Wednesday, June 22, 2011

WEIT: What Makes a Species

The seventh chapter of Why Evolution Is True delves deeply into the nuances of the biological species concept, or BSC. This idea is fundamental to evolutionary biology, and was defined by Ernst Mayr in 1942 as follows:
"A group of interbreeding natural populations that are reproductively isolated from other such groups."
By "reproductively isolated," we mean that individuals in two different species do not generally produce viable offspring. Coyne points out that there's no obvious reason that life should be divided into discrete species—instead it could have all blurred together, so that every point between, say, "cat" and "dog" was equally represented. While "God did it" would be equally compatible with these or any set of circumstances, evolution explains our specific observations perfectly.

In order for geographic speciation (the most common speciation process) to occur, a reproductive barrier such as a mountain range or a body of water must come between two groups of the same species. Once the groups are separated, genes cannot flow between them, so they start to become genetically distinct. This occurs largely due to natural selection as the groups adapt to differing environments, although the "random walk" produced by genetic drift can play a role as well. When the groups diverge far enough, the genomes of one group will be incompatible with the genomes of the other, so they remain distinct species even if the reproductive barrier vanishes.

We can use the principles in the above paragraph to make predictions that we can then go out and test. Lo and behold, what we find matches up precisely with what evolution predicts. First, we should be able to create reproductive barriers in the laboratory. And we do: groups of flies placed in different environments lose the ability to interbreed with other groups, sometimes in a year or less. One of Coyne's own experiments also showed that the more the DNA of two existing fly species differ, the more mating discrimination they exhibit and more likely their offspring are to be sterile.

The second prediction comes from biogeography: species that have similar genes should be found relatively close to one another, but separated by a geographical barrier. And they are: there are seven corresponding species of snapping shrimp on each side of the Isthmus of Panama (which arose to divide the Pacific and Atlantic 3 million years ago). The same holds true for islands: we don't usually find similar species on the same small island because there's not enough isolation, but we do find them on nearby islands in an archipelago. We see this in flightless crickets, Drosophila flies, and lobelia plants. Coyne points out that this pattern is also evidence against creationism:
"After all, there's no obvious reason why a creator would produce similar species of birds or lizards on continents but not on isolated islands."
Third, we should be able to observe reproductive barriers forming and speciation occurring in the wild, albeit very slowly. The example Coyne gives is the orchid Satyrium hallackii, which for now is classified as one species. In northern South Africa they have long nectar tubes that allow only hawkmoths and long-tongued flies to pollinate them, while they have short nectar tubes on the coast, where bees are the only available pollinators. The two groups are genetically isolated, and would probably remain so even if they lived in the same area.

Satyrium carneum, a close relative of the orchid Coyne mentions
Coyne ends by covering two more complex types of speciation, allopolyploidy and autopolyploidy, which involve the accidental duplication of chromosomes. Unlike normal speciation, these processes occur mostly in plants and can result in new species in just two generations. Polyploid speciation allows us to directly observe entire speciation events in the wild, which normally take too long for this to be feasible (although a certain mosquito species is a notable exception).

What we've seen here is a powerful illustration of evolution's explanatory power and scope. The biological species concept as applied to evolution allows us to make predictions with stunning accuracy. In contrast, the creationist concept of "kinds" or "baramins" is poorly defined and grants us no useful insights into the natural world. This is what we mean when we say that evolution is science, while creationism is nothing more than Bible-based wishful thinking.

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