Genetic Drift Defined

A main principle of evolution by natural selection is that the environment favors traits that make individual organisms more fit (better able to get their genes into the next generation). More-fit parents produce offspring who, by virtue of having inherited traits from their parents, are more fit. The most fit of these offspring produce offspring who are even more fit than they, and so on and so forth until . . . well, you get the picture. These changes are not random. More-fit characteristics increase in frequency in subsequent generations at the expense of less fit characteristics.

Here's the rub: Natural selection isn't the only force that determines what genes get into the next generation. It can be doing its thing, stacking the cards in favor of certain characteristics, and then — bam! — a tree falls on the genetically favored organism. In the words of the philosopher Dylan, blame it on a simple twist of fate. If that tree happens to fall on a fast cheetah, there'll be just a few more gazelles for the slower cheetahs to eat and they may leave more descendents than they otherwise would have.

When random processes affect the probability of different traits being present in the next generation, you've got what evolutionary biologists call genetic drift. And it's one of the two major driving forces changing the frequencies of existing genes through time (natural selection being the other). As such, it can

1 Affect whether the frequencies of different alleles increase or decrease in subsequent generations.

1 Result in the frequencies of all but one allele at a given locus decreasing to the point that they are eliminated. When only a single allele is found at a given locus, that allele is fixed; its frequency has gone to a hundred percent and all the other alleles are gone. Until such time as a mutation happens to generate a new allele at this locus, all individuals in the population will be genetically identical at this point in their DNA. No further evolution is possible because, until such a mutation occurs, there is no variation.

The key to understanding genetic drift is to understand what could possibly be random in evolution (quite a bit, actually) and when these random events are evolutionarily significant (sometimes).

¿jjjW^ Natural selection and genetic drift aren't either/or processes. Think of the processes as happening simultaneously and the circumstances (such as population size or the neutrality of a mutation, as explained in later sections) determining which process holds more sway.

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