Drift will influence every allele, even those that are being naturally selected. Its effect is also greater on an allele with a frequency far from one half. The effect of drift on frequencies is greater the smaller a population is. This simplification is not very biological, but it can be shown that real populations behave this way. This could take a very long time for a large population. The frequency in each generation will drift up and down on what is known as a random walk until at one point either all A or all a are chosen and that allele is fixed from that point on. A coin will no longer work to choose the next generation (because the odds are no longer one half for each allele). Thus, the allele frequencies have changed and evolution has occurred. There might be six of one and four of the other, or some different set of frequencies. It is unlikely that the next generation will have exactly half of each allele. Choose that generation randomly by flipping a coin ten times and let heads be A and tails be a. In a stable population, the next generation will also have ten individuals. Imagine a population of ten individuals, half with allele A and half with allele a (the individuals are haploid). In a population of 100, that 1 individual represents only 1 percent of the overall gene pool therefore, it has much less impact on the population’s genetic structure and is unlikely to remove all copies of even a relatively rare allele. If one individual in a population of ten individuals happens to die before it leaves any offspring to the next generation, all of its genes-a tenth of the population’s gene pool-will be suddenly lost. Alleles may or may not make it into the next generation due to chance events including mortality of an individual, events affecting finding a mate, and even the events affecting which gametes end up in fertilizations. Genetic drift occurs because the alleles in an offspring generation are a random sample of the alleles in the parent generation. Drift would be completely absent in a population with infinite individuals, but, of course, no population is this large. Genetic drift is most important in small populations. Genetic DriftĪnother way a population’s allele frequencies can change is genetic drift ( Figure 11.7), which is simply the effect of chance. It should be noted that mutation is the ultimate source of genetic variation in all populations-new alleles, and, therefore, new genetic variations arise through mutation. Whether or not a mutation is beneficial or harmful is determined by whether it helps an organism survive to sexual maturity and reproduce. Beneficial mutations will spread through the population through selection, although that initial spread is slow. Harmful mutations are removed from the population by selection and will generally only be found in very low frequencies equal to the mutation rate. A mutation may produce an allele that is selected against, selected for, or selectively neutral. The change in frequency resulting from mutation is small, so its effect on evolution is small unless it interacts with one of the other factors, such as selection. A mutation can change one allele into another, but the net effect is a change in frequency. Mutation is a change in the DNA sequence of the gene. Mutation is a source of new alleles in a population. Over time, the allele will increase in frequency in the population. If conditions remain the same, those offspring, which are carrying the same allele, will also benefit. If it is an advantage, then that individual will likely have more offspring than individuals with the other phenotypes, and this will mean that the allele behind the phenotype will have greater representation in the next generation. Depending on the environmental conditions, the phenotype confers an advantage or disadvantage to the individual with the phenotype relative to the other phenotypes in the population. Natural selection has already been discussed.
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