Question

How do you explain the deviations from Hardy-Weinberg equilibrium in a population where smoking has effected the frequency of alleles in a population?

Answer 1

Smoking can affect the frequency of alleles in a population in the following two ways:

Reducing fitness: Smoking can lead to reduced fertility in the smokers. This reduces the ability of smokers to pass on their genes and therefore results in lowered fitness in a group as a whole.

Increased mutations: Smoking can also increase the chance of mutation in germ cells, and thus increase the number of deleterious allele in the population.

Now, Hardy Weinberg equilibrium states that the frequency of alleles in a population shall stay same generation after generation, in a large, randomly mating population with no migration and no selection pressure acting on it.

For a hypothetical population, where smoking has affected allele frequencies in the gene pool, one way or other, deviations from Hardy Weinberg principle can be explained in two ways:

Reduction in fitness has upset the equilibrium: Hardy Weinberg equilibrium exists under the assumption that all individuals are equally likely to reproduce and mating is random. Reduced fertility in smokers can upset the equilibrium by deviating from that assumption. If smokers are unable due reproduce with the same robustness as non smokers, mating does not remain truly random and there may develop a bias.

Increased mutation has removed the assumption of no selection pressure: Smoking can increase the chance of mutation by almost 25%. Thus smoking in this population increases the number of deleterious mutations.

At selection equilibrium, rate of forward mutation = rate of back mutation. This means no. of alleles spontaneously shifting from wild type to mutated type is same as the rate of mutation back to wild type allele. However, if smoking has affected the frequency of the alleles, then rate of forward mutation will increase, and the population would no longer be in selection equilibrium. Instead natural selection will become operative in this population.

Thus the possible ways to explain the deviations from Hardy Weinberg equilibrium in a population due to smoking are the disturbance of random mating and the operation of natural selection.