Question

The Kermode Spirit bear is an alternate phenotype of the species Ursus americanus (black bear) in British Columbia that has striking white fur. It was determined in 2001 that a single nucleotide change from G to A results in the replacement of Tyrosine with Cysteine at codon 289 in the melanocortin 1 receptor gene (mc1r; Ritland et al. 2001 Current Biology). This allele (W2) is recessive to the wildtype allele (W1), and occurs as a novel mutation in 1 out of every 10,000 copies. White phenotypes range in frequency from 10% to 25% on various islands, and have been present for long periods of time. Each island has approximately 50 individuals. It has been estimated that the white bears have a 20% advantage over black bears in fishing salmon from rivers due to decreased contrast with the sky.

Assuming these populations started out initially as 100% W1W1 (AA), include output to illustrate your answer with 10 populations running simultaneously for 10,000 generations.

1. 1. What evolutionary force(s) is/are acting on the W2 allele?
   2. If selection, mutation, and drift were the only processes acting on these populations, what would be the eventual outcome(s) for the two alleles in a given population?
2. 3. Try running the simulation for fewer generations. How many generations does it take to fix a novel advantageous allele in all populations? What do these results suggest about the ability of selection to fix a novel advantageous mutation in a small population; in other words, why is selection "having trouble"?
   4. Name one of the parameters you could change to increase the probability of the W2 allele becoming fixed in the population
3. 5. Rerun the simulation, changing the fitness of Aa so that W2 (a) is dominant. With a population size of 50, how does the probability of the W2 allele becoming fixed change if W2 is the dominant allele as opposed to recessive?
   6. Why does this happen?

Answer 1