Cosmopolitan microbes and their cryptic species

The notion that microbial species have cosmopolitan distribution is discussed for protists in light of the considerable genetic variation within nominal species. It is concluded that there is no compelling evidence to indicate that different genotypes ('cryptic species') have biogeographies or that they originated through allopatric speciation. It is suggested that variation in rRNA gene sequences reflects accumulated neutral mutations which do not necessarily reflect functional diversity. The fundamental value of the species concept is that it provides information on the organism in terms of phenotypic properties.