Biology
Natural selection & evolution — quick study summary
Natural selection is Darwin's mechanism for evolution: individuals with heritable traits that improve survival or reproduction in a given environment leave more offspring, so those traits spread. Over many generations, populations change — sometimes diverging enough to form new species. The four conditions: variation, heritability, differential survival/reproduction, and time. Evidence includes the fossil record, homologous structures, biogeography, and observed evolution in real-time (peppered moths, antibiotic resistance).
Key points
- Four conditions: variation, heritability, differential reproduction, time
- Fitness = reproductive success, not 'strongest' or 'fastest'
- Mutations + sexual recombination create the variation selection acts on
- Speciation usually requires reproductive isolation (geographic or behavioural)
- Evidence: fossils, homology, biogeography, antibiotic resistance, peppered moths
Practice quiz
Click each question to reveal the answer.
1. Which of these is NOT a required condition for natural selection?
- Variation in traits
- Heritability of traits
- Equal reproduction across all individuals
- Differential survival or reproduction
Answer: Equal reproduction across all individuals
Natural selection requires UNEQUAL reproduction — individuals with advantageous traits leave more offspring. Equal reproduction means no selection.
2. What does 'fitness' mean in evolutionary biology?
Answer: Reproductive success — how many offspring an individual leaves
Fitness is measured in offspring, not physical strength. A weak but fertile individual is fitter than a strong sterile one.
3. Why are antibiotic-resistant bacteria a clear example of natural selection?
Answer: Resistant individuals survive and reproduce under antibiotic pressure, passing the resistance gene on
Antibiotics act as the selective pressure; bacteria carrying resistance alleles survive and dominate the population within days.
Last reviewed: May 2026