Chapter 23 Reading Quiz 1. 2. 3. 4. 5. What is all of the genes in a population called? In the Hardy-Weinberg equation, what symbolizes the dominant allele? Name one way in which natural populations do not fit Hardy-Weinberg equilibrium. Any change in the allele frequencies of a population is called _____. 10,000 years ago, cheetahs went through what type of genetic drift? 1. 2. 3. 4. 5. Very large population size Isolation from other populations (no migration in or out of group) No net mutations Random mating No natural selection  all genotypes are equal in survival and reproductive success  1. Explain what is meant by the “modern synthesis”.   Comprehensive theory integrating discoveries from different fields (paleontology, taxonomy, biogeography, and population genetics) Emphasized  the importance of populations as units of evolution  central role of natural selection as the primary mechanism of evolutionary change  gradualism as the explanation of how large changes can result from an accumulation of small changes over long periods of time  2. Explain how microevolutionary change can affect a gene pool.   Gene pool  the total aggregate of genes in a population at any one time Microevolution  small scale evolutionary change represented by a generational shift in a population’s relative allelic frequencies  3. In your own words, state the HardyWeinberg theorem.   The frequencies of alleles in the gene pool will remain constant unless acted upon by other agents Describes the genetic structure of nonevolving populations  4. Write the general Hardy-Weinberg equation and use it to calculate allele and genotype frequencies. P2 + 2pq + q2 = 1   The sum of frequencies must = 100% (p + q = 1) When 2 alleles exist, only the frequency of one must be known since the other is derived  1 – p = q OR 1 – q = p  #4 example Ex: One individual in every ten thousand has ‘phenylketonuria’, a deficiency which does not allow the body to process the amino acid phenylalanine. What percent of the population are carriers for this recessive disease? 1 in 10, 000 ; recessive = q2 ; q2 = .0001 ; q = .01 p = 1 – q ; p = 1 - .01 ; p = .99 p2 + 2pq + q2 = 1 ; 2pq = 2(.99)(.01) = .0198 therefore: 2% of the population are carriers for phenylketonuria  More examples… 1. .01% of the Caucasian population has cystic fibrosis. In a sample population of 100,000 white people, how many would be expected to carry the disease? About 1 in 17,000 kids in the UK are born with albinism. This is a recessive disorder. On average, what % of the population would be carriers for albinism? In humans, Huntington’s disease is lethal in utero if both dominant alleles are inherited. The disease, however, is dominant. Therefore, for an individual to inherit the disease, they must be heterozygous for the condition. 7 in 100,000 people have Huntington’s. What are the dominant and recessive frequencies for this disorder? How many people in the 100,000 would be recessive for this condition? 5. Explain the consequences of Hardy-Weinberg equilibrium.   It provides a baseline from which evolutionary departures take place It provides a reference point with which to compare the frequencies of alleles and genotypes of natural populations whose gene pools may be changing  6. Demonstrate, with a simple example, that a disequilibrium population requires only one generation of random mating to establish Hardy-Weinberg equilibrium.   Continued sexual reproduction with segregation, recombination, and random mating would not alter the frequencies of alleles and the gene pool would be in Hardy-Weinberg equilibrium If the gene pool was originally in disequilibrium, only one generation would be necessary for equilibrium to be established (as long as random mating is occurring in the population)  7. List the conditions a population must meet in order to maintain Hardy-Weinberg equilibrium. 1. 2. 3. 4. 5. Very large population size Isolation from other populations (no migration in or out of group) No net mutations Random mating No natural selection  all genotypes are equal in survival and reproductive success  8. Explain how genetic drift, gene flow, mutation, nonrandom mating and natural selection can cause microevolution.  These all cause microevolution because each of these conditions is a deviation from the criteria for Hardy-Weinberg equilibrium  Genetic Drift 9. Explain the role of population size in genetic drift.  Genetic drift  changes in the gene pool of a small population due to chance  if a population is small, its existing gene pool may not be accurately represented in the next generation due to sampling error  chance events may cause the frequencies of alleles to drift randomly from generation to generation  10. Distinguish between the bottleneck effect and the founder effect.   Bottleneck  genetic drift which results from drastic reduction in population size - reduces overall genetic variability in a population since some alleles may be gone Founder  when a few individuals colonize a new habitat and genetic drift occurs - inherited diseases are obvious examples  11. Explain why mutation has little quantitative effect on a large population.  Mutation itself has little quantitative effect on large populations in a single generation since mutation at any given locus is very rare  12. Describe how inbreeding and assortative mating affect a population’s allele frequencies and genotype frequencies.   Inbreeding  results in relative genotypic frequencies that deviate from the frequencies predicted for Hardy-Weinberg equilibrium, but does not alter frequencies of alleles (p & q) in the gene pool Assortative mating  type of nonrandom mating which results when individuals mate with partners that are like themselves in certain phenotypic characters ex: toads commonly mate with those of the same size ex: snow geese (blue with blue, white with white) - results in less heterozygotes than Hardy-Weinberg predicts  13. Explain, in your own words, what is meant by the statement that natural selection is the only agent of microevolution which is adaptive.  It is the only agent which is adaptive, since it accumulates and maintains favorable genotypes  environmental change would result in selection favoring genotypes present in the population which can survive the new conditions  variability in the population makes it possible for natural selection to occur  14. Describe the technique of electrophoresis and explain how it has been used to measure genetic variation within and between populations.    The technique allows researchers to identify variations in protein products of specific gene loci Within  the Drosophila population gene pool has 2 or more alleles for about 30% of the loci examined – bottom line: any two flies will differ in genotype at about 25% of their loci Between  geographical variation in allele frequencies exists among populations of most species – due to  natural selection, genetic drift, localized inbreeding  15. List some factors that can produce geographical variation among closely related populations. 1. 2. 3. 4. Natural selection: environmental factors differ among locals Genetic drift: causes chance variations among different populations Localized inbreeding: subpopulations can appear resulting from a ‘patchy’ environment Cline: one type of geographical variation that is a graded change in some trait along a geographic transect  16. Explain why even though mutation can be a source of genetic variability, it contributes a negligible amount to genetic variation in a population.  Mutations produce new alleles  they are random and rare events which usually occur in somatic cells and are thus not inheritable - only mutations that occur in cell lines which will produce gametes can be passed to the next generation - estimate only 1 or 2 mutations occur in each human gamete-producing cell line  17. Give the cause of nearly all genetic variation in a population.   Sexual recombination provides almost entirely the genetic variation which makes adaptation possible Due to  gametes vary extensively from crossing over and random segregation during meiosis  18. Explain how genetic variation may be preserved in a natural population.    Diploidy  hides much genetic variation from selection by the presence of recessive alleles in heterozygotes (not expressed and not selected against) Balanced polymorphism  the ability of natural selection to maintain diversity in a population Heterozygote advantage  have greater reproductive success (ex: sickle cell & anemia)  19. In your own words, briefly describe the neutral theory of molecular evolution and explain how changes in gene frequency may be nonadaptive.  It states that many variant alleles at a locus may confer no selective advantage or disadvantage  variation in DNA which does not code for proteins may be nonadaptive  20. Explain what is meant by “selfish” DNA. Noncoding DNA has resulted from the inherent capacity for DNA to replicate itself and has expanded to the tolerance limits of each species  The entire genome could exist as a consequence of self-replication rather than providing an adaptive advantage to the organism - “selfish DNA”   21. Explain the concept of relative fitness and its role in adaptive evolution.  Relative fitness  the contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus  every aspect of survival and fecundity (reproductive success) are components of fitness Ex: pink flowers (AA & Aa) produce more offspring than white (aa), therefore AA & Aa genotypes have a higher relative fitness  22. Explain why the rate of decline for a deleterious allele depends upon whether the allele is dominant or recessive to the more successful allele.   Deleterious recessives are normally protected from elimination by heterozygote protection Selection against harmful dominant alleles is faster since they are expressed in heterozygotes  23. Describe what selection acts on and what factors contribute to the overall fitness of a genotype. Selection acts on phenotypes, indirectly adapting a population to its environment by increasing or maintaining favorable genotypes in the gene pool  An organism is an integrated composite of many phenotypic features and the fitness of a genotype at any one locus depends upon the entire genetic context   24. Give examples of how an organism’s phenotype may be influenced by the environment.  physical traits, metabolism, physiology, and behavior are all exposed to the environment and may be selected upon  25. Distinguish among stabilizing selection, directional selection and diversifying selection.   Stabilizing selection  favors intermediate (average) variants by selecting against extreme phenotypes Ex: spider size  large spiders are more easily found by predators; small spiders have difficulty finding and getting food; average size spiders can hide and find food #25 continued…   Directional selection  favors variants of one extreme – shifts frequency curve for phenotypic variations in one direction toward rare variants which deviate from the average Ex: woodpecker beaks – the long beak is always selected over the average and short lengths #25 continued…   Diversifying selection  opposite phenotypic extremes are favored over intermediate phenotypes Ex: limpets (shelled animals) in a tidal area that is dark and light, with no in-between color - dark and light limpets are selected, not the intermediate colors 26. Define sexual dimorphism and explain how it can influence evolutionary change.    It is the distinction between the secondary sexual characteristics of males and females Ex: size, plumage, lion manes, deer antlers, etc… Separate selection process – - have no other adaptive advantage other than attracting mates - showier can contribute more to gene pool  27. Give at least four reasons why natural selection cannot breed perfect organisms. 1. 2. Organisms are locked into historical constraints (descent with modification) Adaptations are often compromises - must be versatile 3. Not all evolution is adaptive - genetic drift; alleles become fixed in small populations 4. Selection can only edit variations that exist - these variations may not represent ideal characteristics - new genes are not formed by mutation on demand 