Let's make a brief digression here, because it's important to appreciate that natural selection isn't the only process of evolutionary change. Most biologists define evolution as a change in the proportion of alleles (different forms of a gene) in a population. As the frequency of "light-color" forms of the Agouti gene increases in a mouse population, for example, the population and its coat color evolve. But such change can happen in other ways, too. Every individual has two copies of each gene, which can be identical or different. Every time sexual reproduction occurs, one member of each pair of genes from a parent makes it into the offspring, along with one from the other parent. It's a toss-up which one of each parent's pair gets to the next generation. If you have an AB blood type, for example (one "A" allele and one "B" allele), and produce only one child, there's only a 50 percent chance it will get your A allele, and a 50 percent chance it gets the B allele. In a one-child family, it's a certainty that one of your alleles will be lost. The upshot is that, every generation, the genes of parents take part in a lottery whose prize is representation in the next generation. Because the number of offspring is finite, the frequencies of the genes present in the offspring won't be present in exactly the same frequencies as in their parents. This "sampling" of genes is precisely like tossing a coin. Although there is a 50 percent chance of getting heads on any given toss, if you make only a few tosses there is a substantial chance that you'll deviate from this expectation (in four tosses, for example, you have a 12 percent chance of getting either all heads or all tails). And so, especially in small populations, the proportion of different alleles can change over time entirely by chance. And new mutations may enter the fray and themselves rise or fall in frequency due to this random sampling. Eventually the resulting "random walk" can even cause genes to become fixed in the population (that is, rise to 100 percent frequency) or, alternatively, get completely lost.
Such random change in the frequency of genes over time is called genetic drift. It is a legitimate type of evolution, since it involves changes in the frequencies of alleles over time, but it doesn't arise from natural selection. One example of evolution by drift maybe the unusual frequencies of blood types (as in the ABO system) in the Old Order Amish and Dunker religious communities in America. These are small, isolated religious groups whose members intermarry—just the right circumstances for rapid evolution by genetic drift.
Accidents of sampling can also happen when a population is founded by just a few immigrants, as occurs when individuals colonize an island or a new area. The almost complete absence of genes producing the B blood type in Native American populations, for example, may reflect the loss of this gene in a small population of humans that colonized North America from Asia around 12,000 years ago.
Both drift and natural selection produce the genetic change that we recognize as evolution. But there's an important difference. Drift is a random process, while selection is the antithesis of randomness. Genetic drift can change the frequencies of alleles regardless of how useful they are to their carrier. Selection, on the other hand, always gets rid of harmful alleles and raises the frequencies of beneficial ones.
As a purely random process, genetic drift can't cause the evolution of adaptations. It could never build a wing or an eye. That takes nonrandom natural selection. What drift can do is cause the evolution of features that are neither useful nor harmful to the organism. Ever prescient, Darwin himself broached this idea in The Origin:
This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection. Variations neither useful nor injurious would not be affected by natural selection, and would be left as a fluctuating element, as perhaps we see in the species called polymorphic.
In fact, genetic drift is not only powerless to create adaptations, but can actually overpower natural selection. Especially in small populations, the sampling effect can be so large that it raises the frequency of harmful genes even though selection is working in the opposite direction. This is almost certainly why we see a high incidence of genetically based diseases in isolated human communities, including Gaucher's disease in northern Swedes, Tay-Sachs in the Cajuns of Louisiana, and retinitis pigmentosa in the inhabitants of the island of Tristan da Cunha.
Because certain variations in DNA or protein sequence may be, as Darwin put it, "neither useful nor injurious" (or "neutral" as we now call them), such variants are especially liable to evolve by drift. For example, some mutations in a gene don't affect the sequence of the protein that it produces, and so don't change the fitness of its carrier. The same goes for mutations in nonfunctioning pseudogenes—old wrecks of genes still kicking around in the genome. Any mutations in these genes have no effect on the organism, and therefore can evolve only by genetic drift.
Many aspects of molecular evolution, then, such as certain changes in DNA sequence, may reflect drift rather then selection. It's also possible that many externally visible features of organisms could evolve via drift, especially if they don't affect reproduction. The diverse shapes of leaves of different tree species—like the differences between oak and maple leaves—were once suggested to be "neutral" traits that evolved by genetic drift. But it's hard to prove that a trait has absolutely no selective advantage. Even a tiny advantage, so small as to be unmeasurable or unobserv-able by biologists in real time, can lead to important evolutionary change over eons.
The relative importance of genetic drift versus selection in evolution remains a topic of hot debate among biologists. Every time we see an obvious adaptation, like the camel's hump or the lion's fangs, we clearly see evidence for selection. But features whose evolution we don't understand may reflect only our ignorance rather than genetic drift. Nevertheless, we know that genetic drift must occur, because in any population of finite size there are always sampling effects during reproduction. And drift has probably played a substantial role in the evolution of small populations, although we can't point to more than a few examples.
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