Selection, allele frequencies and speciation

What you’ll learn

• Explain evolution through mutation and natural selection.
• Distinguish allopatric and sympatric speciation.
• Connect reproductive isolation to reduced gene flow.

Understand the idea

Mutation generates new alleles without anticipating what an organism needs. In a population with inherited variation, environmental conditions cause differences in survival and reproductive success. Individuals carrying advantageous alleles may leave more fertile offspring, so those alleles become more common over generations. Evolution describes changes in population allele frequencies, not an individual changing its genotype through effort.

Allopatric speciation begins when a geographic barrier reduces gene flow. Different selection pressures, mutations and genetic drift can make the separated populations diverge. If reproductive barriers eventually prevent successful interbreeding, the populations can remain distinct even after renewed contact. Physical separation alone does not prove that two species have already formed.

Sympatric speciation occurs without geographic separation. Differences in breeding time, habitat use or mate recognition can reduce interbreeding within the same area; polyploidy can cause rapid reproductive isolation in plants. Isolation restricts mixing of alleles, allowing divergence to persist. Evidence should distinguish a proposed mechanism from an observed change and account for generation time and sample size.

Key formulae

$$\text{allele frequency}=\frac{\text{number of copies of the allele}}{\text{total copies of that gene in the sample}}$$

Worked example

A storm divides an island lizard population into two isolated groups. Explain how this could eventually produce two species.

1. The geographic barrier reduces mating and therefore gene flow between the groups.
2. Different resources or predators can favour different inherited variants; mutation and drift can also alter allele frequencies independently.
3. Over many generations, divergence may produce reproductive incompatibility or different mating behaviour.

Answer: This is a possible allopatric route to speciation, provided reproductive isolation develops rather than merely a visible difference.