Explain how natural selection can lead to adaptive radiation within a lineage of organisms.

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).

Adaptive radiation is a process in which organisms from a single ancestral lineage rapidly diversify into many new forms, each adapted to a different ecological niche. The finches of the Galápagos Islands are a well-known example, where a single ancestral finch species gave rise to multiple species with markedly different beak morphologies.

Model answer (4 marks)

Variation in the ancestral population provides heritable differences.
Individuals better suited to a niche survive and reproduce more.
Subpopulations become isolated and accumulate different alleles.
Reproductive isolation eventually forms, producing new species each in a distinct niche.

Examiner tips

Use the word "variation" and "heritable" early. Show the link between fitness and differential reproduction. Mention isolation and allele accumulation. Finish with speciation as the outcome.

Common mistakes

Forgetting to mention heritable variation. Confusing adaptation with natural selection. Not linking isolation to speciation.

Mark scheme (4 marks)

Variation exists within the ancestral population due to mutation and/or sexual reproduction, providing heritable differences among individuals.
Individuals that are better adapted to a particular ecological niche have higher survival and reproductive success (differential reproductive success / survival of the fittest).
As different subpopulations become isolated (geographically or ecologically) and adapt to different niches, gene pools diverge through accumulation of different alleles / mutations over generations.
Over time, reproductive isolation develops between the diverging lineages, leading to the formation of multiple new species each occupying a distinct niche (speciation).

Key terms in this question

adaptive radiation · natural selection