Evolution & Natural SelectionHESI A2 cheat sheet

Variation, selection, time

9 key facts 3 practice questions Reviewed Oct 2026

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The Mechanism

Variation exists in a population
Some variants survive and reproduce better
Those alleles become more common over generations

Drivers

Mutation creates new variation
Natural selection filters it
Genetic drift — random change, strongest in small populations
Gene flow — migration between populations

Evidence

Fossils, homologous structures, embryology, DNA similarity

Misreading

Individuals do not evolve — populations do

Key takeaway

Variation, selection, time

Before you read

Test yourself first: 3 questions

Question 1 of 3

Natural selection results in:

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