CCEA A Level Biology 5.6 Population Genetics Exam Questions

About CCEA A Level Biology 5.6 Population Genetics

Section 5.6 of Unit A2 2 moves from individuals to populations and evolution. You need the concept of a gene pool, the Hardy-Weinberg equation and principle, the conditions under which it applies, and how mutation, non-random mating, migration and selection change allele frequencies. CCEA also requires the sources of genetic variation (mutation, including base substitution and deletion, chromosome mutation, and sexual reproduction), the idea of fitness, stabilising and directional selection, polymorphism, and how natural selection leads to evolution. The section ends with the species concept and allopatric speciation with reproductive isolating mechanisms. Hardy-Weinberg calculations appear regularly and are easy marks if set out clearly.

What you need to know

• Gene pool The gene pool is all the alleles of all the genes in a population. Allele frequency is the proportion of a particular allele in the gene pool.
• Hardy-Weinberg For one gene with two alleles, p + q = 1 and p^2 + 2pq + q^2 = 1, where p^2 and q^2 are homozygous frequencies and 2pq is heterozygous. The principle says frequencies stay constant if the population is large, mating is random, and there is no mutation, migration or selection.
• Sources of variation Gene mutations (base substitutions and deletions) create new alleles; chromosome mutations change structure or number. Sexual reproduction reshuffles alleles through meiosis and cross-fertilisation. Recessive alleles are maintained in heterozygotes.
• Selection Fitness is an organism's ability to survive and reproduce. Stabilising selection favours the average and reduces variation; directional selection favours one extreme and shifts the mean. Polymorphic populations have two or more distinct forms. Natural selection changes allele frequencies over generations, which is evolution.
• Speciation A species is a group of organisms that can interbreed to produce fertile offspring. In allopatric speciation, a geographical barrier separates populations, different mutations and selection pressures act, and they become reproductively isolated.
• Isolating mechanisms Prezygotic mechanisms include behavioural, seasonal and mechanical isolation; postzygotic mechanisms include hybrid sterility or inviability.

Common mistakes in 5.6 Population Genetics

• Using the frequency of the recessive phenotype as q. The recessive phenotype frequency is q^2; take the square root to find q.
• Saying mutations happen in order to help the organism adapt. Mutations are random; selection acts on the variation they produce.
• Saying individuals evolve. Populations evolve as allele frequencies change over generations.
• Forgetting the conditions for Hardy-Weinberg. State large population, random mating, no mutation, no migration and no selection.
• Describing speciation as just physical separation. Speciation is complete only when populations can no longer interbreed to produce fertile offspring.

Try three 5.6 Population Genetics exam-style questions