Monophyly and Typological species
Most of the definitions of species require that the individual species be monophyletic, i.e. that they be formed in evolution through a unique speciation event. The definition of a typological species, at least in principle, does not entail this limiting requirement. A taxonomist usually considers that a separate species corresponds to a line of organisms that differ from other similar lines in an important phenotype trait. However, what an important phenotype trait consists in is often a matter of the subjective opinion of the scientist. Frequently, a quite inconspicuous trait is chosen, whose presence or absence has some sort of importance for humans. Amongst bacteria, this trait frequently consists in the ability to cause a certain disease or cause a certain symptom of a disease. As the genetic basis for such a trait in the individual lines of organisms need not be very complicated, its occurrence need not be correlated with the overall relatedness of these lines. For example, intestinal bacteria of the closely related species Escherichia coli, which exhibits pathogenic activity for humans, is frequently included under the bacterial genus Shigella. The genus was defined primarily on the basis of its pathogenicity; however, immobility and the inability to cause fermentation of glucose are used as diagnostic traits. In time, it was found that these traits need not always be correlated with the pathogenicity of these bacteria. Consequently, a great many pathogenic strains of Escherichia coli are now known. Molecular taxonomy studies later demonstrated that the genus Shigella is apparently polyphyletic and that its individual lines are variously scattered throughout the phylogenetic (more precisely genealogical) tree of the species E. coli (Pupo, Lan, & Reeves 2000) (Fig. XX.6).
While most modern definitions of species recognize only a monophyly as a species, i.e. a related line of the population derived from a single original parental population, it is almost certain that some biological species are formed polyphyletically, i.e. their members evolve several times independently within various populations. A typical example consists in botanical species formed by polyploidization of some existing diploid species (see XXI.5.2). Polyploid plants exhibit a different phenotype from their diploid ancestors and can also generally not cross with the original species because of differences in the number of chromosomes (however, some species of plants are capable of this across several ploidy levels (Petit, Bretagnolle, & Felber 1999)). However, they are usually capable of productively crossing with independently formed plants with the same ploidity level, i.e. with plants that have the same number of chromosomes. Tetraploids formed by duplication of the genome of a diploidal inter-species cross have relatively the greatest chance of full renewal of fertility. Their chromosomes will most probably not form an aberrant chromosomal arrangement containing tetrades during meiosis, but rather regular chromosomal arrangements containing twice the number of chromosomal pairs compared to the original diploidal species. It is apparent that some polyploids can be formed repeatedly within a species, and can successful reproduce together because of having the same number of chromosomes. As a consequence, they comply with the requirements of the definition of a biological species although, for example, they do not comply with the requirements of the definition of a phylogenetic species.