Question

Industrial Melanism

The peppered moth, Biston betularia, is a speckled moth that rests on tree trunks during the day, where it avoids predation by blending with the bark of trees (an example of cryptic coloration). At the turn of the century, moth collectors in Great Britain collected primarily light forms of this moth (light with dark speckles) and only occasionally recorded rare dark forms. With the advent of the Industrial Revolution and increased pollution, light-colored lichens on the trees died, resulting in strong positive selection for dark moths resting on the now dark bark. The dark moth increased in frequency. However, in unpolluted regions, the light moth continued to occur in high frequencies. (This is an example of the relative nature of selective advantage, depending on the environment.)

Color is controlled by a single gene with two allelic forms, dark and light.   Pigment production is completely dominant, and the lack of pigment is recessive.   We use the letters A and a for these alleles.

1. If this population met the assumptions for Hardy Weinberg Equilibrium predict the genotypic frequencies for the next generation.
2. Now, assume that pollution has become a significant factor and that in this new population 50% of the light moths but only 10% of the dark moths are eaten (natural selection). How many moths would be left for each phenotype? What would be the p and q values of the population after predation? (Assume both genotypes for the dominant phenotype are eaten equally)

Answer 1

Introduction: Biston betularia is a peppered moth found on tree trunks in Great Britain. The original moths were light colored with few rare dark moths. After the industrial revolution, industrial melanism occurred. The dark moths predominated in polluted regions over the light moths whereas the light moths were seen in unpolluted regions. A single gene controls two allelic forms dark and light colors. The production of pigment is dominant allele (A) found in dark moths and absence of pigment is recessive allele (a) found in light moths.

Explanation:

Hardy-Weinberg equilibrium is an important factor in population genetics. The principle states that when the allele frequencies in is constant in a population over generations in the absence of the effects of evolution. The Hardy Weinberg Equilibrium is used to predict the genotypic frequencies for the next generation in dark and light moths.

The Hardy-Weinberg population has two alleles, A and a, that are in equilibrium.

According to the law, the equation is p2+2pq+q2 = 1 where,

p – frequency of dominant dark moth allele "A";

q – frequency of recessive light moth allele "a";

p2 – frequency of dominant homozygous genotype "AA";

q2 – frequency of recessive homozygous genotype "aa"; and

2pq – frequency of heterozygous genotype "Aa";

Also, the sum of frequency of both alleles is p + q = 1.

On the basis of the moth population:

The frequency of light moth “a” is 50%.

Therefore, q = 0.50.

q2 represents the frequency of homozygous genotype "aa"

Thus, q2 = a*a

                 = 0.50*0.50

                 = 0.25

Therefore, q2 = 0.25

The percentage of dark moth population with homozygous recessive allele "a" is 25%.

The frequency of dark moth “A” is 10%.

Therefore, p = 0.10.

p2 represents the frequency of homozygous genotype "AA"

Thus, p2 = A*A

                 = 0.10*0.10

                 = 0.01

Therefore, p2 = 0.01

The percentage of light moth population with homozygous dominant allele "A" is 1%.

Thus, the number of moths left in each phenotype light moth is 25% and dark moth is 1%.

The sum of frequency of both alleles is p + q = 1.

q = √0.25 = 0.5

Therefore, p   =   1 – q

                    =   1- 0.5

                    =   0.5

Thus, the value of p = 0.5 and q = 0.5. This indicates that 50% of dark moth and 50% of light moth may exist after predation.