Explain how variation within a population enables natural selection to occur.

IB DP Biology Standard Level (2023 syllabus) — A4.1 Evolution and speciation · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Individuals in a population show heritable variation in phenotype or genotype.

Some variants give a selective advantage, making those individuals better adapted to survive in their environment.

Better‑adapted individuals are more likely to survive, reproduce and pass on their advantageous alleles to offspring (differential reproductive success).

Over successive generations the frequency of the advantageous alleles increases, so the population becomes better adapted to its environment.

Examiner tips

Use the word ‘heritable’ to show understanding of genetic basis. Show the causal chain: variation → advantage → survival/reproduction → allele frequency change. Keep each point concise and directly linked to the question.

Common mistakes

Failing to mention that the variation must be heritable. Confusing ‘adaptation’ with ‘survival’ without linking to allele frequency. Omitting the step of differential reproductive success.

Mark scheme (4 marks)

Individuals within a population show heritable variation / differences in phenotype or genotype
Some variants / alleles confer a selective advantage, making those individuals better adapted to survive in their environment
Better-adapted individuals are more likely to survive, reproduce, and pass on their advantageous alleles to offspring / differential reproductive success
Over successive generations the frequency of advantageous alleles increases in the population / the population becomes better adapted over time

Key terms in this question

natural selection