Terms to know: gene flow, genetic drift, local adaptation, population bottleneck (bottleneck effect), genetic (heritable) variation, Hardy-Weinberg Proportions (as distinct from Hardy-Weinberg Equilibrium), inbreeding, identity by descent

Questions:

1. Describe two forms of evolution that could result in an allele with adverse (harmful) effects on survival and reproduction becoming more common in a population; explain why each would have this effect.
2. What effect does genetic drift have on: (a) genetic variation (b) genotype frequencies (are they in Hardy-Weinberg proportions or not? Do they change from generation to generation?). Why does genetic drift has these effects?
3. Give examples of how molecular studies which directly examine allele and frequencies through identification of the DNA that makes up genes or the proteins for which the genes encode information could be used to study each of the following: natural selection, genetic drift, and gene flow.
4. Read about Buri's (1956) experiments on random fixation and loss of heterozygosity in fruit flies in your textbook.  In his experiment, what is predicted about the evolution of eye color by a hypothesis that evolution occurs only through genetic drift in his lab populations? What is predicted about the evolution of eye color by a hypothesis that different genotypes differ in fitness so that eye color evolves through natural selection? Which hypothesis (evolution through genetic drift or evolution through natural selection) is supported by Buri's experiments? How do the results of Buri's experiments compare to the computer simulations of genetic drift that we are doing in this class?
5. What is inbreeding?  How is inbreeding related to genetic drift?  Why does inbreeding often result in offspring with low ability to survive and/or reproduce?
6. You are comparing small populations of two different species of frog.  One has occurred in small populations throughout its evolutionary history.  The other historically occurred in larger populations, but because of recent declines in numbers of amphibians resulting from excess exposure to ultraviolet light, now occurs in small isolated populations in areas that are shaded from light.  Inbreeding occurs in both kinds of frog.  In which do you expect to see reduced survival of offspring resulting from inbreeding?  Why would you see this more in one species than in the other?
7. The population of sea otters on the west coast, historically a very large population, underwent a sudden decrease in population size.   Population sizes stayed low for a number of generations, then gradually increased.  Now many of the sea otter populations are as large as they were historically.  (a) what is the term for this pattern of population change?  (b) During this period, what probably happened to genetic variation? (c) How do you predict genetic variation in large populations now to compare to genetic variation that was present in large populations before the population decrease? Explain why.
8. One concern of biologists interested in conserving populations of endangered species is that these species will lose genetic variation.

(a) Explain why these populations are predicted to lose genetic variation.  Which form of evolution is the main cause of this loss of variation?  Give two reasons why this kind of evolution is a problem, if your goal is to conserve this species. (b) It has been suggested that a way to counteract loss of genetic variation in such species is to allow individuals to move among populations, or to move them among populations.  What kind of evolution would be occurring if individuals were moved among populations?  Why would it help counteract loss of genetic variation?  Give one reason why this kind of evolution might help the endangered population AND one reason why this kind of evolution might hurt the endangered population. (c) Suppose you have decided that you want to move individuals among populations of a species to maintain variation.  If you want to move just enough individuals to prevent fixation of different alleles in different populations, at what rate should individuals be moved?
9. What aspects of populations make local adaptation likely?  What makes local adaptation unlikely?
10. Three species of wren (little brown birds), Marsh Wrens, Sedge Wrens, and Gray-breasted Wrens, differ in aspects of their population biology. Sedge Wrens have very high rates of dispersal so that individuals from any area are just as likely to reproduce with birds from very distant areas, nearby areas, or the same area. Marsh Wrens live in intermediately sized populations in marshes, are most likely to stay  and reproduce in the marshes where they were born, although a few individuals do disperse to distant marshes each generation. Gray-breasted Wrens live in small populations in isolated patches of their preferred habitat; they almost never disperse away from the area where they were born. In which of these species (one or more) would you expect high levels of local adaptation? In which would you expect low levels of local adaptation? Explain your reasoning based on what you would expect about within population and between population genetic variation in these three species.
11. Chaffinches (little gray and pink birds) occur throughout Europe, in Northern Africa, and on islands between Europe and Africa. Mainland populations are typically very large; island populations are very small.The map below shows several populations between which you have estimated gene flow.

 

 
 
 
 
 



1. Between which populations would you expect to see fixed differences in allozymes? Why?
2. Between which populations would you expect to see most differences that reflect adaptation to different local conditions? Why?
3. State something that might PREVENT high levels of local adaptation in each of these populations?
12. You compare three species of aquatic snake: Lake Snakes occur in medium sized populations and have dispersal of about 3 individuals between populations each generation, Pond Snakes live in small populations in isolated ponds and an average of only one individual every twelve generations disperses from pond to pond, and Ocean Snakes live in large populations with very high levels of dispersal from population to population.  In which of these snake species should there be the highest level of local adaptation to the specific conditions in which a population occurs?  Explain why; consider the effects that gene flow, genetic drift, and natural selection are likely to have on populations of EACH snake species to justify your answer.
13. Read about the study of migration as a mechanism for evolution in the water snakes of Lake Erie in Freeman and Herron (2001). If all evolution were occurring through natural selection, what would snakes on islands and on the mainland look like?  Why?  If all evolution were occurring through gene flow what would snakes on islands and on the mainland look like? Why? What is the actual appearance of the snakes in these different areas? Explain the actual appearance of snakes in these different areas.
14. Read about the study of migration as a homoginizing evolutionary force across populations of red bladder campion (a flowering plant) on in Freeman and Herron (2001). Note that they study genetic variation among populations -- that is, how different population are from each other genetically.  If gene flow were the only form of evolution occurring, what would be predicted about genetic variation among populations? Why? If genetic drift were the only form of evolution occurring, what would be predicted about  genetic variation among populations? Why? Giles and Goudet conduct their study in an area in which the ages of populations are known.  How are the ages known?  In this situation, the age of populations is predicted to be related to the relative impacts of genetic drift and gene flow on these populations.  Why is this true?  Do the results they observe show the predicted effects of gene flow and genetic drift on genetic variation among populations? Why/why not?
15. In a population of wild roses, at a locus that controls flower color, RR individuals have red flowers, RW individuals have pink flowers, and WW individuals have white flowers. You sample flowers from 80 individuals and find the following numbers of phenotypes: 60 white, 15 pink, 5 red.  Conduct a chi-squared test to determine whether you are at least 95% sure that genotypes differ from Hardy-Weinberg proportions.  Given your chi-squared results, which of the following could be occurring in your population: natural selection, gene flow, genetic drift, mutation?
16. In a population of grasshoppers one gene has alternate alleles F and S. You collect 80 individuals and use molecular techniques to identify the alleles, and find:

Genotype:	FF	FS	SS
# Individuals with Genotype	10	10	60

Use a chi-squared test to determine whether these genotypes are in Hardy-Weinberg proportions.  Based on your results, does it appear likely that natural selection is affecting the this gene?
17. Tail length in a species of fish is controlled by a single locus: TT individuals have long tails, tt individuals have short tails, and heterozygotes have medium length tails.  To test whether tail length is in Hardy-Weinberg equilibrium you randomly sample 100 fish from a population and find that 80 of them have long tails, 15 of them have medium tails, and 5 of them have short tails.
1. What are the genotype frequencies for genotypes TT, Tt, and tt in this fish population?
2. What are the expected genotype frequencies for genotypes TT, Tt and tt if the population is in Hardy-Weinberg equilibrium?
3. Determine whether the observed genotype frequencies differ significantly from the frequencies expected in Hardy-Weinberg equilibrium at the 5% level of significance.
4. Give one hypothesis that would explain why tail length could be in frequencies other than Hardy-Weinberg (answer this question without considering whether you found that the frequencies actually differ from Hardy-Weinberg or not).
18. You have collected tissue from 100 individual adult dogwood trees in a population.  You study an enzyme locus and find the following genotypes (F and S refer to the two enzyme alleles):

Genotype:	FF	FS	SS
# Individuals with Genotype	10	60	30

1. There seem to be a lot of heterozygotes in this population.  Are there significantly more than expected by chance, at the 5% level of significance (i.e. are you at least 95% sure that there are more than are expected by chance?)?
2. What evolutionary processes could result in such a distribution of genotypes?
19. A population of turtles varies in shell thickness: TT individuals have thick shells, Tt individuals have intermediately thick shells, and tt individuals have thin shells.  You measure the shell thickness of 100 individuals and find the following numbers of adult, reproducing individuals with each shell type:

Phenotype:	Thick shells	Intermediate Shells	Thin Shells
# Individuals with Phenotype	77	21	2

1. What is the frequency of the t allele in the population?
2. You hypothesize that thick-shelled turtles have the highest fitness because they are best protected. To test this, conduct a chi-squared test to determine whether or not shell thickness is in Hardy-Weinberg proportions at the 5% level of significance. What can you conclude about whether the population is in Hardy-Weinberg proportions? Does this conclusion support your hypothesis regarding the fitness of different shell thicknesses?