On one view, species are considered to be classes of things, with particular individuals constituting the elements of the class.
On another view, species are considered as
things in the proper sense, ontological entities composed of an aggregate of individual organisms cohering together into a unified whole. A number of factors can affect the cohesion of a group of organisms into a unified thing. The most often cited include shared environmental resources and restrictions (ecological species concept) and shared behavioral traits (mate recognition species concept, for example). However, the most universally recognized mechanism of coherence is the ability to interbreed and produce fertile offspring. Organisms with the capacity to interbreed are cemented together into a single unit of evolutionary significance by shared access to a common pool of genetic resources.
A shared gene pool is at the heart of Ernst Mayr's biological species concept. "
Species are groups of interbreeding natural populations that are reproductively isolated from other such groups"
(Mayr, E. 1969. Principles of Systematic Zoology. New York, McGraw-Hill.)
This formulation is problematic, because populations never interbreed with anything. Individual organisms breed with other individual organisms; populations are regionally situated groups of such interbreeding individuals. Mayr's point is clear enough, but when reformulated to address individuals, an interesting phenomenon arises.
Aggregates of individuals cohere together into a species by the ability to interbreed, but the strength of this 'cement' that bonds individuals into single units of evolutionary significance varies from one kind of organism to another.
The strongest case of cohesion through interbreeding can be found in organisms which exhibit a single mating type. Corn plants are a good example. Every corn plant is hermaphroditic, producing both male and female flowers, both ova and pollen. As such, every individual corn plant is capable of producing fertile offspring by breeding with every other corn plant. Coherence is maximized by the potential for recombination of genetic information from any one individual with that of every other member of the cohesive group.
The coherence afforded by interbreeding is slightly weaker in Cannabis plants. Cannabis exhibits two mating types: male and female, but all female plants possess the ability to turn hermaphrodite. When female plants are stressed by poor environmental conditions, they stop making female flowers and begin to bud male flowers. Rather than producing seeds for distribution in the stressful environment, female plants adopt the evolutionary strategy of a male and release their pollen to the wind in hopes of fertilizing another female living in a spot more suitable for dropping seeds. Due to hermaphroditism in females, every female cannabis plant is capable of mating with every other cannabis plant, both male and female. Males, on the other hand, do not exhibit hermaphroditism, so they are only capable of mating with females. Consequently, cohesion by mating compatibility is weaker in cannabis than in corn.
Humans also have two mating types, but in humans and other bisexual animals inter-fertility is restricted to male -female crosses; same sex parings are always infertile. Each member of the cohesive group is capable of recombining his or her genes with only about half of the other members of the group. As such, coherence though potential inter-breeding is weaker in humans than in both corn and cannabis.
Mushrooms, which exhibit varying numbers of mating types depending on the species, have more complex patterns of mating compatibility. Some mushrooms have one mating type, some have two, and some have many thousands.
Schizophyllum commune, for example, has over twenty thousand mating types. A common mating configuration is found in
Psilocybe cubensis , which has four mating types: A, B, C,and D. Individuals of type A are compatible with those of type B, C, and D, but not with other individuals of type A. Type B is compatible with C, D, and A, but not with other Bs, and so on. Because each
P. cubensis nucleus is compatible with approximately 75% of all other
P. cubensis nuclei, coherence through mating compatibility is stronger in these mushrooms than in humans, but not as strong as in corn plants, which exhibit 100% mating compatibility.
I bring up these examples not for the sake of philosophical nitpicking. Mayr was certainly aware that different kinds of living things have different numbers of mating types, and he mentions these concerns explicitly in a number of places. The point of these four examples is that a lack of mating compatibility does not provide sufficient grounds for claiming that two individuals are different species. In all four examples, a common gene pool is maintained, even though individuals may not have immediate access to all regions of the pool due to an inability to mate with conspecific individuals of incompatible mating types. Mating compatibility provides the strong glue that cements each group of individuals into a single unit of evolutionary significance, but the coherence can be stronger or weaker depending upon the configurations of mating compatibility exhibited by particular species.
Having noted the variation in coherence effected by different configurations of mating types, let us return to Mayr's definition with a bit of charity and understand him as saying:
If individuals from population A exhibit inter-fertility with compatible mating-type individuals from population B, then populations A and B are conspecific. If individuals from population A are not capable of successfully interbreeding with any individuals from population B, then the two populations represent different species. This formulation avoids the criticism presented above, but it fails to describe some very interesting cases of cohesion through shared genetic resources.
Ring species occur when multiple interbreeding populations are stretched out in lines, usually along a geographic feature like the coast of an island or the ridge-line of a mountain range. The phenomenon has been observed in both amphibians and birds.
Moritz, C., C. J. Schneider, et al. (1992) "Evolutionary relationships within the Ensatina eschscholtzii complex confirm the ring species interpretation." Systematic Biology 41: 273-291.Alström, Per (2006): Species concepts and their application: insights from the genera Seicercus and Phylloscopus. Acta Zoologica Sinica 52(Supplement): 429-434.A simplified fictitious example will help to explain the cohesion among interbreeding populations within a ring species. Imagine a square island with four hermit crab populations, each occupying one side of the island. Researchers studying the crabs find that members of population A on the north side of the island are reproductively compatible with members of populations B and D on the east and west sides, but not with crabs from population C on the opposite side of the island. Further mating studies show that crabs from population B on the east side are compatible with individuals from population A to the north and with crabs from population C the south, but not with those from population D on the island's west side. Each population on the island is found to be reproductively compatible with adjacent populations, but not with the population furthest from it on the opposite side of the island. If one uses mating compatibility as the sole criterion for demarcating species, then she must consider populations A and B to be conspecific, and she must further consider populations B and C to be conspecific. However, populations A and C must be considered different species due to a lack reproductive compatibility. This generates a paradox, because one is forced to say that individual crabs in population B are members of two different species!
The paradox can be avoided by considering all four populations of hermit crabs to be members of a single para-coherent group. All four populations share a common gene pool, even though every population in the group is reproductively isolated from some other population in the group. In the short term, DNA from population A is prevented from recombining with DNA from population C due to mating incompatibility, but the barrier to recombination is temporary. Genes can still migrate from population A to population C, but they must do so via the doubly compatible crabs in geographically intermediate population B.
Human rationality tends to operate within a system of bivalent logic, so we create categories that correspond to this black and white kind of thinking. It seems, however, that nature is oblivious to our rational demand for clear cut species categories. The best we can do is carefully observe the evidence nature presents and adjust our logical categories accordingly.
Edited for clarity 08JAN09 - Dan Molter
Edit 14JAN09 - Dan Molter
Title changed to "Biological Species: a Paracoherent Concept"
