The disaster must be one that kills for reasons unrelated to the organism’s traits, such as a hurricane or lava flow.įigure 2. In one fell swoop, the genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population. Genetic drift can also be magnified by natural or human-caused events, such as a disaster that randomly kills a large portion of the population, which is known as the bottleneck effect that results in a large portion of the gene pool suddenly being wiped out ( Figure 2). The effect of drift on frequencies is greater the smaller a population is. 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 version of the gene 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 gene). Thus, the 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 gene. 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 a version of a gene we will call A and half with a version of a gene we will call a. 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 gene. 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. Because the genes in an offspring generation are a random sample of the genes in the parent generation, some versions of a gene may not make it into the next generation due to chance events. Genetic drift is most important in small populations. * Genetic DriftĪnother way the frequencies of certain genes can change is genetic drift ( Figure 1), 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 a mutation in one individual is small, so its effect on evolution is small unless it interacts with one of the other factors, such as selection. In some cases a change in the DNA will change the protein produced. Mutation is a change in the DNA sequence of the gene. Mutation is a source of variation in a population. Over time, the advantageous trait (aka adaptation) will become more common in the population. If conditions remain the same, those offspring, which are carrying the same trait, will also benefit, and pass the genes that give rise to this trait on to their own offspring. If the trait is heritable, then the genes that give rise to the trait will be more common in the next generation. If a certain trait confers an advantage, then the individual possessing the trait may have more offspring than those with other traits. Depending on the environmental conditions, certain traits may confer an advantage or disadvantage to the individuals that possess them, relative to others in the population. Natural selection, discussed in the previous chapter, is the mechanism of evolution that explains how species can become better adapted to their environment. These include mutation, genetic drift and migration. Although natural selection is the mechanism of evolution most commonly discussed, other evolutionary mechanisms also change the frequencies of traits (and the genes that control them) in populations. When certain genes become more or less common in the population over generations, we refer to this change as evolution.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |