Evolution of Niche Width

Abstract

A model is developed to predict evolution in a population which contains a variety of ecologically specialized phenotypes. Individuals of each phenotype are assumed to specialize on a specific region of a resource axis present in the environment. The model, incorporating density-dependent effects on the fitness of the various phenotypes, predicts the number of individuals of each phenotype through time for both asexually and sexually reproducing populations. The lizard species, Anolis roquet, illustrates the kind of population treated by the model. The resource axis is prey size; small lizards preferentially utilize small prey, while large lizards exploit large prey. Field data on this species are used to estimate some of the parameters of the model. The model shows that there is an optimum number of individuals of each phenotype for a given set of resources and a given regime of interphenotypic competition. If the population has this optimum distribution of individuals, then the fitnesses of all phenotypes are equal and the largest population size is achieved. Any other distribution yields too many individuals of certain kinds and too few of others, and a smaller total population size. The model is solved analytically to characterize this optimum distribution thoroughly. The model also shows that an asexually reproducing population, provided it has enough genetic variability, attains the optimum distribution very rapidly, while a sexually reproducing population attains the optimum distribution only slowly if at all. As a result, the sexual population often suffers from overcrowding in some phenotypic classes and undercrowding in others, and has a lower population size. The reason a sexual population may not attain the optimum distribution is that there is always a direct relationship between the shape of the distribution of phenotypes in the whole population and the shape of the distribution of phenotypes among the offspring of each mating within the population. In particular, the variance in the population's phenotype distribution is always approximately twice the variance in the offspring phenotype distribution. Therefore, if a sexual population is to attain the optimum population distribution, its offspring distribution must have a certain shape. Natural selection will mold the offspring distribution to this special shape, but presumably this molding takes a very long time. If this is true, a sexual population in a new or changed environment attains the optimum population phenotype distribution much more slowly than an asexual population. The niche width of a population is the length of the interval on the resource axis from which the population obtains most (say 95%) of its resources. There are two components to niche width: the within-phenotype component, which is due to the variety of resources used by each phenotype, and the between-phenotype component, which is due to the population's having a variety of phenotypes. The model summarized above concerns the evolution in asexual and sexual populations of the between-phenotype component of niche width. Also, an expression is derived which partitions niche width into the within-phenotype and between-phenotype components such that the total niche width is simply the sum of measures for these two components. Ecological release is an expansion in niche width by a population which originated in a packed fauna and which has newly colonized an open environment. A population may undergo release in either the within-phenotype or between-phenotype components of niche width or both. Because asexual populations attain the optimum population distribution of phenotypes more rapidly than do sexual populations, asexual ones have a greater potency for release in the between-phenotype component. This claim seems an important explanation for the colonizing ability of asexual species. The process of faunal buildup on an island is a race between a widening of the offspring phenotype distribution of the first species there and dispersion to the island by members of some other ecologically differentiated species. If the widening of the offspring phenotype distribution, and hence the niche width, is slow enough, vacant regions exist on the resource axis which facilitate establishment of emigrants from elsewhere. In plants, an alternative to achieving ecological release by widening the offspring phenotype distribution is to develop self-compatibility.