Life Forms of Desert Rodents and Convergent Evolution Among the Species of Different Faunas

Abstract

A life form can be defined as a group of species that share a common life history strategy, are similar in general appearance, and have similar adaptations to a particular environment. A life form should therefore be considered as a set of adaptations to a specific environment, so that a species appertaining to a particular life form is the result of certain evolutionary processes. A classification of life forms should therefore generally coincide with a taxonomic classification because the latter (at least, an ideal one!) is also a reflection of evolution. A typical example is demonstrated by ground squirrels. Desert species of this group are very similar in general appearance, have similar biological features, life history traits and are closely related taxonomically. In contrast, there are some species that originate from different phyletic lineages but which appear to enjoy the same life form; such species are usually aboriginals of different geographic regions and are known as “ecological equivalents” or “ecological analogues” (Cloudsley-Thompson 1996). For example, Mares (1993b) noted a profound similarity between Gerbillus and Perognathus, although the former genus belongs to the Old World Gerbillinae and the latter to the New World Heteromyidae. The existence of ecological equivalents demonstrates that the pathways of adaptation have similar evolutionary directions in similar environments, even though the geographical positions of the milieus and primary gene pools of the specializing groups are quite different (Cloudsley-Thompson 1993,1996).