Population Genetics Quiz

PDF page 783: Take a look at your classmates. Individuals of a population often display different phenotypes, or express different alleles of a particular gene. These differences are called polymorphisms. The distribution of phenotypes among individuals, known as population variation, is influenced by several factors, including the population’s genetic structure and the environment (Figure 19.3). Understanding the sources of phenotypic variation is important for determining how a population will evolve in response to different evolutionary pressures. Only those variations that are encoded in an individual’s genes can be passed to its offspring and be a target of natural selection.

PDF page 783: Take a look at your classmates. Individuals of a population often display different phenotypes, or express different alleles of a particular gene. These differences are called polymorphisms. The distribution of phenotypes among individuals, known as population variation, is influenced by several factors, including the population’s genetic structure and the environment (Figure 19.3). Understanding the sources of phenotypic variation is important for determining how a population will evolve in response to different evolutionary pressures. Only those variations that are encoded in an individual’s genes can be passed to its offspring and be a target of natural selection.

PDF page 785: Natural selection and some of the other evolutionary forces can only act on heritable traits, namely an organism’s genetic code. Because alleles are passed from parent to offspring, those that confer beneficial traits or behaviors may be selected for, while deleterious alleles may be selected against. Acquired traits, for the most part, are not heritable. For example, if an athlete works out in the gym every day, building up muscle strength, the athlete’s offspring will not necessarily grow up to be a body builder. If there is a genetic basis for the ability to run fast, on the other hand, this may be passed to a child.

PDF page 786: The theory of natural selection stems from the observation that some individuals in a population are more likely to survive longer and have more offspring than others; thus, they will pass on more of their genes to the next generation. A big, powerful male gorilla, for example, is much more likely than a smaller, weaker one to become the population’s silverback, the pack’s leader who mates far more than the other males of the group. The pack leader will father more offspring, who share half of his genes, and are likely to also grow bigger and stronger like their father. Over time, the genes for bigger size will increase in frequency in the population, and the population will, as a result, grow larger on average. That is, this would occur if this particular selection pressure, or driving selective force, were the only one acting on the population. In other examples, better camouflage or a stronger resistance to drought might pose a selection pressure. Another way a population’s allele and genotype frequencies can change is genetic drift (Figure 19.4), which is simply the effect of 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).

PDF page 787: Figure 19.4 Genetic drift occurs when the gene frequency of a population shifts by random chance (i.e. without a

PDF page 783: As you learn in the chapter that discusses the evolution and origin of species, natural selection works by selecting for phenotypes—and the alleles that determine them—that confer beneficial traits or behaviors. Deleterious qualities are selected against. Genetic drift stems from the chance occurrence that some individuals have more offspring than others and, thus, will pass on more of their genes to the next generation. Small and isolated populations are more susceptible to genetic drift. Natural events, such as wildfires or hurricanes, can magnify genetic drift when a large portion of the population is killed. Because a fire does not distinguish between the genotypes of various organisms, no particular genotype survives the fire better than another. Therefore, the genetic structure of the surviving population may be very different from the genetic structure of the original population. This is called the bottleneck effect. Another scenario in which populations might experience a strong influence of genetic drift occurs when some portion of the population leaves to start a new population in a new location or gets separated by a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of the entire population, a phenomenon called the founder effect. Both the bottleneck effect and the founder effect reduce genetic variation within a population—and genetic variation is the basis for natural selection. When individuals leave or join a population, they carry their alleles with them, resulting in changes in the population’s allele frequencies. Allele frequencies also can change due to mutation in DNA and when individuals do not randomly mate with others; when an individual selects a mate based on phenotype, the genotype is also selected. In summary, any of these conditions can result in deviations from the Hardy–Weinberg equilibrium—and lead to the microevolution of a population.

PDF page 788: Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand, are buffered against the effects of chance. If one individual of a population of 10 individuals happens to die at a young age before it leaves any offspring to the next generation, all of its genes—1/10 of the population’s gene pool—will be suddenly lost. In a population of 100, that’s only 1 percent of the overall gene pool; therefore, it is much less impactful on the population’s genetic structure.

PDF page 783: As you learn in the chapter that discusses the evolution and origin of species, natural selection works by selecting for phenotypes—and the alleles that determine them—that confer beneficial traits or behaviors. Deleterious qualities are selected against. Genetic drift stems from the chance occurrence that some individuals have more offspring than others and, thus, will pass on more of their genes to the next generation. Small and isolated populations are more susceptible to genetic drift. Natural events, such as wildfires or hurricanes, can magnify genetic drift when a large portion of the population is killed. Because a fire does not distinguish between the genotypes of various organisms, no particular genotype survives the fire better than another. Therefore, the genetic structure of the surviving population may be very different from the genetic structure of the original population. This is called the bottleneck effect. Another scenario in which populations might experience a strong influence of genetic drift occurs when some portion of the population leaves to start a new population in a new location or gets separated by a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of the entire population, a phenomenon called the founder effect. Both the bottleneck effect and the founder effect reduce genetic variation within a population—and genetic variation is the basis for natural selection. When individuals leave or join a population, they carry their alleles with them, resulting in changes in the population’s allele frequencies. Allele frequencies also can change due to mutation in DNA and when individuals do not randomly mate with others; when an individual selects a mate based on phenotype, the genotype is also selected. In summary, any of these conditions can result in deviations from the Hardy–Weinberg equilibrium—and lead to the microevolution of a population.

PDF page 788: Genetic drift can also be magnified by natural events, such as a natural disaster that kills—at random—a large portion of the population. Known as the bottleneck effect, it results in a large portion of the genome suddenly being wiped out (Figure 19.5). 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.

PDF page 783: As you learn in the chapter that discusses the evolution and origin of species, natural selection works by selecting for phenotypes—and the alleles that determine them—that confer beneficial traits or behaviors. Deleterious qualities are selected against. Genetic drift stems from the chance occurrence that some individuals have more offspring than others and, thus, will pass on more of their genes to the next generation. Small and isolated populations are more susceptible to genetic drift. Natural events, such as wildfires or hurricanes, can magnify genetic drift when a large portion of the population is killed. Because a fire does not distinguish between the genotypes of various organisms, no particular genotype survives the fire better than another. Therefore, the genetic structure of the surviving population may be very different from the genetic structure of the original population. This is called the bottleneck effect. Another scenario in which populations might experience a strong influence of genetic drift occurs when some portion of the population leaves to start a new population in a new location or gets separated by a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of the entire population, a phenomenon called the founder effect. Both the bottleneck effect and the founder effect reduce genetic variation within a population—and genetic variation is the basis for natural selection. When individuals leave or join a population, they carry their alleles with them, resulting in changes in the population’s allele frequencies. Allele frequencies also can change due to mutation in DNA and when individuals do not randomly mate with others; when an individual selects a mate based on phenotype, the genotype is also selected. In summary, any of these conditions can result in deviations from the Hardy–Weinberg equilibrium—and lead to the microevolution of a population.

PDF page 789: 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 or if a population gets divided by a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of the entire population, which results in the founder effect. The founder effect occurs when the genetic structure changes to match that of the new population’s founding fathers and mothers. The founder effect is believed to have been a key factor in the genetic history of the Afrikaner population of Dutch settlers in South Africa, as evidenced by mutations that are common in Afrikaners but rare in most other populations. This is likely due to the fact that a higher-than-normal proportion of the founding colonists carried these mutations. As a result, the population expresses unusually high incidences of Huntington’s disease (HD) and Fanconi anemia (FA), a genetic disorder known to cause blood marrow and congenital abnormalities.

PDF page 790: Mutations are changes to an organism’s DNA and are an important driver of diversity in populations. Species evolve because of the accumulation of mutations that occur over time. The appearance of new mutations is the most common way to introduce novel genotypic and phenotypic variance. Some mutations are unfavorable or harmful and are quickly

PDF page 791: If individuals nonrandomly mate with their peers, the result can be a changing population. There are many reasons nonrandom mating occurs. One reason is simple mate choice; for example, female peahens may prefer peacocks with bigger, brighter tails. Traits that lead to more matings for an individual become selected for by natural selection. One common form of mate choice, called assortative mating, is an individual’s preference to mate with partners who are phenotypically similar to themselves. Another cause of nonrandom mating is physical location. This is especially true in large populations spread over large geographic distances where not all individuals will have equal access to one another. Some might be miles apart through woods or over rough terrain, while others might live immediately nearby.

PDF page 785: Heritability is the fraction of phenotype variation that can be attributed to genetic differences, or genetic variance, among individuals in a population. The greater the hereditability of a population’s phenotypic variation, the more susceptible it is to the evolutionary forces that act on heritable variation. The diversity of alleles and genotypes within a population is called genetic variance. When scientists are involved in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a population’s genetic variance to preserve as much of the phenotypic diversity as they can. This also helps reduce the risks associated with inbreeding, the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease. For example, a disease that is caused by a rare, recessive allele might exist in a population, but it will only manifest itself when an individual carries two copies of the allele. Because the allele is rare in a normal, healthy population with unrestricted habitat, the chance that two carriers will mate is low, and even then, only 25 percent of their offspring will inherit the disease allele from both parents. While it is likely to happen at some point, it will not happen frequently enough for natural selection to be able to swiftly eliminate the allele from the population, and as a result, the allele will be maintained at low levels in the gene pool. However, if a family of carriers begins to interbreed with each other, this will dramatically increase the likelihood of two carriers mating and eventually producing diseased offspring, a phenomenon known as inbreeding depression. Changes in allele frequencies that are identified in a population can shed light on how it is evolving. In addition to natural selection, there are other evolutionary forces that could be in play: genetic drift, gene flow, mutation, nonrandom mating, and environmental variances.

PDF page 785: Heritability is the fraction of phenotype variation that can be attributed to genetic differences, or genetic variance, among individuals in a population. The greater the hereditability of a population’s phenotypic variation, the more susceptible it is to the evolutionary forces that act on heritable variation. The diversity of alleles and genotypes within a population is called genetic variance. When scientists are involved in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a population’s genetic variance to preserve as much of the phenotypic diversity as they can. This also helps reduce the risks associated with inbreeding, the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease. For example, a disease that is caused by a rare, recessive allele might exist in a population, but it will only manifest itself when an individual carries two copies of the allele. Because the allele is rare in a normal, healthy population with unrestricted habitat, the chance that two carriers will mate is low, and even then, only 25 percent of their offspring will inherit the disease allele from both parents. While it is likely to happen at some point, it will not happen frequently enough for natural selection to be able to swiftly eliminate the allele from the population, and as a result, the allele will be maintained at low levels in the gene pool. However, if a family of carriers begins to interbreed with each other, this will dramatically increase the likelihood of two carriers mating and eventually producing diseased offspring, a phenomenon known as inbreeding depression. Changes in allele frequencies that are identified in a population can shed light on how it is evolving. In addition to natural selection, there are other evolutionary forces that could be in play: genetic drift, gene flow, mutation, nonrandom mating, and environmental variances.

PDF page 785: Heritability is the fraction of phenotype variation that can be attributed to genetic differences, or genetic variance, among individuals in a population. The greater the hereditability of a population’s phenotypic variation, the more susceptible it is to the evolutionary forces that act on heritable variation. The diversity of alleles and genotypes within a population is called genetic variance. When scientists are involved in the breeding of a species, such as with animals in zoos and nature preserves, they try to increase a population’s genetic variance to preserve as much of the phenotypic diversity as they can. This also helps reduce the risks associated with inbreeding, the mating of closely related individuals, which can have the undesirable effect of bringing together deleterious recessive mutations that can cause abnormalities and susceptibility to disease. For example, a disease that is caused by a rare, recessive allele might exist in a population, but it will only manifest itself when an individual carries two copies of the allele. Because the allele is rare in a normal, healthy population with unrestricted habitat, the chance that two carriers will mate is low, and even then, only 25 percent of their offspring will inherit the disease allele from both parents. While it is likely to happen at some point, it will not happen frequently enough for natural selection to be able to swiftly eliminate the allele from the population, and as a result, the allele will be maintained at low levels in the gene pool. However, if a family of carriers begins to interbreed with each other, this will dramatically increase the likelihood of two carriers mating and eventually producing diseased offspring, a phenomenon known as inbreeding depression. Changes in allele frequencies that are identified in a population can shed light on how it is evolving. In addition to natural selection, there are other evolutionary forces that could be in play: genetic drift, gene flow, mutation, nonrandom mating, and environmental variances.