Other single species models

Most other models considering the evolution of the fundamental niche have been explicitly genetic (reviewed in Hedrick 1986; Futuyma and Moreno 1988). In general, they consider quite simple genetics, such as one locus and two alleles, each giving higher fitness in one habitat than the other. They aim in general to predict under what circumstances the population will become fixed for one allele (a specialist population) or retain both (a generalist population), the former case being a 'monomorphism', and the latter a 'polymorphism'. Of course for those interested in specialization, the circumstances favouring monomorphism are the subject of interest. The same models, however, are interesting to another group of scientists: those interested in speciation. They are interested in the conditions that favour polymorphism (Chapter 1). The reason is that once a polymorphism has arisen, with some individuals in a population favouring one habitat and others favouring another, that might eventually lead to the generalist species splitting into two specialist species (Maynard Smith 1966). We will consider a case where speciation is likely to occur in this chapter, and another where speciation already has occurred in Chapter 12.

The predictions of the one locus genetic models are also quite intuitive: if there is temporal turnover in the habitats, a stable polymorphism can arise. This is most people's intuitive reasoning for the advantages of generalism: that it allows an organism to survive changes to its habitat. If, however, there is spatial variation in the distribution of habitats, which is normal, then whether a polymorphism evolves depends on how fitness is affected by population density. If the density of the entire population affects fitness, regardless of which habitat they are in (known as 'hard' selection), specialism is the normal outcome. If, however, it is the density of individuals in each habitat patch that affects fitness (known as 'soft' selection), then a polymorphism can be stable. The reason is again intuitive, since the fitness of each genotype will be frequency dependent, favouring individuals that exploit unoccupied habitats, just as in the ideal-free-distribution model (Chapter 8).

Trade-offs in performance (or preference) in the different habitats are central to all the above models: they assume that a jack-of-all-trades is master of none'. Recently, however, attention has turned to models that do not rely on this assumption. The stimulus, as we shall see later, comes from the rather weak empirical support for the existence of fitness trade-offs. For example, Whitlock (1996) imagined competition between a specialist and a generalist, whose fitness was initially the same in the specialist habitat. However, because it experiences only one habitat, the specialist would more rapidly accumulate alleles beneficial in that habitat due to increased strength of selection. Conversely there would be lower accumulation of deleterious alleles. Thus, the specialist's fitness rises faster than the generalist's in that habitat, such that it becomes the superior competitor over time. Specialization thus enhances the speed of adaptation and reduces genetic load: the same long-term advantages invoked for the maintenance of sex (Chapter 2). A number of other models have since explored specialization without trade-offs (reviewed in Futuyma 2001).All differ from the trade-off models in concentrating more on the constraints imposed by the evolutionary process itself.

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