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Unformatted text preview: BIS2B Fall 2009 (Keen) Lecture 19. Final topics in population genetics and problem-solving practice.
More on kinds of natural selection—previously directional selection covered. New topics = stabilizing, divergent, frequency-dependent and sexual selection. Students should be able to:
predict the effects of different forms of selection on a trait distribution. explain why variation persists within populations even though natural selection operates solve problems in population genetics Selection on Discrete vs. Continuous traits
1.0 frequency 1.0 frequency Peppered Melanic before industrial revolution Peppered Melanic after industrial revolution polygenic, continuous, quantitative, additive, metric, or dosagedependent Compare the effects of strong versus weak selection pressures. In which situation would the mean change most quickly? Directional Selection favors phenotypes at one end of the distribution. Mean Mean Mean Variance Variance Why does the distribution still have a tail, instead of a hard edge, after selection and reproduction. Stabilizing selection favors the mean phenotype and selects against either extreme. What happens to the mean and variance after stabilizing selection? Continuous trait example Discrete trait example. Many problems caused in heart, spleen, etc because of the odd shape of the blood cell. We wonder why this is a stable selection trait that is still in the population. It actually makes one more fit in conditions where there is malaria. When people have sickle cell allele, they are more resistant to malaria. Three genotypes in a population with the malarial parasite
• Hn Hn gets malaria
Sickle Cell Gene Normal Gene • Hs Hn does not get malaria, and has 50% normal hemoglobin molecules • Hs Hs exhibits sickle cell anemia
Heterozygotes • Which genotype has the highest fitness? • Can this genotype reach 100% frequency?
In small populations surrounding areas prominent with malaria, it is possible. Disruptive or divergent selection favors the two extreme phenotypes and selects against the mean.
Extremes are more likely to survive. Mean phenotype has the lowest fitness It is also called diversifying selection
What happens to the mean and variance after selection?
The type of mating favored in this kind is positive assorted mating. Have the extremes mate with other extremes to increase the number. However, if extremes from two different ends of the scale, mate, then they will not have high fitness.
Mean is still the same after divergent selection, but variance increases. Continuous trait example: male head length in dung beetles. Dung beetles roll dung to an area, dig a hole, and then have offspring inside the hole. Small horns, large eyes Large Horns, small eyes No fitness for no horns or medium horn. Horns are needed to fight, big head needed to dig. Not under divergent selection Which trait is under divergent selection? Frequency-dependent selection
Perissodus microlepis Perissodus
Fitness can be high when you are rare. Right-mouthed P. microlepis attack left flank of prey Left-mouthed P. microlepis attack right flank of prey Can bite scales off of other living fish. Side shows which side you can bite and which side is vulnerable. Which predator type will have the highest fitness? What if one type is initially rare? Rare = low frequency
But when you are more fit, you produce more and when you produce more, your frequency goes down and you are no longer rare, thus you become less fit. Do rare types have high or low fitness? Frequency-dependent selection in scale-eating cichlids Frequency-dependent Hori (1993) Science 260: 216 Frequency left-mouthed fish Frequency Dependent Selection causes this trend in the graph. Sample Year between Sexual selection inter-sexual = preference of one sex for traits in the other sex (peahens) Though intra-sexual = competition between members of one sex for the other sex (male elephant seals fight)
within sometimes this is not best for selection. Usually fighting between males. Large tails are not genetically fit for survival, but females prefer the ones with large tails. This causes natural selection to be challenged. How is variation maintained if natural selection is operating?
Enviroment is changing in both space and time. Microcmimates are colder, warmer, etc and favor different individuals. There is also interspecies competition. Arms race for females, to escape predators, etc. You are hired by Insects-R-US to study cockroaches. They have a good insecticide, but a few cockroaches can eat it and live. These roaches carry the T allele. Your job is to estimate allele frequencies after a few generations of exposure to insecticide. You sample roaches and find these genotype frequencies: f (TT) = 0.05: f (Tt) = 0.50; f (tt) = 0.45 • What is the frequency of the T allele in the population: .05 +.25=.3 • A. 0.10 • B. 0.20 • C. 0.30 • D. 0.40 • E. 0.50 What are the expected frequencies of the 3 genotypes under conditions of HardyWeinberg equilibrium?
• A. f (TT) = 0.05: f (Tt) = 0.50; f (tt) = 0.45 • B. f (TT) = 0.90: f (Tt) = 0.05; f (tt) = 0.05 • C. f (TT) = 0.09: f (Tt) = 0.21; f (tt) = 0.70
These are the 3 expected values!!! • D. f (TT) = 0.09: f (Tt) = 0.42; f (tt) = 0.49 Compare your observed and expected values (>.02 apart = different). Which of the following is true? • • • • • A. B. C. D. E.
there are fewer TT animals than expected there is a deficiency of heterozygotes there are more tt animals than expected the observed values match expected the population is in H-W equilibrium You know that animals with the T allele can eat the pesticide and survive. Do your results fit in well with this knowledge? Is it good or bad? Research shows that T allele homozygotes need twice as many gut bacteria as other roaches to digest their food well. How does this help you explain your results? There is a shifting balance. • Suppose that 15 lab mice escaped from their cages in SLB in 2001. The mice interbred and there are now 800 individuals, 90% of which have pink eyes. The handbook of mouse genetics says pink eyes are present in 2% of wild mice. What is the best explanation for the frequency of this trait in the SLB population? • • • • • A. the mice mate assortatively for eye color B. genetic drift acted on this population C. mutations occurred because the pop. was isolated D. pink-eyed mice must have the highest fitness E. the population had no natural predators • Suppose that eye color in mice is determined by one gene with two alleles. There are 720 mice with the genotype pp, 60 mice with the genotype Pp, and 20 mice with the PP genotype. Which equation can you use to calculate the frequency of the P allele? • • • • • A. (20 + 60) / 100 B. (20 + 60) / 800 C. (20 x 2) + (60 x 1) / 800 D. (20 x 2) + (60 x 1) / 1600 E. you can’t determine the allele frequency because the population is not in equilbrium • You have been hired to count the number of spines on 4000 specimens of stickle-back fish collected from a lake over a 10-year period. In year 2, large-mouth bass were introduced into the lake. There are 400 preserved fish for each year. When you plot your data, you find that the mean number of spines has increased from 3 spines to 7 spines over 10 years, but the variance in spine number has not changed. You conclude that spine number:
• • • • • A. B. C. D. E. has been under directional selection has been under stabilizing selection has been under sexual selection has been under divergent selection has not been subject to selection • After graduation, you decide to breed a new relatively long-legged dachshund (wiener dog) to win in the Picnic day races. Suppose you discover that legs account for 30%-40% of the dog’s height, but the 30% occurs in the smallest dogs and 40% occurs in the largest dogs. You estimate the heritability of leg length as 35%. Are you likely to breed a winning dog? • A. yes, leg length is highly heritable in wiener dogs • B. yes, bigger dogs can be crossed with smaller dogs to produce long legs • C. yes, it is a good sign that larger dogs have longer legs • D. yes, the lower the slope of the parent-offspring regression, the better for breeding • E. no, leg length is not likely to respond well to selection. • Marmots are small ground-dwelling rodents that live in colonies. About 50% of the females born in a colony stay in that colony to breed, but 95% of the males born in one colony disperse to another colony to find a mate. Which of the following could explain why genotypic frequencies in individual marmot colonies are not in Hardy-Weinberg equilibrium? • • • • • A. B. C. D. E. founder effects high mutation rates matings among close relatives sexual selection gene flow Maxidilan and population genetics
• Female sand flies bite horses, humans, and other mammals. Sand flies secrete a protein, maxidilan or MD, that dilates mammalian blood vessels making it easier to feed. Sand fly individuals have different versions of MD. The mammalian immune system makes antibodies to MD. When a fly bites a mammal with antibodies, the blood flows too slowly for the fly to get enough food to be able to lay eggs. However, the mammal antibodies only work on the exact version of MD to which a host was exposed. Suppose you find a population of sand flies with only two versions of MD: MD1 and MD2. The genotypic frequencies in the population are: f(MD1MD1)= 0.50, f(MD1MD2)= 0.20 f(MD2MD2)= 0.30 What is the frequency of the MD1 allele in this sand fly population? • • • • • A. B. C. D. E. 0.50 0.25 0.70 0.60 0.40 What are the expected genotypic frequencies assuming that HW conditions are met? • A. f(MD MD )= 0.50, f(MD MD )= 0.20, f(MD MD )= 0.30 • B. f(MD MD )= 0.36, f(MD MD )= 0.48, f(MD MD )= 0.16 • C. f(MD MD )= 0.15, f(MD MD )= 0.70, f(MD MD )= 0.15
1 1 1 2 2 2 1 1 1 2 2 2 1 1 1 2 2 2 Compare the observed and expected genotype :frequencies:
Expected f (MD1MD1)
f (MD1MD2) f (MD2MD2) Observed
m f m .
.30 . Compare the observed and expected:
• Is the population in Hardy-Weinberg equilibrium?
A. YES B. NO • Heterozygous sand flies make a gene product for each allele that they carry. Bearing in mind the effects of mammal antibodies, what will happen to the relative fitness of heterozygous sand flies? • How does this explain the differences you saw between observed and expected genotype frequencies? • The MD1 allele was near 50% about 6 generations ago, but it seems to be increasing. What should happen to its fitness as it becomes more common and why? • What is this kind of selection called? ...
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- Winter '09