Genetic diversity is defined as the number of different alleles of genes in a population. Last week we looked at one method of introducing new alleles into a population – by genetic mutation. Today we will look at other ways that genetic diversity can be increased or decreased, and how it can be measured.
Increasing genetic diversity
There are a couple of ways that new alleles can be introduced into a population. The first is genetic mutation. The second is gene flow. Gene flow occurs when an individual from another population migrates into the first population, and reproduces to produce offspring carrying alleles which are new to that population. Don’t forget that meiosis produces new combinations of alleles, but it does not introduce brand new alleles.
Reducing genetic diversity
To reduce genetic diversity, alleles must be removed from the population. This can occur due to a genetic bottleneck – when there is a large reduction in the number of individuals in a population. A genetic bottleneck could occur due to a number of reasons such as a natural disaster, disease, or hunting. It could also happen if a small number of individuals leave the population to form a new population (called the founder effect). When the small number of individuals reproduce, their offspring will only have the reduced number of alleles compared to the original larger population. The diagram below shows an example of a genetic bottleneck happening to a population of a flower species due to a flood.
Measuring genetic diversity
Genetic diversity can be measured by looking at either the genotype or phenotype of organisms. By looking at the base sequence of an organisms DNA, you can determine which alleles of a gene it has (the genotype). The greater the number of different alleles in a population, the greater the genetic diversity. Phenotype (the observable characteristics coded for by the alleles) can be used to measure the variation of characteristics in a population. A larger number of phenotypes means greater diversity.
There are a couple of calculations that can help to quantify genetic diversity. One is the heterozygosity index. This shows the proportion of individuals in a population which are heterozygous at a particular locus on a chromosome (one gene). More heterozygous individuals (heterozygotes) means more genetic diversity.
You can also calculate the proportion of polymorphic gene loci. If a gene locus is polymorphic it means that there are different alleles found at the same locus. The more polymorphic gene loci that a population has, the greater the genetic diversity.
- Genetic diversity can be increased by mutations or gene flow, and can be decreased by a genetic bottleneck.
- It can be measured by looking at genotype or phenotype, and by doing calculations to look at the number of heterozygous individuals or polymorphic gene loci.