Evolution of Niche Width

Progressive specialization, that is, narrowing of niche width in the course of evolution, is forced by interspecific competition and intraspecific optimalization, and thus represents an expected evolutionary trend. The opposite process, that is, an extension of niche width, is observed mostly after entering a new environment without competitors, allowing utilization of a wider spectrum of resources. This process is called 'ecological release' and may be underlined both by the extension in within- and between-phenotype component of species ecological variation (importance of the two contributing modes vary widely among species). Species niches can widen also because of 'phenotypic plasticity' (heritable genotype-environment interactions directing the trait in the early ontogenesis), and can vary even purely behaviorally, as an immediate response to an altered resources or species structure.

Although sometimes there is an obvious constraint on expanding a species' niche - for example, physiological constraints like freezing of body fluids or presence of a competing species - we often see no apparent reason why species niches stay restricted to a fraction of a resource which continuously varies in space. One possibility is that gene flow from the central large population adapted to average conditions restricts adaptation to marginal conditions: the alleles neutral or nearly neutral in the main population which are deleterious in the marginal populations will sweep through the small marginal population, thus preventing the adaptation. The argument, however, does not extend to a population on a continuous gradient, as it requires significant asymmetry in frequency and quality of the habitats. Often we actually do not have a good understanding of why a species' realized niche (and consequently its geographic range) stays limited without adaptive response to the environmental variation. Constraints on genetic variance or genetic drift leading to a weaker response to selection represent possible causes.

On the other hand, asexual reproduction or self-fertilization can provide an advantage in adapting to marginal conditions - both because small populations are still viable (as there is no need to find a mating partner) and because gene flow does not restrict adaptation to marginal conditions. Indeed, it is found in many plants and animals adapting to extreme, marginal habitats (classic animal examples are Daphnia pulex or freshwater snail Campeloma). However, although lack of recombination in asexuals means that locally favorable gene combinations are maintained, adaptive evolution in asexual species is significantly slowed down as beneficial combinations have to arise in each strain independently. It appears that high levels of, but not obligatory, self-fertilization or asexual reproduction (parthenogenesis and vegetative reproduction) are commonly advantageous for adaptation to marginal habitats.

In some cases, we observe an apparent regularity in the evolution of niche width and position. The classical example represents cycles of species dispersal, specialization, and local adaptation (and eventual extinction) observed on various archipelagoes, called 'taxon cycles'. They were originally described by Edward O. Wilson on Melanesian ants, but were best documented by Robert Ricklefs and his co-workers on Caribbean birds. In the first stage, an immigrant, which is mostly a species with a high dispersive ability, colonizes coastal or disturbed areas. Then the species spreads across the island, adapting to the new resources and expanding its niche (quite likely as a consequence of a release from competitors, predators, and parasites). In the next step, the species becomes more specialized, and its distribution becomes spottier. The narrowing of its niche may be driven by an immediate advantage of an adaptation to a local resource or immigration of new generalist competitor. Finally, species distribution becomes very fragmented, ending in local endemism, and ultimately extinction.

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