1510 BIOL S12 Case Study Evolution by Natural Selection
1/26/12
Case Study: Evolution of coat color in rock pocket mice in the Pinacate lava flows
The volcanic crater (left) is 1500 m diameter and 100 m from rim to bottom. The dark patches are
lava flows that occurred 1.7 x 106 years ago on a light landscape dominated by sandstone. In the
inset (lower right), natural color variants of rock pocket mice are shown against the lava and
sandstone rock backgrounds.
1. What features of this system might be important for the survival and reproduction of mice?
Owls?
2. On what rock backgrounds are mice at a greater disadvantage in terms of predation, and
why?
Focal population: Assume a light coat color population of 5000 females and sufficient males
occupies the area inside the marked circle. Female average reproductive rate is 5 offspring per year.
The total population size doesnt change over time.
3. Is this location amenable to mouse survival in the presence of predators, and why?
4. How many offspring are produced in the entire population each year, on average? Show
your work.
Genetics: Coat color is a phenotype (a physical trait) encoded by a gene called MC1R that has two
variants, or alleles, light (L) and dark (D). Each mouse has two copies of MC1R, one inherited from
each parent. A genotype is the pair of alleles that a mouse carries. When even one copy of the dark
allele is present, mice have dark coats.
5. Fill in the possible genotypes (ex: LL) and phenotypes (light or dark), and indicate with a *
the genotypes in the focal BIG population.
Genotype
Phenotype
THE QUESTIONS: Can a dark mouse arise d e novo in a light population? How likely
would that be?
New variation is generated by mutation. Mutation rate has three components in this scenario. The
product of these gives the probability that a mouse would be dark (mutation rate).
= 2 x 109 = 2 mutations per billion DNA sites in mice mutation rate of a DNA site
MC1R has several sites that could mutate to make a dark coat = 10
Number of MC1R copies per mouse = 2
6. Whats the mutation rate to dark coat color for a mouse? Show your work.
7. How often does a dark mutant arise in the light-colored population?
P(dark mouse in population) = mutation rate * reproductive rate * population size
Calculate the probability and then translate your answer into the # of mutations that
arise in 1000 years.
8. How many dark mutations could have arisen independently since the lava flow formed?
# independent dark mutants = P(dark mouse in population) * age of lava flow
9. What are the specific requirements for evolution by natural selection in this case?
10. Whats the selective agent?
11. Which coat color is more fit in the focal population? If the focal population lived on the
craters sandstone floor, how would that change your answer?
12. Once dark mice exist in the population, do you predict that all mice in the focal population
would eventually be dark? On what time scale would that occur? Brainstorm in your group
to think of a plan (experiment or model) to determine how long it would take?
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