12.3 Random Change. evolution defined in genetic terms as any change in gene (and allele) frequencies within a population or species

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1 evolution defined in genetic terms as any change in gene (and allele) frequencies within a population or species 12.3 Random Change The Hardy Weinberg principle demonstrates that, under a set of specific conditions, a given gene pool remains unchanged from generation to generation. The underlying conditions are critically important. By providing the set of conditions under which genetic change would not occur, the Hardy Weinberg principle helps identify key factors that can cause evolution, a change to the gene pool of a population or a species. The following are the key factors: When a population is small, chance fluctuations can cause changes in allele frequencies. When mating opportunities are nonrandom, individuals that are preferred as mates will pass on their alleles in greater numbers than less preferred mates. When genetic mutations occur, new alleles may be created or one allele may be changed into another, thereby changing the frequencies of both new and original alleles. When individuals migrate, they remove alleles from one population and add them to another. When natural selection occurs, individuals with certain alleles have greater reproductive success than others do, thereby increasing the relative frequency of their alleles in the next generation. Real populations can be affected by any of these situations, resulting in changes to allele frequencies. genetic drift changes to allele frequency as a result of chance; such changes are much more pronounced in small populations Genetic Drift When populations are small, chance can play a significant role in altering allele frequencies. For example, assume only 1 in 50 cricket frogs carries a particular allele, C 1 (Figure 1). In a large population of individuals, you would expect 200 to carry the allele. If severe weather conditions led to the random deaths of half the population, you would expect about 100 of the 5000 survivors to be carrying the C 1 allele; therefore, the allele frequency would not be expected to change. However, if the initial frog population were endangered, with only 100 individuals, you would expect only two to possess the C 1 allele. If half the members of this population died, there would be a good chance that either both the C 1 carriers would die thereby eliminating the C 1 allele entirely or both would survive, thereby instantly doubling the allele frequency of C 1. This pattern, while an extreme example, demonstrates genetic drift, a change in the genetic makeup of a population resulting from chance. Figure 1 The remaining populations of the endangered Blanchard s cricket frog, Acris crepitans blanchardi, once found on Pelee Island in Lake Erie, are very vulnerable to the effects of genetic drift. 550 Chapter 12 NEL

2 Section 12.3 (a) 1.0 AA in five populations (b) 1.0 Frequency of Allele A 0.5 allele A lost from four populations Generation (25 stoneflies at the start of each generation) Figure 2 In small populations (a), genetic drift can result in dramatic changes in allele frequencies, while in larger populations (b), genetic drift is not usually significant. Frequency of Allele A 0.5 allele A neither lost nor fixed Generation (500 stoneflies at the start of each generation) Figure 2(a) illustrates genetic drift in a population of 25 stoneflies. The frequency of allele A fluctuates wildly from generation to generation. In five trials, the A allele became fixed at 100% in 22 generations or fewer, while in the other four trials, the A allele was lost entirely, being reduced to 0 in 36 generations or fewer. In a larger population of 500 stoneflies, as shown in Figure 2(b), the allele frequency remained relatively stable even after 50 generations had passed; there was no trend toward fixing of the allele. Significantly, in small populations, genetic drift can lead to fixation of alleles, thereby increasing the incidence of homozygous individuals within a population and reducing its genetic diversity. When a severe event results in a drastic reduction in numbers, a population may experience a bottleneck effect (Figure 3). When this form of genetic drift occurs, a very small sample of alleles survives to establish a new population. Their relative frequencies may differ from those of the original population and additional genetic drift may result in further deviations in the gene pool. This is known to have occurred with the northern elephant seal (Figure 4). bottleneck effect a dramatic, often temporary, reduction in population size usually resulting in significant genetic drift parent population NEL bottleneck (drastic reduction in population) surviving individuals next generation Figure 3 A dramatic, sometimes temporary, reduction in the size of a population can result in a bottleneck effect. Figure 4 The northern elephant seal population was reduced by overhunting to 20 individuals in the 1890s. Although the population had rebounded to over individuals by 1974, genetic testing of 24 loci exhibited total homozygosity. Mechanisms of Evolution 551

3 TRY THIS activity Demonstrating Chance See for yourself how random chance works in small populations. Model the allele frequency in a new population by tossing a six-sided die 30 times. Record your results. Examine the results of your classmates. (a) How often did you roll a three? Did your response differ from the one sixth you would expect by chance? (b) How many times did you roll a three in your first six tosses? What ratio did this produce? (c) Relate the variations in the frequencies of the number three to variations in allele frequencies that occur when small founder populations form. founder effect genetic drift that results when a small number of individuals separate from their original population and find a new population self-pollinating plants plants that habitually fertilize themselves and produce viable offspring When a few individuals from a large population leave to establish a new population, the resulting genetic drift is a founder effect. The allele frequencies of the new population will not be the same as those of the original population and may deviate further as the new population expands. Founder effects seem to be common in nature, such as when a few seeds carried by a bird or by winds to a distant volcanic island may germinate and rapidly establish a large population. With self-pollinating plants, an entire population can be established from a single fertile seed. Founder effects can also be seen in human populations. Members of the Amish community in Pennsylvania are all descendants of about 30 people who emigrated from Switzerland in One of the founders had a rare recessive allele that causes unusually short limbs. The frequency of this allele in the current Amish population is about 7%, compared to a frequency of 0.1% in most populations. Founder effects have been documented in the wild. In 1982, Peter Grant and Rosemary Grant from Queen s University witnessed the establishment of a new population of large ground finches (Geospiza magnirostris) in the Galapagos Islands. The Grants had been studying Darwin s finches on one island, Daphne Major, and had observed juvenile large ground finches visiting the island every year for 10 years. In 1982, however, three males and two females remained on the island to breed. In early 1983, they produced 17 young birds, which became the founders of a new population. The population that they established has remained ever since. Careful measurements of inheritable traits by the Grants indicated that the founding population has a different genetic composition from that of the original large population of Geospiza magnirostris from which the founders came. gene flow the movement of alleles from one population to another through the movement of individuals or gametes neutral mutation has no immediate effect on an individual s fitness; most neutral mutations are silent or occur in noncoding DNA Gene Flow When organisms migrate, leaving one population and joining another, they alter the allele frequencies of both. Such gene flow occurs frequently in most wild populations. For example, prairie dogs live in dense colonies consisting of a few dozen members. For much of the year they prevent other prairie dogs from joining their colony. In late summer, however, mature male pups are permitted to enter new colonies, thereby affecting both gene pools. Gene flow can also occur when individuals of adjacent populations mate without moving permanently. In these ways, genetic information is shared between populations. Unlike genetic drift, gene flow tends to reduce differences between populations. Mutation Mutations are the only source of additional genetic material and new alleles. Mutations may arise as a result of unrepaired changes in DNA sequences or chromosome breakage and rejoining. Although most mutations occur in somatic (body) cells, these mutations cannot be inherited and, therefore, do not play a role in evolution. However, any mutation that occurs in a gamete has the potential to be passed on to later generations, thereby entering the gene pool. These mutant alleles and any new phenotypes they produce become the source of new raw material for natural selection. What effects can mutations have and how frequently do they occur? Inheritable mutations can be neutral, harmful, or beneficial. Because mutations are random changes to the genetic code, they are much more likely to be neutral or harmful than they are to be beneficial. A neutral mutation is one that has no immediate effect on 552 Chapter 12 NEL

4 Section 12.3 an individual s fitness, or reproductive success. A harmful mutation reduces an individual s fitness and usually occurs when a cell loses the ability to produce a properly functioning protein or when major chromosomal changes adversely affect meiosis and mitosis. A beneficial mutation, which occurs when a cell gains the ability to produce a new or improved protein, gives an individual a selective advantage: increased reproductive success. Types of Mutations Different types of mutations vary in their ability to affect the phenotypes of individuals and their impact on the evolution of populations. Point mutations are changes in single base-pairs along the DNA molecule. When a point mutation occurs in a eukaryotic organism s genome where DNA is noncoding, it will be neutral. When a point mutation causes an amino acid substitution in a coding region of the DNA, a new gene product and, therefore, a new phenotype is produced. The change might be deleterious (or lethal), hindering the proper functioning of the final protein product. In other cases, the change may have no significant effect on the protein s function. Rarely, a point mutation could result in a protein with an improved or new function that benefits the individual. Small insertions and deletions that occur within functioning genes almost always produce a nonfunctioning gene; such mutations are usually harmful. Because they are rarely beneficial, these mutations do not play a major role in evolution. While large-scale inversions are often neutral mutations, they are a useful tool for evolutionary biologists as their presence can be inferred by examining chromosome banding patterns (Figure 5). Gene duplication occurs when unequal crossing over during meiosis results in an additional copy of one or more genes being inserted into a chromosome. This kind of mutation is important because it is a source of new genes. At first, these duplicated genes are just extra copies and add redundancy to the genome, providing no advantage to the individual. However, the new DNA is then free to mutate and, potentially, gain a new function. Such duplication events can ultimately produce entire gene families with very similar structures but altered functions. For example, many species have a family of genes called histones. These gene copies, numbering in the hundreds, are all very similar in structure and located very close together on the same chromosome. The small differences in their DNA sequences are thought to result from point mutations that take place after duplication events. In addition, nonfunctioning pseudogenes genes that are duplicated and later lose their ability to be transcribed provide very strong evidence for evolution. How common are mutations? The best experimental evidence suggests that point mutation rates range from about 1 in cell divisions in species with a very small genome (e.g., bacteria) to one or more in each gamete in species with a large genome. Because they rarely result in obvious changes to an individual organism s phenotype, they are not readily observable. Many harmful mutations result in the death of the gamete or individual before birth. Despite these difficulties, researchers sequencing an entire genome are finding evidence of frequent gene duplication. In humans, a gene coding for one enzyme glyceraldehyde-3-phosphate dehydrogenase used in glycolysis occurs in a single functioning copy and 20 nonfunctioning pseudocopies. The 300-base-pair sequence called Alu, which appears to have originated as a copy of a gene coding for ribosomal RNA, serves no function but is present in copies and constitutes about 5% of the entire genome. (a) fitness general term referring to lifetime reproductive success of an individual harmful mutation an inheritable change in a cell s DNA that impairs the proper operation of a gene product or regulatory function or adversely affects mitosis or meiosis beneficial mutation an inheritable change in a cell s DNA that results in an additional or enhanced gene product or regulatory function gene duplication a mutation leading to the production of an extra copy of a gene locus, usually resulting from unequal crossing over pseudogenes DNA sequences that are homologous to functioning genes but are not transcribed (b) Figure 5 A chromosome before (a) and after (b) a mutation. Large inversions can result in the reversal of banding patterns along chromosomes. NEL Mechanisms of Evolution 553

5 INVESTIGATION Agents of Change (p. 577) Population size, genetic drift, and natural selection all affect allele frequencies. How can their influence be predicted? SAMPLE problem Polyploidy, a mutation that results in three or more sets of chromosomes, occurs when unreduced (diploid) gametes join to form a cell containing one or more entire extra sets of chromosomes. Instead of being diploid (2n), some organisms are tetraploid (4n) or even octoploid (8n) or more. The fern, Ophioglossum reticulatum, contains an astonishing 1260 chromosome pairs (630n). Polyploidy, the most dramatic form of mutation, provides an organism with an immediate doubling of genetic material. This type of mutation has played a major role in the evolution of plants; the majority of ferns and almost half of all flowering plants are polyploids. Animals are rarely polyploids. E. coli Mutation Rate The population of the microorganism E. coli living in your large intestine could be conservatively estimated at 10 billion bacteria. Experimental evidence suggests that these bacteria undergo cell division at a rate of once every hour and experience a mutation rate of 1 per 5000 divisions. How many mutations can this population of bacteria expect to experience in one year? Solution divisions d 36 5d y 1 mutation (17 billion) mutations divisions Note: It is assumed that the population does not increase in size and that the number of surviving bacteria remains constant. Therefore, the number of cell divisions in each generation remains the same. Answer million Practice 1. A population of 10 million free-tailed bats lives in a large cave. Assume that each year 5 million baby bats are born but the population remains the same. If the mutation rate averages 0.4 mutations per gamete, how many mutations would likely occur in 200 years? DID YOU KNOW? Miracle Grain Triticale is a grain of hybrid origin produced by crossing rye and wheat. The hybrid lacked homologous pairs of chromosomes and was infertile. Researchers treated the hybrid with the drug colchicine, which prevented the first meiotic cell division and resulted in the production of diploid (2n) gametes. When these gametes fertilized each other, they produced synthetic tetraploids (4n). These tetraploids contain a diploid set of homologous chromosomes for both rye and wheat within the same individuals. The hybrids are fertile and yield a high-quality grain. SUMMARY Random Change Evolution occurs when the allele frequencies of a population change over time. Genetic drift and gene flow produce changes in allele frequencies and affect genetic diversity. The source of all new genetic information is mutation. Gene duplications are the main source of new genetic material. As extra copies, they are free to mutate without the likelihood of causing harm. Although rare in individual cells, mutations are numerous in large populations over many generations. 554 Chapter 12 NEL

6 Section 12.3 Section Questions Understanding Concepts 1. Use the term allele frequency to explain how biologists define and quantify evolution within a population. 2. Relate two ways in which alleles can become fixed in a population. 3. Define genetic drift and genetic flow, offering two examples to illustrate each definition. 4. Suggest three types of organisms that might produce founder populations. Explain the process that results in this effect. 5. Explain why harmful mutations play virtually no role in the evolution of populations. 6. When a mutation causes gene duplication, it often has little or no immediate effect. How do such mutations play a role in evolution over longer periods of time? 7. For each of the following situations, explain whether the Hardy Weinberg equilibrium would be maintained generation after generation: (a) a population of African violets maintained by a plant breeder (b) the population of the black fly, Simulium venustum, in northern Ontario (c) a racoon population living in the Humber Valley in west Toronto (d) a newly discovered bird population on a remote island off the coast of British Columbia 8. How do pseudogenes offer compelling evidence in support of evolution? 9. Consider the Practice Question about mutation in bats on the previous page. If beneficial mutations are very rare, accounting for only one in every 3 million, how many beneficial mutations are likely to occur in the bat population for the same 200-year period? 10. The world s population of cheetahs is almost identical genetically (Figure 6). Male cheetahs are known to have low sperm counts and the species in general has a low resistance to many infectious diseases. All cheetahs are thought to be homozygous at over 99.9% of their gene loci. Explain how a severe genetic bottleneck effect in the past could account for these observations. Applying Inquiry Skills 11. During the fall migration, several Canada geese stop at a river near a good food source and then nest there the following spring. Because of the abundance of food, this population of geese stops migrating. What effects, both immediate and long-term, might this situation have on the gene pools of the original and founder populations? 12. If variation in species were solely a result of genetic recombination during sexual reproduction, how would that limit the evolution of species? 13. It is thought that a billion prairie dogs once populated an area of more than 100 million ha. Their current territory has been reduced and fragmented to less than 1% of this original space. Predict the impact of these changes in habitat on the prairie dog gene pool, as well as on the evolution and survival of the species. Making Connections 14. Find and describe an example that does not appear in this text in which the founder effect has altered the allele frequency of a human population. 15. Why might evolutionary biologists be more concerned with the study of population genetics than the study of the simple inheritance of alleles by offspring from their parents? 16. Wildlife biologists in British Columbia estimate that fewer than 100 Vancouver Island marmots, Marmota vancouverensis, were alive in (a) Research this endangered species using print and electronic sources to determine the cause(s) of the severe bottleneck effect in their population. (b) What efforts, if any, are being made to maintain the genetic diversity of this species? GO Figure 6 All cheetahs today are virtually identical genetically. NEL Mechanisms of Evolution 555

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