Difficulty of defining "species" and identifying particular species

The Greenish Warbler demonstrates the concept of a ring species.

It is surprisingly difficult to define the word "species" in a way that applies to all naturally occurring organisms, and the debate among biologists about how to define "species" and how to identify actual species is called the species problem.

Most textbooks follow Ernst Mayr's definition of a species as "groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups".^[5]

Various parts of this definition serve to exclude some unusual or artificial matings:

• Those which occur only in captivity (when the animal's normal mating partners may not be available) or as a result of deliberate human action.
• Animals which may be physically and physiologically capable of mating but do not normally do so in the wild, for various reasons.
• Animals whose offspring are normally sterile.

The typical textbook definition above works well for most multi-celled organisms, but there are several types of situations in which it breaks down:

• By definition it applies only to organisms that reproduce sexually. So it does not work for asexually reproducing single-celled organisms and for the relatively few parthenogenetic multi-celled organisms. The term "phylotype" is often applied to such organisms.
• Biologists frequently do not know whether two morphologically similar groups of organisms are "potentially" capable of interbreeding.
• There is considerable variation in the degree to which hybridization may succeed under natural conditions, or even in the degree to which some organisms use sexual reproduction between individuals to breed.
• In ring species, members of adjacent populations interbreed successfully but members of some non-adjacent populations do not.
• In a few cases it may be physically impossible for animals that are members of the same species to mate. However, these are cases in which human intervention has caused gross morphological changes, and are therefore excluded by the biological species concept.

Horizontal gene transfer makes it even more difficult to define the word "species". There is strong evidence of horizontal gene transfer between very dissimilar groups of prokaryotes, and at least occasionally between dissimilar groups of eukaryotes; and Williamson^[6] argues that there is evidence for it in some crustaceans and echinoderms. All definitions of the word "species" assume that an organism gets all its genes from one or two parents which are very like that organism, but horizontal gene transfer makes that assumption false.