Mechanisms of Evolution

So far this semester we have focused on how different types of selection drive evolution. Selection is the only mechanism of evolution that drivesadaptive evolution – evolution that causes populations to become better adapted to their environments. However, not all evolution is adaptive. Evolution is changes in allele frequencies i populations over generations. The allele frequency describes the proportions of all of the possible alleles in the population’s gene pool. The gene pool is the sum of all the alleles in a population. Changes in allele frequencies can be driven by more factors than just selection. Here we will investigate the two other mechanisms of evolution that both involve randomness: mutation and genetic drift.

Mutation

Mutation is a random change in the DNA sequence. Mutation is the ultimate source of genetic variation in all populations—new alleles, and, therefore, new genetic variations arise through mutation. A mutation creates a new allele, and thus by definition results in a small change in the allele frequencies within the population in which it occurs.

The genetic changes that mutation causes can have one of three outcomes on the phenotype. One possibility is that the mutation affects the organism’s phenotype in a way that gives it reduced fitness—lower likelihood of survival or fewer offspring. These harmful mutations are called deleterious mutations and are generally removed from the population by selection. Harmful mutations will generally only be found in very low frequencies equal to the mutation rate. Another possibility is that a mutation may produce a phenotype with a beneficial effect on fitness. Beneficial mutations will typically become more frequent due to natural selection and will spread through the population. Finally, many mutations will also have no effect on the phenotype’s fitness, called neutral mutations. The prevalence of neutral mutations is unaffected by selection, and only subject to the other mechanisms of evolution.

Whether a mutation is beneficial, harmful, or neutral is determined by the environmental context in which it exists. If it helps an organism survive and reproduce in its environment, it is beneficial and therefore an adaptation. Mutations may also have a whole range of effect sizes on the organism’s fitness that expresses them in their phenotype, from a small effect to a great effect.

Mutations are random

It is important to understand that the variation that natural selection works on is already in a population and does not arise in response to an environmental change. For example, applying antibiotics to a population of bacteria will, over time, select a population of bacteria that are resistant to antibiotics. The resistance, which a gene causes, did not arise by mutation because of applying the antibiotic. The gene for resistance was already present in the bacteria’s gene pool, likely at a low frequency. The antibiotic, which kills the bacterial cells without the resistance gene, strongly selects individuals that are resistant, since these would be the only ones that survived and divided. Experiments have demonstrated that mutations for antibiotic resistance do not arise as a result of antibiotic application.

In a larger sense, evolution is not goal directed. Species do not become “better” over time. They simply track their changing environment with adaptations that maximize their reproduction in a particular environment at a particular time. Evolution has no goal of making faster, bigger, more complex, or even smarter species, despite the commonness of this kind of language in popular discourse. What characteristics evolve in a species are a function of the variation present and the environment, both of which are constantly changing in a nondirectional way. A trait that fits in one environment at one time may well be fatal at some point in the future. This holds equally well for insect and human species.

Genetic Drift

Genetic drift refers to changes in allele frequencies across generations that occurs purely by random chance. By chance, some individuals will have more offspring than others—not due to an advantage conferred by some genetically-encoded trait, but just because one male happened to be in the right place at the right time (when the receptive female walked by) or because the other one happened to be in the wrong place at the wrong time (when a fox was hunting). Sometimes chance events, such a natural disaster, influence which individuals survive and reproduce and thus influence the allele frequencies in the next generation. Thus, evolution has occurred due only to random chance. While natural selection results from aspects of an organism’s environment exerting “selective pressure” (e.g., the desert environment favors the spines of the cactus and the long ears of the fox), genetic drift is not a result of selective pressures. Genetic drift, like natural selection, is occurring all of the time in all environments. Drift will influence every allele, including those that are being naturally selected. Thus, it can be difficult to determine which process dominates because it is often nearly impossible to determine the exact cause of change in allele frequencies at any given time.

Genetic drift exerts a particularly strong effect in small populations, such as those that colonize islands or or the few individuals that remain after large-scale disruptions (e.g. earthquakes, fire). You are already familiar with the statistical principle (sampling bias) underlying this: the random loss of 20 iguanas from a large population of 1 million iguanas is bound to result in a smaller impact on allele frequencies than the random loss of 20 iguanas from an island population of 100 iguanas. Genetic drift occurs because the alleles in an offspring generation are a random sample of the alleles in the parent generation. 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. 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. 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.

Imagine a population of ten haploid individuals, half with allele A and half with allele a. In a stable population, the next generation will also have ten individuals. Choose that generation randomly by flipping a coin ten times and let heads be A and tails be a. It is unlikely that the next generation will have exactly half of each allele. There might be six of one and four of the other, or some different set of frequencies. Thus, the allele frequencies have changed and evolution has occurred. A coin will no longer work to choose the next generation (because the odds are no longer one half for each allele). 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. This could take a very long time for a large population. This simplification is not very biological, but simulations and studies have shown that real populations behave this way. The effect of drift on frequencies is greater the smaller a population is. Its effect is also greater on an allele with a frequency far from one half. Genetic drift causes random changes in allele frequencies and these changes are larger when populations are small.

Bottleneck effect

Natural events, such as an earthquake that randomly kills a large portion of the population, can magnify the effects of genetic drift. These situations, where a large portion of the gene pool is suddenly and randomly eliminated, is called the bottleneck effect (Fig 5.1). At once, the survivors’ genetic structure (i.e., allele frequencies) becomes the entire population’s genetic structure, which may be very different from the pre-disaster population. Almost certainly rare alleles are lost from the population, resulting in a decrease in the variability in the population. The survivors reproduce to create the next generation, which can have dramatically different allele frequencies compared to the population before the bottleneck event occurred. Bottleneck effects are a special case of genetic drift where the effects of drift are particularly strong because a population’s size has been dramatically reduced.

Founder Effect

Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the population leaves to start a new population in a new location. It is likely that the small number of individuals who found the new population have different allele frequencies than the entire population due to random chance (sampling error). Thus, the new population can end up having very different allele frequencies than the original source population due to genetic drift – this is called the founder effect.

References

Adapted from:

Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/1-introduction

and

Fowler, S., Roush, R., & Wise, J. (2022). NSCC Academic Biology 1050. Nova Scotia Community College. Retrived from https://pressbooks.nscc.ca/biology1050/