How Science Works: Evolution by R. John Ellis

Author:R. John Ellis
Language: eng
Format: epub
Publisher: Springer Netherlands, Dordrecht

Genetic Drift

Darwin thought that natural selection was the main cause of evolution but since his time another mechanism has been discovered, called genetic drift. This process is defined as the change in gene frequency between generations caused by random sampling effects. The word “drift” is unfortunate because it could be interpreted to imply direction, but the effect is purely random, unlike natural selection, which is highly non-random.

Suppose two men are in the forest, collecting wood. One man has a set of genes conferring high evolutionary fitness i.e. he is capable in principle of fathering six children, but has fathered only one so far. The other man is less fit, because his genes do not result in him being able to pass so many copies of his genes to the next generation and he has no children. Suddenly a storm blows up and a tree falls on the first man, killing him. The second man goes on to father two children, so the genetic composition of the next generation is different from what it would have been if the first man had survived to father five more children. A purely random event has changed the genetic composition of the population produced by these two men.

Biologists continue to argue about the relative effects of natural selection and genetic drift in evolution, but what is agreed is that genetic drift will be more important in small populations than in large populations. A human example of genetic drift involves a small group of people that crossed the Bering Strait between America and Russia about 10,000 years ago, at the end of the last Ice Age. This group gave rise to the Native Americans that live in the USA and South America today. The observation that these Native Americans almost totally lack the gene for the protein determining blood group B suggests that this founding group was very small in number because about 16% of the entire world’s population of humans possesses this blood group. This type of genetic drift is called the founder effect for obvious reasons, and is commonly seen in island species that are descended from a group of organisms too small in number to contain a representative sample of all the genes present in that species on the mainland.

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