Adaptation, allele frequencies and speciation

What you’ll learn

• Relate anatomical, physiological and behavioural adaptations to a niche.
• Apply Hardy–Weinberg relationships under stated assumptions.
• Explain how selection and reproductive isolation can lead to population divergence.

Understand the idea

A niche describes how an organism uses resources and interacts with its physical and biological environment. Adaptations may be anatomical, such as a thick cuticle; physiological, such as producing concentrated urine; or behavioural, such as activity during cooler hours. These features can improve reproductive success in a particular environment, but an adaptation can carry costs and need not be beneficial everywhere. Individuals do not acquire inherited adaptations because they decide they need them.

Mutation creates new alleles without targeting the organism’s future needs. Natural selection changes allele frequencies when heritable differences affect survival and reproduction. A favourable allele can become more common as its carriers contribute disproportionately to later generations. For a two-allele locus, Hardy–Weinberg relationships predict genotype frequencies from allele frequencies under assumptions including random mating, a large population and negligible mutation, migration and selection. The model supplies a baseline; a departure requires investigation rather than automatically proving one particular cause.

Reproductive isolation reduces gene flow between populations. Different selection pressures, new mutations and chance changes can then produce different allele frequencies and genetic combinations. Accumulated differences may eventually prevent successful interbreeding, supporting the formation of distinct species. Geographic separation can initiate this process, but geography alone does not prove that speciation has occurred. Evidence about reproductive compatibility and genetic divergence is needed.

Key formulae

$$p+q=1$$

$$p^2+2pq+q^2=1$$

$$p=\frac{2n_{AA}+n_{Aa}}{2N}$$

Worked example

A large randomly mating population is in Hardy–Weinberg equilibrium for an autosomal locus. The recessive genotype aa has frequency 0.09. Find q, p and the expected heterozygote frequency.

1. Under the stated model, q² = 0.09, so q = 0.30.
2. The other allele frequency is p = 1 − 0.30 = 0.70.
3. Expected heterozygote frequency is 2pq = 2 × 0.70 × 0.30 = 0.42.

Answer: q = 0.30, p = 0.70 and heterozygote frequency = 0.42.