01:119:115 Lecture Notes - Lecture 20: Allele Frequency, Gene Pool, Sexual Reproduction

A population of finches lives on an island some distance from the mainland, where another population of finches of the same species resides. In both populations, the size of the birds' beaks is influenced by two alleles at the beak locus, B and b. On the island, the frequency of the B allele is 0.9, and the frequency of the b allele is 0.1. On the mainland, both alleles are equally frequent (0.5).

a. If the genotypes at the beak locus have the following fitnesses on the island: w_BB = 1, w_Bb = 0.95, w_bb = 0.9, what is the change in the frequency of the B allele on the island in one generation due to natural selection?

b. What will be the change in the frequency of the B allele on the island in one generation as a combined result of migration and natural selection?

c. If the migration rate from the mainland to the island is 0.05 (meaning that 5% of individuals in the island population migrate in from the mainland every generation), what is the change in the frequency of the B allele in one generation due to migration? Assume there is no migration from the island back to the mainland.

Hemoglobin and Fitness Instructions Directions: Neutral Evolution

1. Obtain 20 beans of two different colors (e.g., white and red). Count out 16 white and 4 red beans. The white beans represent the Hn allele and the red beans represent the Hs allele. This is the genetic makeup of your starting population. (Note: You can use any objects that can readily be categorized into two groups, such as coins, colored rocks, or paper clips.)

2.Calculate the frequency of both alleles [f(Hn) and f(Hs)] and record them in Table 1. In our experiment frequency is a measure of how many copies of a given allele exist in the gene pool (i.e., a proportion). Use decimal values. 


3.Arrange the beans into pairs. These pairs represent the genotype of each of 10 individuals in the population. Record the number of individuals with each genotype [f(Hn Hn), f(Hn Hs), and f(HsHs)] in Table 1. 


4.Now imagine that the individuals are living and reproducing with each individual reproducing at the same rate (i.e., all individuals produce two copies of each of their alleles into the next generation). Obtain enough beans to represent the next generation— the offspring generation—and then let the parental generation “die”. 


5.Calculate the frequency of each allele in the offspring generation and record it in Table 1. 


Answer the questions that follow in Table 1. 


Table 1

Answer the following questions to help you understand the exercise:

What happened to the frequency of the common allele? 


What happened to the frequency of the rare allele? 


What happened to the frequency of the common and rare alleles when the starting frequencies were different from yours

What happens to allele frequencies from one generation to the next if there are no evolutionary forces acting on the population? 


A population of finches lives on an island some distance from the mainland, where another population of finches of the same species resides. In both populations, the size of the birds' beaks is influenced by two alleles at the beak locus, B and b. Finches with the BB genotype have the largest beaks, bb have the smallest beaks, and Bb individuals have a beak size intermediate between the two homozygotes. On the island, the frequency of the B allele is 0.3, and the frequency of the b allele is 0.7. On the mainland, both alleles are equally frequent (0.5).

(a) If the migration rate from the mainland to the island is 0.1 (meaning that 10% of individuals in the island population migrate in from the mainland every generation), what is the change in the frequency of the B allele on the island in one generation due to migration? Assume there is no migration from the island back to the mainland.

(b) The genotypes at the beak locus have the following fitness on the island: w_BB = 0.95, w_Bb = 0.98, w_bb = 1. Which allele is the most fit? What size beak is selectively favored on the island? What do you predict will happen to the frequencies of these alleles in the long term?

(c) The island experiences a severe drought for two consecutive years, which kills a plant species on the island that makes small seeds eaten by the finches. As a consequence, the food available decreases and the average size of edible seeds on the island increases greatly. Birds with large beaks are much better able to crack large seeds and thus better able to survive the drought. Which allele is the most fit? How do you predict the allele frequencies of B and b will change during this drought?