Genetic Diversity (AQA AS Biology): Revision Note

Genetic diversity

• Organisms of the same species have very similar genomes, but two individuals (even twins) will have differences in their DNA base sequences

• Considering the size of genomes, these differences are small between individuals of the same species

• The small differences in DNA base sequences between individual organisms within a species population are called genetic variation

• Genetic variation is transferred from one generation to the next, and it results in genetic diversity within a species population

• Genetic diversity is defined as the number of different alleles of genes in a population

• Mutation results in the generation of new alleles and contributes to genetic diversity or the size of the gene pool

  • The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype (because the genetic code is degenerate)

  • New alleles are not always seen in the individual in which they first occur

  • They can remain hidden (not expressed) within a population for several generations before they contribute to phenotypic variation

Effects of genetic diversity

• There needs to be some level of genetic diversity within a population for natural selection to occur

• Differences in the alleles possessed by individuals within a population result in differences in phenotypes

• Environmental factors affect the chance of survival of an organism; they, therefore, act as a selection pressure

• Selection pressures increase the chance of individuals with a specific phenotype surviving and reproducing over others

• The individuals with the favoured phenotypes are described as having a higher fitness

  • The fitness of an organism is defined as its ability to survive and pass on its alleles to offspring

  • Organisms with higher fitness possess adaptations that make them better suited to their environment

• A population with a large gene pool or high genetic diversity has a strong ability to adapt to change

• If a population has a small gene pool or very low genetic diversity, then they are much less able to adapt to changes in the environment and so can become vulnerable to extinction

  • Cheetahs are an example of a species with a small gene pool

  • They experienced a very large decline in numbers approximately 10,000 years ago, which left small, fragmented populations of individuals remaining

  • There was no mixing between populations, and large amounts of inbreeding occurred

  • Low genetic variation means the species are less likely to be able to respond (survive) in the event of any environmental changes