1. Answer both of the following questions regarding the Hardy-Weinberg Equilibrium (8 pts total):
a. List two assumptions of the Hardy-Weinberg equilibrium and provide examples (real or hypothetical) of the consequences to genetic diversity of a population when each of these assumptions is violated.
The assumptions are that there is no natural selection, no mutations, no gene flow, large population size and random mating.Violations of each lead to the following consequences for genetic diversity:Natural selection decreases genetic diversity within a population.Mutations increase genetic diversity within a population.Gene flow increases genetic diversity within a population.Small population sizes lead to genetic drift, which decreases genetic diversity within a population.Nonrandom mating may not alter allelic diversity, and may increase or decrease genotypic diversity, depending on the type of nonrandom mating.
b. Explain how these two mechanisms you discussed in part “a” work together or against one another to cause or prevent, respectively, changes in allele frequencies.
You must explain how the two mechanisms work together or counteract each other. Simply repeating what you wrote in part a is not sufficient. For example, mutations increase allelic diversity, which then can be acted on by natural selection, perhaps eliminating detrimental mutations or fixing beneficial ones in the population.
Mutations and gene flow may work together - mutations that occur in one subpopulation may be moved via migration and may be incorporated into another population through mating of the migrating individual with another individual.
Mutations and genetic drift work against one another, one to increase diversity, another to decrease it. This is general, and you may imagine how drift may fix a mutation in the population, randomly. Also, it is assumed that the mutations are selectively neutral.
Another example would be genetic drift and natural selection - they may work against one another or with one another. Alleles that are being selected for may actually decrease in frequency in small populations subject to drift, or they may be fixed through drift even if another allele leads to equal fitness.
2. Define autopolyploidy and state it’s significance to evolutionary biology (5 pts).
A type of polyploidy involving the multiplication of chromosome sets from only one species. Autopolyploids may arise from the fusion of diploid gametes that have resulted from the nondisjunction of chromosomes at meiosis. Autopolyploids often have larger cells than the parent plants but are morphologically very similar. They may, however, be at least partially isolated from each other reproductively and occupy different ecological niches. A possible case of instant speciation.
3. Describe one way that reproductive isolating mechanisms may arise during parapatric speciation (5 points).
First, you must understand that parapatric speciation generally occurs along a cline or gradient. There must be strong selection for traits along the cline, and little to no gene flow, in order for reproductive isolation to occur. Therefore, of the reproductive isolating mechanisms we discussed most will fit, but you need to provide a plausible scenario by which isolation will arise. For instance, the example of yellow monkey flower was that the copper tolerance locus is linked to the locus that causes hybrid inviability (or is the same locus). Strong selection for the copper tolerance allele in copper-contaminated soils and selection against individuals that have that allele in non-contaminated soils, coupled with hybrid inviability (which negates any gene flow) leads to reproductive isolation. Another example might be selection for individuals that have a habitat preference associated with the cline, such that they tend not to migrate to other areas along the cline. This would be an example of both spatial and behavioral pre-zygotic mechanisms.
4. Discuss in detail the most common means by which speciation is thought to occur. Provide at least two examples (hypothetical or real) supporting your discussion (8 pts).
Allopatric speciation must be defined or described as spatial separation of populations, separated by a barrier that is impassable or few cross it. This spatial separation causes the populations to evolve separately for a period of time, during which any of the mechanisms of evolution may act to cause the two populations to evolve along different paths. When, and if, the populations ever come together again, they must have evolved reproductive isolating mechanisms that prevent them from mating in order for allopatric speciation to have occured. We discussed several examples, including Hawaiian fruit flies, Darwin's finches, Hawaiian honecreepers, and more.
5. How does evolutionary biology explain variation in virulence of different pathogens (disease-causing organisms)? How can we apply this knowledge to public policy? (8 pts)
You must consider how different modes of transmission may affect or influence the virulence of a pathogen. For instance, direct transmission from host to host, requiring host contact, would likely lead to evolution of low virulence. High virulence is expected more with modes of transmission that use an intermediate vector (such as mosquitoes) or media (wastewater). We can apply this to public policy by providing clean water to communities, which decreased the virulence of cholera, or by providing needle exchange programs and condoms to influence the virulence of HIV. Many of you discussed the use or overuse of antibiotics. Although that can influence the evolution of pathogens, there is no direct relationship to virulence and we can't predict that virulence will increase or decrease along with the evolution of resistance to antibiotics. If you used that as an example you received partial credit, even though you did not directly address variation in virulence.
6. Discuss one of the following two questions (8 pts):
a. Describe Darwin’s argument that natural selection is the mechanism of biological evolution of a population. Include some of the evidence he used to support this part of his thesis presented in On the Origin of Species. Be specific!
Darwin's influences were many, including the ideas that evolution did occur (LaMarck), populations grew exponentially (Malthus), geologic processes were uniform and gradual (Hutton, Lyell), and many of the examples he used influenced his thinking (artificial selection of pigeons, finches on Galapagos Islands, biogeography). His argument that Natural Selection was the mechanism boiled to 5 facts & 3 inferences. Individuals produce many offspring; if all survived population would increase exponentially forever. Most populations are stable in size, and although they can fluctuate no population increases indefinitely. Natural resources are limited; there are not enough resources for all to survive, so there must be a struggle for resources and few survive. In addition, he noted that individuals vary, yet resemble parents. Although he did not know the mechanism, he concluded that much variation is heritable. Survival therefore is not random; it depends on inherited characteristics. Those with best fit to environment are more likely to survive & reproduce. This unequal ability leads to gradual changes in populations; favorable characteristics accumulate.
The mechanism can affect biological evolution by favoring certain characteristics over others. The gene pool of the population changes over time to accumulate more and more copies of the alleles that code for adaptations that give their possessors higher fitness. That is, those individuals have a better ability to survive and they produce more offspring that themselves survive to reproduce.
b. Discuss how relative and absolute dating methods are used to determine the age of the Earth, and how both point to a very old age (determined to be about 4.6 billion years).
Stratigraphy and fossil evidence are relative methods that were used to show that the Earth and it's inhabitants have changed over time. Given that geologic processes are uniform and gradual, the many strata observed in cliffs and the Grand Canyon must have been laid down over long periods of time. The fact that many of the species of fossils are extinct and that the fossils in strata closer to the surface (are more recent) appear more similar to extant creatures is further evidence that the earth is much older than previously supposed. Use of radiometric data provides absolute dates to many rock formations and fossils. Using the fact that radioactive decay of a sample of radioactive isotope is constant and measurable, along with various facts about the formation of the rock from which a sample is derived, allows us to accurately data rocks from some strata. Use of multiple techniques validates estimates of age. All of these data point to an extremely old age for the Earth.
7. Consider diversity in a species of bumblebee that lives in an urbanized area that is fragmented by many highways and roads. The bees can cross the highways, but they typically don’t. They pollinate many species of plants that also have subdivided populations (6 pts).
a. What would you predict might be a consequence to among-population variation and within-population variation for populations isolated in such a manner?
Among population variation should increase, and within-population variation should decrease (or may remain unchanged).
b. If the bumblebee species underwent a speciation event, what would likely be the mechanism and mode of speciation?
The mechanisms cannot be gene flow, but could be genetic drift (which is most likely), natural selection (also somewhat likely, depending on conditions - the separation is not likely to be great enough to lead to environmental differences). Mutation followed by drift or selection is plausible. The mode of speciation is either allopatric or parapatric, depending on whether you considered the road a barrier or if you described a scenario where the populations were separated along some kind of gradient.
8. Consider the graph below and answer each of the following three questions (7 pts).
a. Describe the mechanism that causes the trajectories shown. Be specific with regards to what is happening to each genotype.
The mechanism is natural selection. In this example, AA is favored over both Aa and aa. This is evident by the loss of both of those latter genotypes. Could be stated as selection for AA or selection against aa and Aa. Selection against aa appears to be greater than against Aa, as evidenced by the trajectories.
b. What is the equilibrium condition?
Fixation of the A allele, AA genotype is found in all individuals.
c. Show, using the Hardy-Weinberg equilibrium, that the population is not in equilibrium at generation 40.
Using the mathematical approach will only work if you compare the expected at an earlier or later generation with the observed at generation 40. Because the genotypic distributions are clearly changing on the graph, and have not yet reached equilibrium, the population is not in equilibrium at generation 40.
9. What are two of the major drawbacks to application of the Biological Species Concept (5 pts)?
The BSC is problematic for asexual species, species that self-fertilize, populations separated spatially and temporally, and in determining whether two populations can actually mate.
10. In a population of the plant Phlox the alcohol dehydrogenase (ADH) locus has three alleles “a”, “b” and “c”; none is dominant to the others. The following genotypic frequencies were found in a population; aa = 0.30, ab = 0.10, bb = 0.10, ac = 0.05, bc = 0.25, and cc = 0.20. (SHOW WORK – 9 points)
a. If the population consists of 100 individuals, determine the actual number in the population showing each genotype.
Simply multiply each proportion by 100 to get frequency.
b. Calculate the allele frequencies (set a = p, b = q, and c = r).
Use the equation [(2 x #aa) + #ab + #ac] / (2 x N) = [60+10+5] / 200 = 0.375 for a.b = 0.275c = 0.35
c. Calculate the expected genotypic frequencies for the ADH locus in Phlox under Hardy-Weinberg equilibrium assumptions.
p2 = (0.375) x (0.375) = 0.14 (compare to aa = 0.30)2pq = 2 x (0.375) x (0.275) = 0.21 (compare to ab = 0.10)2pr = 2 x (0.375) x (0.35) = 0.26 (compare to ac = 0.05)q2 = (0.275) x (0.275) = 0.08 (compare to bb = 0.10)2qr = 2 x (0.275) x (0.35) = 0.19 (compare to bc = 0.25)r2 = (0.35) x (0.35) = 0.12 (compare to cc = 0.20)
d. Is there evidence for evolution occurring in this population? If so, what microevolutionary mechanism may be in operation, judged by actual vs. expected genotypic frequencies? (If not, state that evolution is not occurring)
Yes, there is no consistent pattern, so a likely mechanism is genetic drift. DO NOT SAY THAT DRIFT IS SELECTING FOR A PARTICULAR GENOTYPE - IT IS NOT DRIFT THEN, IT IS NATURAL SELECTION!!! Selection is also possible, either for cc and aa, or against ab and ac. Positive assortative mating is possible, although it not a consistent pattern.
11. Refer to the figure below to answer these three questions (6 points total).
a. A taxonomic group (such as a Family) that included species A, B, and C would be Polyphyletic when referring to the common ancestor of B and C.
b. Could a taxonomic group that includes species A, B, and D ever be monophyletic? What else would need to be true?
Yes, if C, E, and F were also included, and one referred to the most distant common ancestor.
c. The statement “species B and C evolved from species A” is TRUE/FALSE (circle one). Why?
False. A is at the end of the lineage. It is not likely to be the same species as the common ancestor of A, B, and C. A evolved from that common ancestor, just as B and C did.
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