Question

In studying sub-populations of a small rodent that has a country wide distribution in the widely varying habitat of Costa Rica, Irena and Irwin (out intrepid evolutionary biologists) became interested in a gene with two alleles where one of the alleles (A1) produced an enzyme variety that protected mice from the venom of the “bushmaster” (a highly venomous snake) and the other allele (A2) produced an enzyme that protected the mice against the venom of the “fer de lance” (another highly venomous snake).

28. In habitats where both snakes are present in significant numbers, the genotype frequencies for were measured to be f(A1/A1) = 0.1; f(A1/A2) = 0.6; f(A2/A2) = 0.1. This represents a...

    1. a) population in Hardy-Weinberg equilibrium showing the evolutionary pattern of selection for the heterozygote genotype.

    2. b) population that is in Hardy-Weinberg equilibrium showing evidence of selection against the heterozygote.

    3. c) population not in Hardy-Weinberg equilibrium showing the evolutionary pattern of selection for the heterozygote genotype

    4. d) population that is not in Hardy-Weinberg equilibrium showing evidence of selection against the heterozygote.

29. If aggressive human intervention removed all of the snakes in the habitat described in the question above, which of the following would you not expect to happen to the allele and genotype frequencies in the population?

    a) They would stay at exactly the levels they were at when the snakes were removed. b) One allele or the other could become fixed depending on the size of the rodent

    population.
    c) The number of heterozygotes would decrease. d) They would fluctuate randomly due to drift.

30. In studying habitats where both snakes are present in significant numbers, Irwin and Irena found that over many generations, the allele frequencies tended to move first in one direction and then in the other but always tended to move back toward allele frequencies of f(A1) = 0.5 and f(A2) = 0.5. This could be an example of...

    a) frequency dependent selection b) selection for a dominant
    c) selection for a recessive
    d) underdominance

31. In one particular habitat with a small rodent population size but where both snakes were present in significant numbers, the genotype frequencies were measured to be relatively close to f(A1/A1) = 0.38; f(A1/A2) = 0.24; f(A2/A2) = 0.38. This could be an example of...

    1. a) overdominance

    2. b) inbreeding

    3. c) directional selection

    4. d) mutation-selection balance

Answer 1

Ans. 1) population not in Hardy-Weinberg equilibrium showing the evolutionary pattern of selection for the heterozygote genotype.

For population to be in Hardy Weinberg equilibrium there are 5 condition:

1. no mutation
2. random mating
3. no gene flow
4. infinite population size
5. no selection.

If any of the above conditions is not follow then the population will not be stable and thus won't be in Hardy Weinberg equilibrium. As in question it is clearly mention that rodents population are small. Also it is mentioned that due to presence of snakes the heterozygous population are more stable and large in no. This explains that rodent population are naturally selected by presence of snakes. As the two conditions are broken in this example, therefore it is no in Hardy Weinberg equilibrium. And heterozygous frequency is highest that mean heterozygous rodent are selected.

Ans. 2) They would stay at exactly the levels they were at when the snakes were removed.

When both snakes are removed then rodents of all genotype can survive as their predators are absent. So their are chances that frequency of heterozygous may decrease, or the frequency of other two homozygous rodent may increase, depending upon who is more fit. But they won't be same as were before the presence of snakes.

Ans. 3) frequency dependent selection.

As it is mentioned that the frequency show directional selection and maintain frequency of both allele at 0.5. This means that fitness of either of the allele, dominant and recessive, changes depending on their frequency.

Selection for dominant or recessive is not true as here recessive allele are also equally selected.

Underdominance also favour the homozygous dominant and recessive not the heterozygous but there is not complete loss of heterozygous allele. Frequency of both dominant and recessive are equal but the heterozygous will jave lower than both of the. But won't be zero.

Ans. 4) inbreeding.

Inbreeding favour the homozygous allele over heterozygous. In this, organism which are genetically closely related to each other are mated. Inbreeding can result in purging of deleterious alleles from a population through purifying selection. Inbreeding is a technique used in selective breeding.

It is not overdominance selection as in this heterozygous alleles are selected over homozygous.

In directional selection, any one homozygous allele frequency is high, other homozygous have lower frequency.

Mutation selection balance there is equilibrium in deleterious allele occur when the rate at which deleterious alleles are created by mutation equals the rate at which deleterious alleles are eliminated by selection.