V. R. Ferris

Evolution, phylogeny and systematics

The recent history, current status, and outlook for understanding the phylogeny and evolutionary relationships of the cyst nematodes as a group comprise the focus of this chapter. Arguments about taxonomic features and taxonomic nomenclature will be discussed only insofar as they are necessary to an understanding of broader issues of phylogeny and evolution. Other taxonomic details can be found in Chapter 4 on Taxonomy and Identification.

Biogeography of soil nematodes

Abstract Biogeography is the study of distributions of organisms, plus an attempt to explain the distributions. Two approaches to explanation of biogeographic patterns may be termed ecological biogeography and historical biogeography. Most nematologists have taken the ecological approach, with a goal of determining why a particular species is restricted to certain areas, and not present in nearby areas. Historical biogeography is based on the premise that present-day patterns of taxa result largely from the history of the taxa and of the areas of the earth in which they have lived. Nematologists generally adhere to the classic view of dispersal in which a center of origin is postulated and long-range dispersal over barriers is invoked. The dispersal mechanism is often assumed to be man himself. Challenges to this approach exist in the form of methods which infer the biogeographic history from phylogeny. Vicariance biogeographers postulate fragmentation of widespread ancestral biotas, resulting from geological, climatic or other disjunctions, and further allopatric speciation among descendant biotas. Distributions amenable to general explanation can be distinguished from those which require unique dispersal events. Biogeography of soil nematodes is hampered by present limitations in systematics, including our inability to determine species limits with certainty.