Question

1. Which mechanisms of evolution (selection, allele flow, genetic drift, mutation) tend to increase genetic diversity in a population, and which tend to decrease it?

2. Why is the loss of genetic diversity especially worrisome for endangered species?

Answer 1

1. Genetic diversity refers to the variation of genes within species. The differences could be in alleles (different variants of same genes), in entire genes or in chromosomal structures. The genetic diversity enables a population to adapt to its environment and to respond to natural selection. There are several factors which may increase or decrease genetic diversity as a part of mechanism of evolution. These are gene flow, genetic drift, natural selection, mutation, etc.

(a) Gene flow or gene migration is the movement of alleles from one population to another. Due to gene migration, novel alleles/genes are introduced into a population. The new gene alleles are integrated to the new population in which a section of population has migrated, thus increasing genetic diversity.

(b) Genetic drift generally operates in small populations and small populations are more likely to experience the loss of diversity over time by random or by chance event of alleles movement. The random drift of an allele induce loss of other novel alleles at the same locus resulting in loss of genetic diversity. In small populations, inbreeding or mating between individuals with similar genetic characteristics are frequently observed which tend to fix the common alleles without undergoing variations. Genetic drift results in bottle neck effect which also decreases the genetic diversity in a population.

(c) Variations resulting from genetic recombination in the populations gene pool allows natural selection to operate on the traits that enable a population to adapt to changing environment. Selection can act for or against a trait to adapt with changing environment. It may increase genetic diversity if traits produced via mutation are beneficial or may decrease genetic diversity if a deleterious allele is selected by chance. The more gentic diversity a population has, the greater chances of the population to adapt and survive.

(d) Random mutations generate genetic variations. A mutation will increase genetic diversity over a short time as new alleles are added to the gene pool, but persistent presence of it is entirely dependent on selection and drift. However, if mutations are beneficial for an organism/species, these are selected by the nature and thus have a long term effect on increasing genetic diversity.

2. Genetic diversity enables a population to adapt to its environment and to the changes occuring in the environment via responding to natural selection. If a species has more genetic diversity, it can adapt better to the changed environmental conditions. Lower diversity in a species leads to uniformity, as in the case with large monocultures of genetically similar crop plants. The total genetic diversity of a community will be greater if there are many species, as compared to a situation where there are only a few species. So the relation between genetic and species diversity is very delicate. Changes or loss of genetic diversity leads to loss of biological diversity because it is the key driving force towards the process of speciation and formation of ecotype. Thus, loss of genetic diversity is worrisome for endangered species.