Genetic Drift
Why are Blue Jays blue? Or, more to the point, why are all Blue Jays blue?
Both of these questions are worth thinking about. First, if natural selection rules, if every aspect of every species results from a superior adaptation, then the blue-ness of the Blue Jay must reflect some superior adaptation. A strict “Darwinist,” and Darwin himself, would have to say that Blue Jays are blue because that is a superior adaptation for the bird’s particular ecological niche. But since other North American songbirds are red, yellow, brown, and black, that seems to be an unlikely proposition.
Second, why aren’t there any red (or any other color) Blue Jays? Why are Blue Jays, and almost all wild species, uniform in color and other specific traits? If blue isn’t necessarily a superior color for a North American songbird (as the evidence would suggest that it isn’t) then why are ALL Blue Jays blue. Why don’t we see varieties of color? Or varieties of other traits?
The answer lies in the concept of ‘genetic drift,’ the tendency of a finite population to become fixed with one character state (color, for example). Notice that we’re talking about a finite population. In an infinite population, another evolutionary concept, the Hardy-Weinberg principle, states that neutral gene variations (alleles) will persist indefinitely.
But all populations are finite. And that’s what accounts for genetic drift. Just by random action (assuming no adaptive advantage), one color, or trait, will eventually crowd out all other variations.
Take a look at the Excel spreadsheet above. Suppose it represents a population of birds. In the first generation of ten individuals, they are all different colors. (Maybe it would be more accurate to think of them as asexually reproducing bacteria. But it doesn’t matter. Sexually reproducing populations are still subject to genetic drift. But a simple, asexual example is easier to envision.)
In the first generation, there is one bird (or bacterium) of each color. In any following generation, each individual can have 0, 1, or 2 children. (In the example spreadsheet, Excel’s random number generator does this.) In Generation 2, the blue, black, and white populations doubled; while, by chance, no colors went to zero (became extinct). Now look at Gen 3. Green, brown, and orange all petered out; once they became extinct, they had no more descendents. Also in Gen. 3, you can see that Blue, Black, and White continued to thrive. This is all just by chance. We assume no evolutionary advantage to any color. You can see the trends. By Gen. 6, Black was over half the total population (8 of 15 individuals) and there were only 5 color variants left. Gens 9 and 10 tell the story, as Blue and Red ewnt extinct, leaving Black as the sole color variant of the species.
For fun, I link here to the Excel spreadsheet itself. I set it to “manual recalculation.” If you open it up, you might not see the exact results shown. Hit the ‘F9′ key to re-calc, and you will see various other possibilities. It’s a random number generator, so it never gives the same result twice. (Hit ‘F9′ repeatedly and look at the results of each re-calc. Sometimes all ten colors become extinct; other times there are still 2 or 3 colors persisting in Gen. 10. It doesn’t matter. You can always see the trend toward consolidation. This is “genetic drift.”)
It’s really a very important concept in evolution. All Blue Jays are blue, not necessarily because blue is a better adaptation for them, but because other color genes got crowded out, via genetic drift. (Wow! That last sentence is such a gross generalization. I hope it is not inaccurately over-simplified.) Genetic drift and natural selection both drive evolution.
Incidentally, Charles Darwin knew nothing of DNA, genetics, or the theory of genetic drift. Thus, the use of the term “Darwinist,” is quite revealing of the speaker’s poor understanding of evolutionary theory. Nonetheless, a riposte like, “Darwinist? Why do you characterize modern evolutionary theory as “Darwinist,” when it encompasses the theory of genetic drift and many other concepts that Darwin did not know?” will get you absolutely nowhere.
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