A2-LEVEL AQA BIOLOGY NOTES 
TOPIC 2: populations

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Species exist in one or more populations.

• Population = group of organisms of the same species occupying a particular space at a particular time that can potentially interbreed
  

Gene pool = the range of different alleles existing for a gene locus within a population or species
Allele frequency = the frequency with which a specific allele occurs for that gene within a population or species
The Hardy–Weinberg Principle
Hardy-Weinberg Principle is a mathematical model which predicts the ongoing frequencies of alleles and genotypes within a closed, freely and randomly breeding population.

• p+ q  = 1
  
• P and q are the probability of 2 different alleles  (one usually recessive, one usually dominant)
  
• P2 + q2 = 1
  
• No change in the allele frequencies within a population
  
• Assuming:
  
  • Large population
    
  • No migration or immigration
    
  • No mutation to new alleles
    
  • Random mating, with respect to the genotype of the organism