expense of fitness in other habitats, it will by definition be a specialist. If fitness is approximately the same in all habitats, it will be a generalist. The basic

Evolutionary Trade Offs Models

Fig. 9.1 Levin's (1968) model of the evolution of specialization. Fitness in two habitats may be described by a trade-off, which may be convex or concave. A convex trade-off leads to the evolution of a generalist, which can use both habitats quite well (black circle), because its total fitness from both habitats is greatest then. A concave trade-off leads to specialization on the most common habitat (white circles), because any alternative generalist would achieve reduced total fitness.

Fig. 9.1 Levin's (1968) model of the evolution of specialization. Fitness in two habitats may be described by a trade-off, which may be convex or concave. A convex trade-off leads to the evolution of a generalist, which can use both habitats quite well (black circle), because its total fitness from both habitats is greatest then. A concave trade-off leads to specialization on the most common habitat (white circles), because any alternative generalist would achieve reduced total fitness.

solutions are pretty intuitive: if the trade-off is concave then organisms evolve to maximize their fitness in one habitat,whichever is the most common. If the trade-off is convex, such that a generalist is almost as fit in both habitats as a specialist, then as long as both habitats are reasonably common, generalists will evolve.

According to Levin's model, the evolution of specialism is thus predicted to rely on the presence of a fitness trade-off, the more severe the better, and will also depend on the frequency of alternative habitats,with increasing bias in frequency of one habitat tending to lead to specialism in that habitat. Of course, this model just considers the fundamental niche, and is not explicitly genetic.

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