Adaptive dynamics

The techniques used in Heino's model are interesting. In a normal ESS approach there are no explicit assumptions about population dynamics made in determining the final evolutionary state. If, however, we explicitly want to predict the effect of evolution on population dynamics, we are more likely to have to face up to this challenge. Adaptive dynamics adds in this interaction. In brief, the fitness of new mutants is assessed in terms of their population dynamic interaction with residents. If at the end of that interaction, the mutant has a higher fitness than the resident, the mutant becomes the resident strategy and the resident is displaced. This continues until a so-called 'singularity' is reached (Figure 8.6). Much of the time a singularity may be a fitness maximum (and ESS),thus representing a single resting state. However, in some circumstances the singularity may, counter-intuitively, be a fitness minimum. This means that evolution converges to a point that is less fit than all surrounding strategies. This is surprising to evolutionary biologists who are used to the analogy of evolution as climbing an adaptive landscape, a view that is consistent with the notion of an ESS. However, this

Phenotype Phenotype

Fig. 8.6 Adaptive dynamics leading to evolutionary branching. A lineage normally moves (arrow) over an adaptive evolutionary landscape by climbing to areas of higher fitness (a), eventually reaching an adaptive peak which is fitter than all other local phenotypes. However, when the adaptive peak is reached, the dynamical interactions between the resident genotypes and mutants may allow any alternative new mutant to invade, which means that the resident genotype is now at a fitness minimum (b). Under certain conditions, two divergent phenotypes can then coexist, the start of a polymorphism or even speciation.

Phenotype Phenotype

Fig. 8.6 Adaptive dynamics leading to evolutionary branching. A lineage normally moves (arrow) over an adaptive evolutionary landscape by climbing to areas of higher fitness (a), eventually reaching an adaptive peak which is fitter than all other local phenotypes. However, when the adaptive peak is reached, the dynamical interactions between the resident genotypes and mutants may allow any alternative new mutant to invade, which means that the resident genotype is now at a fitness minimum (b). Under certain conditions, two divergent phenotypes can then coexist, the start of a polymorphism or even speciation.

intuitive notion assumes that the landscape is fixed, at least during the timescales involved in any one study. Adaptive dynamics changes all that. Invasion of new mutants is assessed in relation to the ecological dynamics of mutants and residents, at each step of the adaptive walk. The dynamics of evolution change the ecological dynamics at each step, and the changing ecological dynamics change the local fitness landscape because fitness can be density- or frequency-dependent. Thus, it is perfectly possible for evolution to proceed towards a fitness minimum because the ecological interaction that determines evolution towards that point is different to the interactions that occur once the point is attained. After climbing to what seems like a peak, sometimes the peak sinks and becomes a trough.

What happens in the neighbourhood of a fitness minimum is particularly interesting, for then any alternative mutant strategy is fitter than the resident: in other words, selection is disruptive. This can lead either to a coexisting polymorphism, or even speciation in sympatry. This so-called 'evolutionary branching' can occur repeatedly. Thus, taking into account the population dynamic interactions involved during evolution can lead to very different predictions about the asymptotic state of evolution, which are not only intuitively appealing but seem to offer explanations for observable phenomena. For example, polymorphic strategies are commonly seen in traits that relate to population level phenomena, such as body size, dispersal rate, dormancy rate, and so on. In addition, the degree of evolutionary branching has obvious implications for speciation, and has been implicated as a major source ofsympatric speciation (Dieckmann and Doebeli 1999).

The justification for adaptive dynamics at present is purely logical; it is not yet known to increase predictability. For an evolutionary ecologist, however, there is excitement in exploring the interaction between evolution and population ecology. The perceived need in medical fields, caused by the evolution of disease, has led to the notion of Darwinian medicine (medicine with evolution in mind),and in fishing circles to Darwinian fisheries (fishing with evolution in mind). The next few years should unravel how many areas of population ecology Darwin's name will attach to.

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