Evolution of Populations Study Guide - Answer Key

Evolution of populations 1. ‘Complete these definitions or ideas that are central to understanding the Hardy-Weinberg theorem. a. Population: A group of individuals of the same species that live in the same area and interbreed to produce fertile offspring b. Gene pool: The total aggregate of all the alleles for all of the loci for all of the individuals in a population c. Genetic drift: d. Allele frequency: e. Genotype: f. Phenotype: 2. Suppose that there is a population of plants that have flower color that is controlled by one gene. Where R is completely dominant and codes for a red flower over r is recessive and encodes a white flower. A flower that is heterozygous will have a genotype of Rr and a phenotype of RED 3. Consider the example of the medium ground finch (Geospiza fortis), a seed-eating bird that lives on the Galápagos Islands. In 1977, the G. spiza population endured a long period of drought. Of 1200 birds, only 180 survived. The surviving finches had larger, deeper beaks than the finches that died. Soft, small seeds were in short supply during the drought. Large, hard seeds were more abundant. Finches with large, deep beaks could crack the large seeds and thus were able to survive the food shortage during the drought. A common misconception is that individual organisms evolve over their lifetimes. However, evolution is the changes in the population overtime. a. Natural selection does act on individuals. Explain how individuals did not evolve and natural selection acted on the individual finches in the example above. Individual finches did not evolve. Each bird had a beak of a particular size, which did not grow larger during the drought. b. Explain how the finch population evolved from one generation to the next in the example above. The proportion of birds with large beaks in the population increased from generation to generation because birds with large beaks were better able to survive the drought and reproduce successfully 4. In this chapter we discuss changes in allele frequencies in a population from one generation to the next. This type of evolution is called MICRO EVOLUTION 5. There are three (3) mechanisms that can change allele frequencies in a population. State them below. a. Natural Selection b. Genetic Drift c. Gene flow 6. Individual variation occurs in all species and often reflects genetic variation, differences among individuals in the composition of their genes. However, not all phenotypic variation is heritable. A person who builds their muscles over their lifetime will not pass that to their offspring. Phenotype is the product of an inherited genotype and environmental influences. Only the genetic component (the alleles) of variation has evolutionary consequences. a. Evolution depends on genetic variation. Name two processes that produces new alleles and new genes 1. Mutations Most point mutations are harmless. Much of the DNA in eukaryotic genomes does not code for protein products. Because the genetic code is redundant, some point mutations in genes that code for proteins may not alter the protein’s amino acid composition. o Even if there is a change in an amino acid as a result of a point mutation, it may not affect the protein’s shape and function. On rare occasions, a mutant allele may actually make its bearer better suited to the environment, increasing its reproductive success. o 2. Gene duplications Duplicated segments can persist over generations and provide new loci that may eventually take on new functions by mutation and subsequent selection. The result is an expanded genome with new genes that may take on new functions b. What is the limitation of the two processes. Meaning these processes could happen in different types of cells, but what cell line will make new alleles that are heritable. In multicellular organisms, only mutations in cell lines that form gametes can be passed on to offspring. c. What is the effect of sexual reproduction in populations? In sexually reproducing populations, most of the genetic variation results from the unique combinations of alleles that each individual receives. Variant alleles originated from past mutations. However, sexual reproduction shuffles variant alleles and deals them at random to produce unique individual genotypes. Three mechanisms contribute to the shuffling: crossing over, independent assortment of chromosomes, and fertilization. The combined effects of these three mechanisms ensure that sexual reproduction rearranges existing alleles into new combinations each generation, providing the genetic variation that makes evolution possible. 7. The Hardy-Weinberg theorem states that in a population that is _NOT_ evolving, the allele frequencies and genotype frequencies remain constant from one generation to another. 8. What are the assumptions of Hardy-Weinberg theorem? 1. 2. 3. 4. 5. No mutations. The gene pool is modified if mutations alter alleles or if entire genes are deleted or duplicated. Random mating. If individuals pick mates with certain genotypes, or if inbreeding is common, the mixing of gametes will not be random and genotype frequencies will change. No natural selection. Differential survival or reproductive success among genotypes will alter allele frequencies. Extremely large population size. In small populations, chance fluctuations in the gene pool will cause allele frequencies to change over time, a process called genetic drift. No gene flow. Gene flow, the transfer of alleles due to the migration of individuals or gametes between populations, will change the frequencies of alleles. 9. In the following scenario, state which assumption of the Hardy-Weinberg Law is being violated and give the basis for your choice. a. In a particular region of the coast, limpets (a type of mollusc) live on near-shore habitats that are uniformly made up of brown sandstone rock. The principle predators of these limpets are shorebirds. The limpets occur in two morphs, one with a light-colored shell and one with a dark-colored shell. The shorebirds hunt by sight and are able to see the light limpets on the dark sandstone easier than the dark limpets. Natural selection is occurring, limpets with light coloration are more easily spotted and eaten by shorebirds. 3. No natural selection. Differential survival or reproductive success among genotypes will alter allele frequencies. b. In Chen caerulescens, a species of goose, the white body form (the snow goose) and the blue body form (the blue goose) occasionally coexist. In these areas of contact, white-by-white and blue-by-blue matings are much more common than white‑by‑blue matings. There is mate preference. 2.Random mating. If individuals pick mates with certain genotypes, or if inbreeding is common, the mixing of gametes will not be random and genotype frequencies will change. 10. A population of soybean plants has incomplete dominance for leaf color. 49 individuals are Homozygous plants (Cy Cy) and yellow, 111 individuals are Heterozygous and yellow-green (CYCG ), and 56 individuals are homozygous recessive (CGCG) and green. a. How many copies of the flower color allele are there in the soybean population? ( 49 x 2) + (111 x 2) + (56 x 2) = 432 b. What percentage of the plants in the population are: Homozygous Dominant (Cy Cy): 49 / 216 = 22.6% Heterozygous (CYCG ): 111/ 216 = 51.4% Homomzygous recess. (CGCG): 56/216 = 26% c. Calculate the frequency of the CY(p) and CG (q) allele in the population. Cy = 49 individuals, each has two CY alleles + 111 individuals have one Cy allele = 98 + 111 = 209 / 432 = 48.4 = p Cg = 56 individuals, each has two CG alleles + 111 individuals have one Cg allele = 112 + 111 = 223 = q = 51.6 = q 11. Natural selection is based on differential survival and reproductive success. a. In order for natural selection to occur individuals in a population must….. vary in their heritable traits. b. Individuals with variations better suited to the environment will tend to…… produce more offspring than those with variations that are less well suited. c. As a result of selection, how does it affect the alleles that are passed on to the next generation? alleles are passed on to the next generation in frequencies different from their relative frequencies in the present population 12. What effects can natural selection have on populations? For example, what types of selection can occur in a population Directional selection, stabilizing selection, disruptive selection 13. Describe how each different type of selection affects a population? Directional selection is most common during periods of environmental change or when members of a population migrate to a new habitat with different environmental conditions. Directional selection shifts the frequency curve for a phenotypic character in one direction by favoring individuals who deviate from the average. o For example, an increase in the size of seeds available as food led to an increase in beak depth in a population of Galápagos finches. Disruptive selection occurs when environmental conditions favor individuals at both extremes of the phenotypic range over those with intermediate phenotypes. o For example, two distinct bill types are present in Cameroon’s black-bellied seedcrackers. o Larger-billed birds are more efficient in feeding on hard seeds, and smaller-billed birds are more efficient in feeding on soft seeds. Birds with intermediate bills are relatively inefficient at cracking both types of seeds and thus have lower relative fitness Stabilizing selection favors intermediate variants and acts against extreme phenotypes. Stabilizing selection reduces variation and maintains the status quo for a trait. o Human birth weight is subject to stabilizing selection. o Babies much larger or smaller than 3–4 kg have higher infant mortality than average-sized babies. 14. A plant biologist has been carrying out a long-term study of local buckwheat (chapparal plant) in southern California. She has 20 years of data showing that a Buckwheat has a variable average leaf size of 4 cm, with some plants having larger leaves of up to 7 cm in width and some plants having smaller leaves, as small as 1 cm in width. She has noticed that over the last 5 years of drought, the average leaf width of buckwheat has decreased to 2.2 cm width. a. Tell me which type of selection processes in taking place. Directional selection b. Explain how you know. Average leaf size has become smaller over time. Draw a line graph representing the population over the first 15 years and the population after selection. # of individuals c. Leaf width d. On a new graph like the one above, what would the population look like if there was disruptive selection? 15. Genetic drift occurs when changes in gene frequencies from one generation to another occur because of chance events. a. Genetic drift will have an effect in a small population. b. What two situations will result in genetic drift? Describe the how each situation works. 1. founder effect occurs when a new population is started by a small number of individuals who do not represent the gene pool of the larger source population. At an extreme, a population could be started by a single pregnant female or a single seed with only a tiny fraction of the genetic variation of the source population. Genetic drift continues from generation to generation until the population grows large enough for sampling errors to be minimal. Founder effects have been demonstrated in human populations that started from a small group of colonists. o 2. The bottleneck effect occurs when the numbers of individuals in a large population are drastically reduced by a disaster. By chance, some alleles may be overrepresented and others underrepresented among the survivors. Some alleles may be eliminated altogether. Genetic drift continues to change the gene pool until the population is large enough to eliminate the effect of chance fluctuations. The bottleneck effect is an important concept in conservation biology of endangered species o 16. This summer fires were raging in the western United States (California, Oregon, Washington). The burned areas in Washington are now experiencing large amounts of rain and the burned areas are having major mud slides. In one region, a mudslide has wipe out 80% of a population of purple pansies (plant). The surviving purple pansies in the population have fewer flowers than the original population. Number of flowers in controlled by a specific gene. a. What is most likely the cause of the change in the population. Genetic drift due to a bottleneck b. Explain your reasoning. The population experienced a dramatic decrease in numbers due to flooding and mudslides. No particular genotype had a higher survival rate due to inherited traits. Individuals with the gene for less flower became more frequent in the population by random chance. 17. There are four key points about genetic drift. List them below 1. Genetic drift is significant in small populations. 2. Genetic drift causes allele frequencies to change at random. 3. Genetic drift leads to a loss of genetic variation within populations 4. Genetic drift can cause harmful alleles to become fixed. 18. Explain the mechanism of Gene flow. Gene flow is the transfer of alleles among populations due to the migration of fertile individuals or gametes. Gene flow tends to reduce differences between populations. o If extensive enough, gene flow can combine neighboring populations into a single population with a common gene pool. o Gene flow is an increasingly important agent of evolutionary change in human populations. o With increased human mobility, mating is more common between previously isolated populations, leading to an exchange of alleles and reducing genetic differences between human populations. a. If a nearby wildflower population consisted entirely of white flowers, its pollen (CW alleles only) could be carried into another hypothetical wildflower population. o What would be the result of this gene flow between the two populations? The result would be an increase the frequency of CW alleles in the hypothetical population in the next generation. 19. Examine the scenario below. Small island A contains three separate populations of a single species of cherry tree. The seed size varies between trees. That is, some trees produce seeds that are all in the small size ranges, others produce seeds all in the middle size ranges, and others produce seeds in the large size ranges. A small population of mice is introduced to the island. The mice eat cherries and are the only predators on the cherry trees. When the mice eat a cherry, they completely digest it and the pit or seed inside it. The mice choose medium and large seeds and leave the smallest seeds uneaten. For each scenario: a. Decide whether or not natural selection is operating. In doing this, indicate whether there is variability in the population(s). If no, what does this imply about evolution? If yes, what is the nature of the variation? For example, what characteristics must the variation have for selection to operate on it? Natural selection is operating. Mice are selective choosing medium and large seeds based on some triat. b. Is there any indication that members of the population(s) differ in fitness? If no, what does this imply about the operation of natural selection? If yes, describe the difference in fitness. Cherry trees differ in fitness based on the size of the cherry seed. Medium and large seeds are selected against and small seeds are less likely to be eaten. Trees that produce cherries with smaller seeds are going to leave behind more offspring then trees who’s seeds are larger and leave less offspring.