Species, Concepts Of

cladistic A classification based entirely on monophyletic taxonomic groupings within a phylogeny; taxonomic units that are paraphyletic or polyphyletic are rejected. A ‘‘cladist’’ is one who practices ‘‘cladistics,’’ usually in the sense of using parsimony to adjudicate between data from multiple characters in the construction of a ‘‘cladogram,’’ which is an estimate of the true phylogeny. cohesion The sum total of forces or systems that hold a species together. The term is used especially in the interbreeding and cohesion species concepts. Cohesion mechanisms include isolating mechanisms in sexual species as well as ‘‘stabilizing’’ ecological selection, which may cause cohesion even within asexual lineages. disruptive selection Selection acting to preserve extreme phenotypes in a population. Speciation usually involves disruptive selection, because intermediates (hybrids between incipient species) are disfavored (see also stabilizing selection). DNA bar coding A means of delimiting species via DNA sequence clustering, usually from mitochondrial DNA. gene flow Movement of genes between populations, usually via immigration and mating of whole genotypes, but sometimes single genes may undergo horizontal gene transfer via transfection by microorganisms. gene pool The sum total of the genetic variation within a reproductively isolated species population; this term is mostly used by supporters of the interbreeding species concept. genomic cluster A synonym for genotypic cluster. genotypic cluster In a local area, a single genotypic cluster (or species) is recognized if there is a single group of individuals recognizable on the basis of multiple, unlinked inherited characters or genetic markers. A pair of such genotypic clusters (or species) is recognizable if the frequency distribution of genotypes is bimodal. Within each genotypic cluster in a local region, allele frequencies will conform to Hardy–Weinberg equilibrium, and the different unlinked loci will be in approximate linkage equilibrium. The presence of more than one species or genotypic cluster can then be inferred if the distribution of genotypes is bimodal or multimodal, and strong heterozygote deficits and linkage disequilibria are evident between the clusters. isolating mechanisms The sum total of all types of factors that prevent gene flow between species, including premating mechanisms (mate choice), and postmating mechanisms (hybrid sterility and inviability). Modern authors deny that these ‘‘mechanisms’’ have necessarily evolved to preserve the species’ integrity as originally assumed, though this may sometimes be the case in reinforcement of