BIO 206W (CH 6,7,)
Explain the various ways in which loci can be maintained in a polymorphic state?
- Alleles for a polymorphic locus are functional (neutral theory) -Mutation-selection balance -Heterozygote superiority -Selection for/against different alleles at different times, place, ecological circumstances and in males vs. females -Frequency dependent selection
If population is in HWE for a particular locus, what 2 features of the population for that locus remain unchanged?
- no change in frequencies of alleles over generations - the frequencies of genotypes must remain as p^2 + 2pq + q^2
5 evolutionary forces
1. natural selection 2. mutation 3. gene flow 4. genetic drift 5. non random mating
If only mutation is affecting the frequency of an allele at a particular locus, what sort of evolutionary outcome do you expect over many generations (i.e., Fig. 6.27)?
A small difference in the frequency of that allele would be expected. Over very long periods of time, mutation can eventually produce appreciable changes in allele frequency. Does so slowly.
What is the coefficient of inbreeding (F)? What is the relationship between heterozygosity of an inbred population and that in a random mating population (i.e., be able to explain the formula HF = HO(1 - F))?
F= coefficient or inbreeding - probability that 2 alleles in an individual are identical by descent HF= the relationship between the heterozygosity of inbred population HO= random mating population
Define genetic drift and describe under what circumstances it has its greatest effect on frequencies of alleles and why that is the case. If only genetic drift is affecting a locus, what will eventually happen to the frequency of each allele if there are just two alleles for the locus?
Genetic drift - any random change in frequency of alleles in a population and has greater impact in smaller populations. If there are 2 alleles then the frequency of each will slowly drift to .5. Genetic drift is evolution that happens by chance
What is the evidence (based on transgenic strains of E. coli as well as epidemiological data) for why the frequency of the ∆F508 allele of the cystic fibrosis transmembrane conductance regulator locus may be maintained at a rate higher than what a mutation/selection balance would yield (i.e., explain the data in Figs 6.31 a) and b) with regard to typhoid fever, caused by Salmonella typhii.
Homozygous F508 cells were almost totally resistant to infiltration by S. typhi. while homozygous wild type cells were highly vulnerable. Heterozygous cells were partially resistant; they accumulated 86% fewer bacteria than did wild type cells
Under what unique circumstance can selection result in no change in the frequency of alleles, but a change in frequencies of genotypes does occur?
If a population is subjected to natural selection it causes a change in frequency of alleles and genotypes
What would you predict would happen to the distribution of the banding phenotypes if suddenly there were only gene flow (and no selection or other evolutionary "forces") influencing those populations? In other words, how would you expect the histograms in Fig 7.6 to change under those circumstances?
If there was no gene flow there wouldn't be any other forces at play so the allele frequencies will stay the same through generations
Compare the frequencies of the two alleles above to what they would be if selection actually favored heterozygotes for the locus. Be able to explain why the frequencies of the recessive lethal alleles in question 6 would reach their respective "equilibrium" frequencies. In other words, explain figure 6.21.
If they formed a heterozygote, the frequency would go to .5 instead of 1 or 0. V- viable L- lethal VV or VL = alive LL = dead Populations evolved for 15 generations, predicted V would rapidly rise and then more slowly continue to rise to 0.94 (this didn't happen) V reached an equilibrium, or unchanging state at frequency 0.79 so the lethal was 0.21 How does natural selection maintain this? HETEROZYGOTE SUPERIORITY (overdominance)
Describe the experiment in which investigators created populations of 8 randomly chosen females and 8 randomly chosen males to establish a new population for 107 experimental populations over the course of 19 generations. What happened to the frequency of the allele they were measuring over the course of time (i.e., generations) and why did it change?
It is genetic drift bc it is randomly choosing eye color. Some generations had an increase in frequency and some decreased. At the end of the experiment 30 generations lost the allele so frequency is 0 and 28 generations had frequency of 1, very close to 1:1 ratio you would predict w genetic drift They are losing heterozygosity
Define Effective Population Size. What factors make it less than N and why do they tend to do so?
Ne is the actual number of reproducing adults (effective population) and it is always smaller than absolute number of individuals (N) Factors that make Ne less than N: -number of years to reproductive age -average age in population -sex ratio of individuals that reproduce -non-random mating -any factor that reduces genetic contribution of some individuals to future generations
Describe the relationship between infection with HIV and the 32 CCR5 allele. If the 32 CCR5 allele can exist at a frequency as high as 0.2 in some human populations (e.g., Ashkenazi Jewish populations in northern Europe), why hasn't the allele increased in frequency in most (any?) human populations?
People who have 32/32 survive AIDS but 32/+ and +/+ dont
gene pool
all of the alleles in a breeding population
Explain frequency-dependent selection and be able to explain the experimental results shown in Fig. 6.24. How do those results demonstratefrequency-dependent selection? What is causing the selection in that experiment?
Yellow vs purple flower alleles; one is considered rare. Bees switch between flower they decide to pollinate; if they are unsuccessful at one, they switch to another flower or different color. If 10% yellow and 90% purple then the yellow will get the most attention from the bees or visit more frequently. -male - lower fitness of yellow when more common - females - higher fitness of yellow when its more rare
What is the hypothesis for why there is greater genetic differentiation across geographic distances for Y chromosome loci compared to either autosomal or mitochondrial loci in humans. In other words, what may be causing the apparently higher rates of gene flow for autosomal and mitochondrial loci compared to those on the Y chromosome?
all people acquire autosomal and mitochondrial, only people with Y chromosome get that loci
Explain why data for Pacific field crickets (fig. 7.12) illustrate an example of genetic drift.
because the crickets came from New Guinea and Australia, they has to have traveled on boat so each island the boat arrived to would get a small population of crickets. This means that each population is different. Cricket populations from Hawaii carried less allelic diversity than from Oceania and Oceania carried less allelic diversity than Australia.
For a locus that has only two alleles, describe how selection is expected to act on a recessive allele that is lethal in the homozygous state and what the equilibrium frequency of that allele will be (all else equal) if the frequency of both alleles (the other one is non-lethal) is 0.5 at the outset of the experiment. Why does the frequency of the recessive allele stabilize at that frequency?
because there are only 2 alleles and one is lethal, it wouldnt be possible to show up at a full 1, only at .5 frequency
Describe the factors we think caused the steep decline in Florida Panther populations even after they had been protected by the Endangered Species Act. What solutions can be used to help populations such as the Florida Panther?
florida panthers have substantially lower heterozygosity than other populations of cats. inbreeding depression reduced their reproductive success and caused heart defects, low sperm count...etc Solutions: migrants from other populations (texan pumas) were brought in which brings alleles that can be reintroduced and then reverse effects of drift and eliminate inbreeding depression
Define gene flow (remember "migration" (as used in the text) = dispersal) and explain how selection and gene flow may account for the distribution of water snake genotypes for different banding phenotypes on the Ohio mainland and the islands in Lake Erie (i.e. be able to explain Fig. 7.6).
gene flow - the alleles flow through the generations and migrate with the snakes as the move place to place and reproduce. Migration=dispersal. Variation in color pattern within and between populations. Unbanded snakes are more cryptic on island rocks than banded
population
group of interbreeding organisms and their offspring, or a group of individuals living in the same area if they are asexual species
Describe the potential effects of inbreeding depression. Why do we think inbreeding depression has such effects? Be able to explain how to calculate F for a particular set of relatives.
inbreeding depression- as inbreeding coefficient increase the number of offspring who don't survive increases Effects: -lower survival -lower fertility and fecundity (#) -lower LRS (lifetime reproductive success) -asymmetrical development
HWE model algebraical equation
p^2 + 2pq + q^2
What is the relationship between a change in the proportion of heterozygotes and population size (i.e., be able to explain the formula Hg + 1 = Hg [1-(1/2N)])? In the experiment described in question 6) above what was paradoxical about the decline in heterozygosity? What are possible explanations for the paradox?
the frequency of heterozygotes indeed tends to fall rapidly in small populations and slowly in large populations. Eventually one allele or the other will become fixed in every population and the average frequency of heterozygotes will fall to 0 and .5 wont happen as a final allele frequency only 1 to 0.
