Genetic drift as cause of evolutionary change

By Branden Holmes (Surroundx) on November 22, 2010

There are two ways in which evolutionary change may occur. The first is of course natural selection, made famous by Darwin in the Origin (he coined the phrase). The second process is random sampling, better known by its popular name genetic drift. Natural selection deals with advantageous and disadvantageous mutations, whereas genetic drift governs how neutral and nearly neutral mutations will be inherited from one generation to the next. However, genetic drift is only an evolutionary force in small populations. This is because it is the random changing of the frequencies of different alleles at a given locus, or position where a gene resides upon a chromosome. For alleles to become fixated (where every individual in the population possesses that particular allele) by coincidence is much more probable given a small initial population because there are less individuals for the allele to spread to.

Neutral (or nearly neutral) mutations can be classed into one of two categories: genotypic or phenotypic. Genotypic mutations are those mutations which are not expressed in the final phenotype, or physical appearance of the organism. This may be for one of three reasons, the first of which is mutations to pseudogenes, which are the remnants of genes which once did something but are now not used. And since they are unused, mutations can accumulate in these areas of the genome. The second type is similar to pseudogenes. These are copies of portions of DNA which are sometimes inadvertently made. And because they specify the same thing as the section of code from where they derived, these extra copies are useless. The third type is synonymous mutations. Because the genetic code is degenerative (different sequences of nucleotide bases can convey the same information) these mutations cannot be "seen" by natural selection because they do not create any new variation.

Phenotypic mutations are those mutations which are expressed in the phenotype but still convey little or no selective advantage or disadvantage. The exact proportion of neutral:advantageous mutations has been in dispute for a long time, with most biologists considering the vast majority to be advantageous. While some such as the late Stephen Jay Gould go so far as to state that non-adaptive TRAITS are common.

As an illustrative example of genetic drift: despite the many criteria that we humans use to select potential mates, eye colour is not among them, or at the most only very rarely. Thus, whatever eye colour our partner has is as good as random. Eye colour thus fluctuates at random within our species. But our population size, nearly seven billion people, is too large for any particular colour to be expunged from the gene pool, or for any particular colour to become fixated. Because genetic drift is not cumulative until it causes an allele to become fixated, it is only classed as evolution (any cumulative genetic change in a population) when such allele's become fixated. The random changing of allele frequencies, such as eye colour, are NOT instances of evolution because they are not cumulative.

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About Branden Holmes
I am an amateur evolutionist interested in the theoretical side of the subject.

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