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Aug 01 2007

Evolution-17: How species diverge

(Please see here for previous posts in this series.)

When my daughter was quite young, about five or so, the question of where people came from came up in a mealtime conversation. Naturally we told her that human beings had evolved from ancestors who were monkey-like and then became human-like. She sat there for a while silently digesting this interesting bit of new information and mulling it over in her mind. It seemed clear that she was not at all disgusted or even bothered by the thought that we were related to the monkey family. That kind of revulsion seems to be something that has to be acquired, often nurtured by religions.

But something was bothering her and she finally articulated it, asking “But when that happened, wouldn’t the mother monkey notice that her child looked different?”

She had hit upon an issue that many skeptics of evolution raise. They argue that there is a contradiction if we assume that we had evolved from an ancestor species that was so different from us that we could not interbreed with that species. Surely, the argument goes, doesn’t evolution imply that if species A slowly evolves into species B, then there must be a time when the parent is of species A while the child is of species B? Isn’t it a ridiculous notion for parent and child to belong to different species?

The answer is that it is perfectly possible that as we go from generation to generation, for each child to be the same species as the parent, while of a different species from a distant ancestor. In fact, we have living examples of such a phenomena,
In The Ancestor’s Take (p. 302), Richard Dawkins uses the example of the ring speciation of the herring gull and lesser black-backed gull to illustrate how this happens. In Britain, these two kinds of birds don’t hybridize even though they meet and even breed alongside one another in mixed colonies. Thus they are considered different species. But he goes on to say:

If you follow the population of herring gulls westward to North America, then on around the world across Siberia and back to Europe again, you notice a curious fact. The ‘herring gulls’, as you move around the pole, gradually become less and less like herring gulls and more and more like lesser black-backed gulls, until it turns out that our Western European lesser black-backed gulls actually are the other end of a ring-shaped continuum which started with herring gulls. At every stage around the ring, the birds are sufficiently similar to their immediate neighbors in the ring to interbreed with them. Until, that is, the ends of the continuum are reached, and the ring bites itself in the tail. The herring gull and the lesser black-backed gull in Europe never interbreed, although they are linked by a continuous series of interbreeding colleagues all the way around the other side of the world.

Dawkins gives a similar example of this kind of ring speciation with salamanders in the Central Valley of California, which is ringed by mountains. If you start with salamanders at one end of the valley and proceed clockwise around the mountain range to the opposite side of the valley, the salamanders change slowly, at each stage being able to interbreed with the neighbors. The same thing is true when you go counterclockwise from the same starting point. But when you arrive at the opposite point of the valley where the two chains of evolution arrived at by going in different senses meet, you find they are now two different species.

The herring gulls and salamanders are the examples separated in space (which we can directly see now) of the same argument separated in time (which we can only infer) that says that as descendants are produced, they form a continuum. Each generation, while differing slightly, can interbreed with its previous generation, but over a long enough period of time, the two end points of the continuum need not be able to interbreed.

Thus it is possible for one species to evolve into another and for an organism to be intermediate between two species.

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