Allele Frequency (AQA A-Level Biology): Revision Notes
Allele Frequency
What is allele frequency?
Allele frequency refers to how often a particular allele occurs within a gene pool of a population. The gene pool contains all the alleles of all the genes present in all individuals within a population at any given time.
In a sexually reproducing population, any individual can potentially breed with any other member of their species. This means the alleles of any individual can combine with alleles from other individuals during reproduction, contributing to the overall genetic diversity of the population.
The gene pool represents the total genetic variation available within a population. Think of it as a "genetic library" containing all possible allele combinations that can be inherited by offspring in that population.
How selection affects allele frequencies
Selection directly influences allele frequencies within populations through environmental pressures. When environmental conditions change, they affect the probability that specific alleles will be passed on to future generations.
Organisms carrying alleles that provide a selective advantage under current environmental conditions are more likely to survive and reproduce successfully. This increases the frequency of favourable alleles in the gene pool over time, while reducing the frequency of less advantageous alleles.
Environmental factors do not create new alleles - they simply influence the frequency of alleles that already exist in the gene pool. Mutation can introduce new alleles, but this is a separate, random process that generally occurs at a steady background rate.
Evolution and allele frequencies
Evolution by natural selection can be defined as a change in allelic frequencies within a population over time. As selection pressures act on different phenotypes, the underlying allele frequencies shift to reflect which genetic variants are most successful in the current environment.
This process explains how populations adapt to their surroundings. Those individuals with allele combinations that enhance survival and reproductive success contribute more genetic material to subsequent generations, gradually altering the genetic composition of the entire population.
This definition of evolution emphasises that evolution is fundamentally about genetic change at the population level, not changes within individual organisms during their lifetime.
Key Concepts Summary
Key Points to Remember:
- Allele frequency measures how common specific alleles are within a population's gene pool
- Selection changes allele frequencies by favouring certain variants over others, but doesn't create new alleles
- Evolution occurs when allele frequencies change over time within populations
- Environmental factors influence which alleles provide survival advantages, driving natural selection