The Evolutionary Significance of the Ecological Niche

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Abstract
As a result of comparative studies on mesic - and xeric -inhabiting isopods it was found that differences in environmental factors in the 2 niches resulted in behavioral and physiological adjustments of the 2 species. Venezillo arizonicus inhabiting a xeric environment characterized by high insolation is physiologically adapted to its habitat. Armadillidium vulgare inhabiting a more diverse mesic environment shows mainly behavioral adaptations. On this basis, it is suggested that each ecological niche is composed of 2 different parts: an inner or "habitual" part, where moderate and constant conditions prevail, and a peripheral part where conditions tend to be more extreme and of shorter duration. It is suggested that natural selection works in the peripheral part of the niche, the animal being already adapted to the conditions in the habitual part. The extreme conditions that prevail in arid habitats are a powerful selective force. The striking similarity in the faunas (especially reptilian) of various desert areas can possibly be explained by the similarity of the selective forces. The result is convergent evolution of unrelated forms due to similar forces acting in the extreme part of the niche.