Variation within populations
Members of the same species share very similar genomes, although their DNA base sequences are not completely identical. The relatively small sequence differences found between individuals give rise to genetic variation within a population.
Within a population, organisms belong to the same species, occupy the same area and are able to breed with one another. A species can be divided into separate populations.
Genetic diversity is the variety of different alleles of genes found in a population. Because genetic differences can be inherited, alleles can persist between generations and contribute to the population's overall diversity.
Mutation and new alleles
Mutation provides new versions of genes. When a new allele enters the gene pool, the range of alleles within the population can increase.
The effect of a newly formed allele depends on the change involved. It may improve the phenotype under particular conditions, reduce its suitability, or produce no observable phenotypic difference. Degeneracy of the genetic code means that some changes in base sequence do not change the amino acid specified.
The phenotype produced by an allele may also not be visible immediately. An allele can persist without being expressed for several generations before it contributes to detectable variation among individuals.
Why the size of the gene pool matters
| Higher genetic diversity | Lower genetic diversity | |
|---|---|---|
| Range of alleles | Many alternative alleles occur within the population. | The population contains a more limited set of alleles. |
| If conditions change | Individuals are more likely to exist whose alleles suit the new conditions. | There is less genetic variation available for selection to act upon. |
| Overall effect | The population has a greater capacity to adapt. | Failure to adapt is more likely, increasing vulnerability to extinction. |
Cheetahs: a severe fall in cheetah numbers about 10,000 years ago left small populations separated from one another. Limited mixing between these groups, together with extensive inbreeding, resulted in a restricted gene pool. Consequently, cheetahs contain relatively little genetic variation with which to respond when environmental conditions alter.