The diversity of ecological niches even among closely related species is enormous and demands explanation. What is the reason for such diversity? We have already mentioned one of the most important factors, the interspecific competition, which pushes ecological niches of species far away, to avoid niche overlap. More specifically, natural selection prefers such phenotypes of competing species which utilize different resources than those which share them. Competition thus leads to the increase of resource range utilized by a given taxon, and this process is faster when other taxa do not constrain this diversification. Indeed, the increase of the breadth of utilized resources in the course of evolution is fastest in such situations where other taxa with similar requirements are absent. For example, ecomorphological diversification of Galapagos finches and Hawaiian hon-eycreepers has been much faster than the diversification of related taxa on the mainland, where the utilization of new resources was constrained by other taxa already utilizing them. Availability of resources almost always increases diversification rate, indicating the role of interspecific competition for this process.
Interspecific competition is not, however, the only force driving niche diversification. Each species has its own evolutionary history, and thus can adapt to different resources by an independent process of evolutionary optimization, as phenotypes which are more efficient in transforming obtained energy into offspring are favored by natural selection. If there are several mutually exclusive ways to achieve this, it is likely that each species will go by a different route due to evolutionary contingency, and niche diversification will follow without competition. Notably, optimization does not lead to an advantage of the whole species in terms of the resource utilization, but only to an individual advantage regardless of the evolutionary fate of the whole species. As evolution is opportunistic, species can evolve to extremely specialized forms in terms of either habitat utilization or food preference, which is apparently disadvantageous for future species persistence in an everchanging world.
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