Speciation

Speciation is the formation of one or more descendant species from an ancestral species. Species are here understood to be groups of interbreeding organisms whose members are generally incapable of breeding with members of other such groups.

Speciation is a crucial aspect of the evolutionary process. Little progress was made in understanding speciation as long as it was felt that species evolve naturally as time goes by, in a gradual progressive manner, through simple natural selection. The feeling ever since Darwin was that, were the fossil record complete and fully studied, the nature of the evolutionary process is such that it would be impossible to draw lines between ancestral and descendant species (the notion of "phyletic gradualism"; see also Punctuated Equilibria).

However, the discreteness of species finally earned the serious attention of evolutionary biologists. Geneticist Theodosius Dobzhan-sky developed the concept of "isolating mech-anisms"—that is, a list of factors that would interrupt the reproductive process between different groups within the same species, a process he felt was the necessary first step toward forming one or more descendant species from an ancestral species.

Biologists draw a distinction between "sym-patric" species (species living in the same place at the same time), and "allopatric" species—contemporaneous species that live in different places. Controversy has raged for years on whether or not it is possible to disrupt reproductive connections sufficiently in sym-patry so that two fully fledged species could evolve out of a single species. The consensus for most of the twentieth century was "no"— although examples involving parasites who live on obligate hosts are among some of the categories of sympatric speciation that have begun to gain acceptance in recent years.

Allopatric speciation remains the central part of speciation theory—simply because all biologists agree that separation of parts of a single species into two or more regions such that contact is rare or impossible gives a head start to the process of establishing two sets of populations that can no longer interbreed. The further idea is that plant or animal species will evolve in isolation so that enough genetic differences accumulate that, should the geographic barriers ever break down and members of both sets of isolated populations come back into contact with each other, they will either (1) attempt to mate and be successful—forming hybrids that are perfectly normal—in which case speciation has not occurred; or they will (2) breed successfully, but the hybrids (generally recognizable as such) form a thin line along a zone of contact; whether or not full hybridization eventually occurs (and, again, speciation fails), or whether the hybrid zone eventually disappears (leaving two different, noninterbreeding species) depends on future events, and it is not certain in such instances whether speciation will succeed or fail; or they will (3) attempt to mate, but their offspring will not be viable (hardy); or they will (4) attempt to mate, but without any offspring at all, as too many genetic differences will have accumulated to allow fertilization to occur; or (5) they won't even recognize each other as appropriate mating partners in the first place. Possibilities 3 and 4 fall into Dobzhansky's "post-mating isolation" categories, while possibility 5 is an example of "premating" isolation— considered an even stronger form of isolating mechanism in the speciation process.

Indeed, some biologists have pointed out that selection can simply change the normal mate recognition signals (whether visual, vocal, or chemical) when populations are physically isolated—just to keep mating going on within each of the separated populations. If these signaling systems change enough, should contact ever reoccur, the members of the two groups will simply not recognize each other as prospective mates at all.

Many paleontologists have, in recent years, come to the more controversial conclusion that much if not most anatomical change in evolution occurs during speciation events, and, further, that speciation events tend to occur in bunches within single regional ecosys-tems—usually following an episode of extinction of many species from physical environmental factors (see Evolution).

In any case, it is known that speciation can be exceedingly rapid. For example, Lake Victoria, home to hundreds of species of (now severely endangered) cichlid fishes, is known to have dried up almost completely only some 12,000 years ago. It is thought that the fishes living in the lake prior to the drying event all became extinct—with surviving species coming down to replenish the lake's ecosystem from the tributaries that empty into Victoria's basin. Thus biologists are forced to conclude that most of the hundreds of species now living in Lake Victoria evolved from a relatively few ancestral fish species in the scant time of 10,000 to 12,000 years! Such species "flocks" have always proven difficult to understand in terms of standard allopatric speciation theory: how does reproductive isolation occur within a single lake?

It turns out that many of the fish species currently living in Lake Victoria breed only in particular places; for example, some need a pebbly bottom, which occurs only sporadically around the margin of the lake. This has led some biologists to suggest that these local breeding populations, with the fish so closely tied to relatively small patches of substrate, are in fact miniversions of allopatric environ ments, where physical isolation of even a few thousand years is sufficient for behavioral, anatomical, and genetic divergence—and therefore true speciation—to occur. Other biologists prefer to imagine isolation occurring in the tributaries themselves, with periodic extinctions weeding out species living in the lakes, and new species evolving outside the system and periodically invading the waters of the lake. Other lakes of the great East African Rift System have similar flocks of cichlid fishes—and the mystery of their speciation patterns has yet to be fully resolved.

Finally, it should be noted that botanists, in particular, have suggested additional ways that speciation can occur. Polyploidy, a genetic condition much more common in plants than in animals, can result when two related species hybridize with one another. The number of chromosomes in cases of polyploidy doubles (that is, rather than having the chromosomes match up as in normal sexual reproduction); the offspring, with twice the number of chromosomes of either parent, can mate only with other polyploid individuals produced the same way. But they can, then, mate perfectly well with one another—yielding an example of speciation through hybridization.

—Niles Eldredge

See also: Evolution; Evolutionary Biodiversity; Natural Selection; Punctuated Equilibria; Species

Bibliography

Eldredge, Niles. 1999. The Pattern of Evolution. New York: W. H. Freeman; Futuyma, Douglas J. 1997. Evolutionary Biology. Sunderland, MA: Sinauer; May-nard Smith, John. 1993. The Theory of Evolution. Cambridge: Cambridge University Press; Mayr, Ernst. 2001. What Evolution Is. New York: Basic Books.

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