THE CONCEPT OF THE BIOSPECIES:
a distribution and a dynamic process.
With the publication of the Origin, there has been a fundamental change in our understanding of the meaning of the species. Darwins monumental work provided the final completion of a gateway through which we could step out of the static cosmology of the past into a world that was dynamic, fully alive, and far more real than we have imagined. The idea of immutability of species has been rooted in the essences of Greek philosophy from Aristotle, through St. Thomas Aquinas and the scholastics, and then through the Renaissance Classics, and carried into our time by the most prestigious institutions of learning, such as the University of Oxford in England. But let us see the biological implications of this change in our thinking.
For a long time, the concept of species was static, based on morphological similarities according to a single (monotypic) norm of variation. If an organism did not fit the described set of morphological traits, then it was not of the same species. As to taxonomy, the species was the lowest rank of classification. Variations were recognized but they had to remain within a close range to the monotype. Today, we call this kind of static species concept the Morphospecies. The biospecies, on the other hand, may be defined as follows. Members of the biospecies are represented by actually or potentially interbreeding populations, which are reproductively isolated from other such groups. (Cain, 1954.) In other words, sharing a common gene pool is the key of specific status.
To see how much the morphospecies concept is inadequate to describe the real nature and diversity of life on earth, consider the meaning of biospecies as illustrated by the few examples given here.
The grackles are black, somewhat larger than thrush sized birds with iridescent colors of green, purple, and bronze. The colors are produced by refracted light and are not due to actual pigments in the feathers. In older field guides to the birds the two types, the purple (Quiscalus quiscula) and the bronzed grackles (Quiscalus versicolor) were described as two different species with a third, the Ridgeway's grackle added to the list. The range of the purple grackle extends from north to south on the eastern part of the United States, and that of the bronzed grackle is in an east to west distribution. The two ranges overlap in the northeast where Ridgeway's grackle is found. It turns out that the purple and the bronzed grackles are of the same species, and Ridgeway's grackle is their viable hybrid. The originally two distinct species now are listed as the subspecies, or geographical races of the same polytypic species. It should also be noted that the coloration of the Ridgeway's grackle consists of iridescent bars showing a greater degree of variation than either of the parental purple or bronzed grackles. An increase of variation in the hybrid represents a case of secondary hybridization indicative of some genetic differences between the two subspecies. (In case of primary hybridization the intergrading is smooth, without any increase in variation in the hybrid. This indicates a close genetic similarity. Interbreeding between human races is characterized by features of primary hybridization.) The example of grackles shows that a species can be polytypic.
The leopard frog (Rana pipiens) is distributed from north to south in the eastern part of the United States. The vernacular name refers to the coloration of the frog which is green with yellowish spots, while the scientific name refers to the sound of the song they produce. Being an amphibian, the leopard frog is bound to many isolated pockets of water with a somewhat reduced rate of migration between local populations. This fact allows for genetic diversification to develop between different localities. It is interesting to see that adjacent populations show the features of primary hybridization along the entire length of the distribution, but if very distant members of the distribution are brought together artificially, they may not even interbreed at all. The geographical extremes of the distribution are reproductively isolated as if they were not members of the same species. The explanation is that genetic differences between adjacent populations are small, but they do add up through distance to a sufficient degree to result in hybrid sterility. We look upon the extreme forms of distribution as belonging to two semispecies of the same species. The concept reflects the dynamic nature of the species. It is a process term, not like the static morphospecies.
We find four different species of rosella parrots in Australia. Platicercus venustus lives in the northwest, P. icterotis in the southwest, P. adscitus in the northeast, and P. eximius in the south east, including Tasmania. The ranges of these four species of rosella parrots are distinct except for the ranges of P. adscitus and P. eximius which overlap. They, however, do not interbreed in this sympatric area but behave as two distinct species. The others do not interbreed because of geographical isolation separated by deserts. Relying on morphological similarities, it may be concluded that the four rosella parrot species are at the end of a speciation process, but based upon morphological similarities, they may be classified into one Superspecies, the first taxonomic rank above the species.
The fruit flies, Drosophila pseudoobscura and D. persimilis are called sibling species because they are close to identical in outward appearance and yet they are reproductively isolated in sympatric areas. The isolating mechanism is most likely to be physiological and behavioral. There are many examples of sibling species among fruit flies, notably in the D. willistoni group. Other well studied examples are among the mosquitoes in the Anopheles maculipennis and A. gambiae groups.
Considering the above examples, it may be concluded that the rank, species,
is a process term, it is polytypic, dynamic, and is not the lowest taxonomic
rank because the processes of speciation require such terms as subspecies or
geographical races, semispecies, and sibling species, marking the end of speciation
by the term superspecies. We call this species concept the biospecies. The primary
criterion of specific status is this matter of interbreeding, the sharing of
a common gene pool, and the ability to produce viable hybrids. In an attempt
to describe the members of a biospecies, we do not consider morphology alone
but everything living organisms have from genetics to behavior. The other side
of the coin of sharing genes is reproductive isolation. The gradual development
of such isolation, most commonly through geographical distance, is part of the
dynamism of the speciation process.
A.J. Cain. Animal Species and their Evolution. Hutchinsons University Library, London. 1954.
Ernst Mayr. Animal Species and Evolution. Harvard University Press. 1965.
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