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Races are defined according to any identifiable characteristic, and also by gene frequencies. "Race differences are relative, not absolute".
A physiological race, or forma specialis is a group of individuals that do not necessarily differ in morphology from other members of the species, but have identifiably different physiology or habits.Parasitic species frequently have races that are adapted to different hosts.
An ecological race is an ecotype, part of a species that is adapted to a different local habitat.
If the races are sufficiently different or if they have been tested to show little genetic connection regardless of phenotype, two or more groups/races can be identified as subspecies or another infraspecific rank, and given a name. According to Ernst W. Mayr, "a subspecies is a geographic race that is sufficiently different taxonomically to be worthy of a separate name."[unreliable source?]
^I. Buddenhagen (2009). "Understanding Strain Diversity in Fusarium oxysporum f. sp. cubense and History of Introduction of 'Tropical Race 4' To Better Manage Banana Production". ISHS Acta Horticulturae828: 193–204.
^ abcWalker, P.M.B., ed. (1988). "Entry for Race". The Wordsworth Dictionary of Science and Technology. W. R. Chambers Ltd. and Cambridge University Press.
^ abcdRieger, R.; Michaelis, A.; Green, M.M. (1968). A glossary of genetics and cytogenetics: Classical and molecular. New York: Springer-Verlag. ISBN9780387076683.
^ abWalker, P.M.B., ed. (1988). "Entry for Physiological Race". The Wordsworth Dictionary of Science and Technology. W. R. Chambers Ltd. and Cambridge University Press.
^Ernst Mayr (1970). Populations, Species, and Evolution: An Abridgment of Animal Species and Evolution. Cambridge, Mass: Belknap Press. ISBN0-674-69013-3.