AQA A Level Biology复习笔记4.4.1 Genetic Diversity

• Organisms of the same species have very similar genomes, but two individuals (even twins) will have differences between 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 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 (due to the fact that the genetic code is degenerate
  • New alleles are not always seen in the individual that they first occur in
  • 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 posses 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
  • This left small, fragmented populations of individuals remaining
  • There was no mixing between populations and large amounts of inbreeding occurred
  • This is problematic for conservation as low genetic variation means the species are less likely to be able to respond (survive) in the event of any environmental changes

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