Introduction and Learning Objectives
Previously we examined how the genetic composition of a population is studied. In this tutorial we will examine the conditions that can alter the genetic composition of a population. This theme is central to evolution. Genetically stable populations (those in Hardy-Weinberg equilibrium) do not evolve, however, genetically unstable populations are undergoing evolutionary change. We will examine those conditions that affect genetic stability and contribute to evolutionary change. By the end of this tutorial you should have a basic understanding of:
- How the founder effect and bottleneck effect relate to genetic drift
- How gene flow, mutations, and mating behavior can affect genetic stability
- How selection can influence allele frequency
- Summarize how the factors that can cause a population to move away from HW equilibrium lead to evolution
- Discuss how both a population bottleneck and a founder event can lead to genetic drift, and its effect on allele frequencies
- Describe how mutation can be a source of new alleles in a population, and why some mutations can be neutral, rather than harmful or beneficial
- Illustrate how migration can affect the genetic structure of a population
- Explain why non-random mating can change the genotype frequencies in a population, but not the allele frequencies
Microevolution versus Macroevolution
Evolution is a continuous process, but major evolutionary changes are rare events. Major evolutionary changes include the origin or feathers in birds or jaws in vertebrates. These major evolutionary changes that mark the appearance of a new species (or genera and higher taxonomic levels) are macroevolutionary events.
Evolution can occur within a species. Think about the human allele for sickle-cell disease: populations living in areas with high incidences of malaria are better adapted to the presence of that apicomplexan because of the higher frequency of this allele in those populations. Evolution of human populations has occurred in the past, is occurring today, and will continue to occur. Changes that lead to alterations in allele frequencies are microevolutionary events. At the most basic level, evolution is a change in allele frequencies and/or genotype frequencies in a population from generation to generation. A population is evolving even if allele frequencies are fluctuating at just one gene. Therefore, at the population level, evolution is a nearly continuous process.
We have discussed how natural selection adapts organisms to their environment and will consider this in more detail in Tutorial 35. As you learned from the example of the sickle-cell allele, this adaptation results in a change in allele frequency. However, natural selection is only one mechanism that can change allele frequencies.
From a geneticist's point of view, a population composed of hundreds or thousands of individuals is less likely to evolve than a small population. The larger the population, the more of a buffer there is against random variations in allele frequencies; an infinite size population would be almost completely resistant to random fluctuations. In contrast, small populations are especially vulnerable to the loss of genetic variability due to random events; in fact, they may evolve into extinction.
Small populations are much more likely to experience genetic drift (random fluctuations of allele and genotype frequencies) than are large populations. If a population has an allele frequency of p = 0.25 or 1/4, then the allele frequency of q is 1 - 0.25 = 0.75 or 3/4. (Remember, the sum of all individual allele frequencies for a particular gene must add up to 1.) Therefore, for each p allele, there are 3 q alleles in the population (or a 1:3 ratio). If a population consists of 1000 individuals and 1000 X 2 = 2000 alleles, then 500 of the alleles in the gene pool would be p (2 X 250) and 1500 of the alleles would be q (2 X 750). If the alleles in the next generation are counted, they may have drifted by five alleles (e.g., to 505 and 1495), but the new allele frequencies would be 0.253 and 0.748, which are very close to the original values of 0.25 and 0.75. Although there is very small measurable change, it would take many generations before one allele became fixed in the population and the other eliminated.
However, if the population in the example above contained only 10 individuals (2 X 10 = 20 alleles) and the initial allele frequencies were again p = 0.25 and q = 0.75, then there would be 5 p alleles (2 X 2.5) and 15 q alleles (2 X 7.5) in the population. If the next generation also drifted by five alleles, then there would be 10 p alleles (5 + 5) and 10 q alleles (15 - 5). Therefore, in one generation the allele frequencies would go from 0.25 and 0.75 to 0.5 and 0.5. If both alleles are selectively neutral (have no effect on fitness), then each faces a probability of being lost in the course of several generations, and eventually one allele will become fixed in the population. Heterozygosity at that gene would become zero and the genetic variation within the population would be reduced.
Next we'll examine the two major situations that restrict population size and lead to genetic drift.
The Bottleneck Effect
One situation that can result in genetic drift is the bottleneck effect. A bottleneck event (e.g., earthquakes, fires, over-hunting) decimates a population and results in only a small number of individuals surviving. In a bottleneck event, the remaining, random survivors may not have the same allele and genotype frequencies as the original population. In the new population, some alleles may be found at higher frequencies, whereas others may be found at lower frequencies or even lost altogether.
In a small but rapidly reproducing population, genetic drift will typically affect the population for a number of generations until the population size becomes large enough that genetic drift is negligible. While natural disasters have historically been the cause of bottleneck effects, overzealous hunting can also cause this effect.
Many endangered species, such as the cheetah (Figure 1) and the American bison, provide examples of bottleneck events and reduced genetic variation. Among cheetahs, all living individuals are genetically very similar, which means this surviving population has less genetic variation upon which selection can act. Even when a species, such as the cheetah, is protected following a bottleneck event, the loss of genetic variability may make the species more likely to go extinct. Genetically homogeneous populations are more prone to catastrophes because a given insult (e.g., a disease or a predator) can sweep through the entire population quickly; without variation, there is nothing for natural selection to act on. Therefore, conservation efforts must not only preserve numbers, they must also promote genetic variability. Only when some individuals are better suited to the environment than others, can evolution increase the adaptation of a population to its environment. Perhaps the potential to undergo natural selection is one of the best measures of a healthy population.
Figure 1. Cheetah (Click image to enlarge)
There is reason to think that bottleneck effects might play a major role in extinction. There is evidence that major catastrophic events (e.g., meteor strikes) might have led to major bottleneck effects in Earth's past. Such events might not have necessarily killed off all members of a species (e.g., dinosaurs), but might have decimated their population enough such that they became genetically weakened due to a loss in variability; eventually these populations died out. One book with a catchy title, Extinction: Bad Genes or Bad Luck? by David M. Raup, explores this idea.
A disease that kills only individuals with a genetic predisposition is not an example of a bottleneck event, in which individuals survive regardless of their adaptations/fitness, but would be an example of natural selection at work.
The Founder Effect
Another situation that can result in genetic drift is the founder effect. A founder event occurs when a few individuals (founding individuals) become geographically separated from the original population and form a new population. The alleles present in the founding individuals make up the gene pool of this new population. Most likely, the founding individuals will have the same allele and genotype frequencies of the original population, nor will they possess all of the variation present in the original population. Because it will take several generations before their population increases much in size, genetic drift would continue to influence the allele frequencies in the population. This effect can be seen when remote islands are colonized.
An example of the founder effect in human populations occurred on the island of Tristan da Cunha (Figure 2). This island was settled in the 1800's by fifteen British immigrants. There have only been a couple of migrations since; today there are just seven surnames on this island that has about 250 inhabitants. Expectedly, this leads to problems associated with inbreeding (the mating of closely related individuals). One notable problem is the occurrence of retinitis pigmentosa (a rare form of blindness), which occurs at a much higher frequency in this population than in the original population. Some questions in the tutorial quiz will use this genetic disorder to illustrate how the founder effect alters the frequency of alleles in a small human population.
The Founder Effect: Test Your Understanding
During the early 1700’s, a small group of pacifist Protestants fled Germany to avoid religious persecution. This group, the Dunkers, settled in the farmland of eastern Pennsylvania and has been relatively isolated from other groups of people living in the area (they have strict rules about marriage outside of the group). The original group was comprised of 50 families.
The current Dunker population is genetically distinct from the rest of the United States (and modern Germany) in terms of the ABO blood group system. In the Dunker population, the frequency of the IA allele is .30 (30%) and the frequency of the O blood type is .16 (16%). In the general US and German population the frequency of the IA allele is .40 and the frequency of the O blood type is .25 (25%).
- What are the frequencies of the IB and i alleles in both populations?
- Use your knowledge of the founder effect to explain the differences in ABO blood group allele frequencies between the 2 populations and indicate whether or not this is an example of adaptive evolution.
Populations remain genetically stable if no alleles enter or leave the population. Recall, a change in allele frequencies due to interactions with outside populations is termed gene flow. If a population was fixed for one allele at a gene and a gamete carrying an alternative allele was able to fertilize an individual from that population, then the allele frequencies would change due to the introduction of this new allele and the population would undergo microevolution. Migration of individuals among populations leads to gene flow, as individuals immigrating into the population bring new alleles, while those emigrating take alleles with them.
As you learned in Tutorial 31, a mutation is a change in an organism's DNA. Mutation will introduce a new allele into a population, causing small changes in the allele and genotype frequencies. Because mutations are the ultimate source of all genetic variation, over time they can have a significant influence on the genetic structure of the population, but the mutation rate usually is very low so it is not a strong evolutionary force. In general, , most mutations found in a population appear to be “neutral”, having little to no effect on the phenotype. Because several different combinations of nucleotides may code for the same amino acid (recall the redundancy in the genetic code), a mutation in the DNA of a protein-coding gene may not change the amino acid sequence of the protein. Conversely, some mutations can have a major effect on the phenotype. These mutations almost always result in a nonfunctional or less efficient product Very rarely, a mutation may result in a protein that is slightly more efficient, or that performs a new function. In order for the frequency of the mutant allele to increase as a result of natural selection, the advantage would have to be great enough to affect the individual's reproductive success. Therefore, although an assumption when applying the Hardy-Weinberg equation is that no net mutations occur, most mutations have little overall effect on the allele frequencies of a population.
Random versus Nonrandom Mating
Random mate selection favors genetic stability. Nonrandom mate selection often occurs in natural populations because an individual is more likely to choose a mate from his or her vicinity than from more distant corners of the population. Frequent near-neighbor matings tend to subdivide the population's gene pool into subpopulations that have less genetic diversity than the entire gene pool of the population.
Inbreeding (the mating of closely related individuals) was described earlier in this tutorial. Self-fertilization ("selfing") occurs when an individual fertilizes its own gametes (a common occurrence in plants). Both of these mating processes decrease the level of heterozygosity in the offspring. Normal healthy individuals can carry several or many deleterious alleles. Although each individual may carry some deleterious alleles, unrelated individuals are likely to be carriers of different harmful alleles. Therefore, most of the resulting offspring will be healthy carriers as well. If the parents are closely related, however, it is more likely that they are carrying the same deleterious alleles. Some of the recessive deleterious alleles will occur in the homozygous state, exposing the allele to selection.
Similarly, individuals may choose mates that have some of the same phenotypic traits (e.g., body size or height) as themselves. This is termed assortative mating. Although these kinds of characters are complex, involving numerous genes, they are still usually heritable. Mating of genetically similar (although not necessarily related) individuals tends to increase the amount of homozygosity in the population. However, nonrandom mating does not change allele frequencies because it does not include introducing new alleles into the population, or the loss of alleles.
Genetic Stability and Selection Pressure
This tutorial has focused on how non-adaptive changes, some due to random events, can affect allele frequencies, thus affecting the genetic stability of a population. Even deleterious alleles may increase in frequency due to chance events (recall the Tristan da Cunha example).
One of the conditions for maintaining Hardy Weinberg equilibrium is that all individuals have the same reproductive capacity and fitness. Even minor deviations from this equality will lead to some individuals producing more offspring than others, leading to a change in allele frequencies. Any alleles that directly or indirectly affect an individual's ability to survive, mate, or reproduce may respond to selection pressure. Under very strong selection pressure (e.g., when bacteria are exposed to a powerful antibiotic), a mutation conferring some form of resistance to the antibiotic can increase in frequency in the population very quickly. In fact, antibiotic resistance (read "A Growing Threat") is becoming a major health risk, and both pesticide and herbicide resistance are major concerns to agriculturists. As you will learn, the example of antibiotic resistance is a result of natural selection because the surviving, resistant individuals are better adapted to their environment.
Similarly, an allele that makes an individual more vulnerable to disease or predation is likely to be under very strong selection pressure and may decrease in frequency very rapidly, until it is either very rare in the population or purged completely from the gene pool. We will discuss selection in more detail in Tutorial 35.
Each of the phenomena discussed in this tutorial acts to change the frequency of alleles and/or genotypes in an evolving population. A population in which all of these forces are negligible or absent will remain in a state of equilibrium and allele frequencies will remain the same across generations. Few populations remain in equilibrium for very long, primarily due to the large amount of genetic variation in natural populations. Frequently, individuals differ from one another in many ways (e.g., from neutral molecular differences in amino acid sequences to differences in appearance, ability, and behavior). Even under relatively strong selection pressure or very small population sizes, it can take many generations for an allele to become fixed or lost. Even under controlled laboratory breeding programs, it can take many generations for a recessive deleterious allele to be removed from the population. Once an allele becomes sufficiently rare in the population, it is unlikely to be found in the homozygous state. Natural selection (one of the strongest and most directional of the evolutionary forces) can only act on the phenotype. Therefore, a recessive allele can linger for a very long time in a population if it is only rarely exposed to natural selection.
Similarly, although an individual may have a low fitness level, this may not be indicative of his or her genotype. Not all variation is heritable, and environmental influences (e.g., weather, nutrition, and availability of potential mates) may overwhelm subtle differences in genotypes. A well fed but genetically inferior animal may have greater reproductive potential than a genetically superior but undernourished animal. In this case, natural selection would not lead to a population better adapted to its environment.
This tutorial examined population genetics, with reference to microevolution. Microevolution refers to the change in allele frequencies in a gene pool of a population over generations. A population is a group of interbreeding individuals. Over many generations, these changes can lead to new species. However, different populations of the same species can be undergoing different types of microevolutionary changes but still interbreed; hence, by definition, they are still members of the same species. Keep in mind, microevolution describes small changes in a population over successive generations, and does not necessarily mean that two populations will eventually become new species. (Although this can happen over many generations.)
The Hardy-Weinberg equation can be used to determine the allele frequency in a population. Thus, it provides a useful tool to describe the degree of microevolution that is taking place over successive generations. Not all populations are undergoing microevolution at all times. In certain stable environments, many populations show no evidence for microevolution. (Although if the conditions change, then microevolution can take place.)
There are a number of conditions that predictably decrease microevolution; namely: large populations that are more genetically stable compared to smaller populations; no migration/immigration by which new alleles are introduced or removed from a population; no mutations to introduce new alleles; random mating that allows all individuals equal access to all alleles and; no natural selection, which causes changes in allele frequencies due to fitness differences between individuals.
Conversely, opposing conditions increase microevolution; namely: small populations are genetically unstable because they are susceptible to genetic drift; migration/immigration that allows alleles to enter or leave a population; mutations that introduce new alleles; nonrandom matings that cause unequal access to all alleles; and natural selection that changes allele frequencies based upon individuals' reproductive capacity and fitness.
After reading this tutorial, you should have a working knowledge of the following terms:
- assortative mating
- bottleneck effect
- founder effect
Questions? Either send your instructor a message through ANGEL or attend an office hour (the times are posted on ANGEL).