Evolution and Natural Selection
Darwin's Theory of Evolution by Natural Selection
Core Principles
Charles Darwin (1859, On the Origin of Species) proposed that evolution occurs by natural selection. The mechanism rests on four observations:
- Variation: individuals in a population vary in heritable traits
- Overproduction: more offspring are produced than can survive
- Selection: individuals with favourable traits survive and reproduce more
- Inheritance: favourable traits are passed to offspring
Over generations, allele frequencies shift — this is evolution. Alfred Russel Wallace independently developed the same idea at the same time.
Types of Natural Selection
Three directional modes of selection:
- Directional: selection favours one extreme phenotype (e.g., antibiotic resistance, darker moths during industrial melanism)
- Stabilising: selection favours the average phenotype, reduces variation (e.g., human birth weight)
- Disruptive: selection favours both extremes, increases variation (e.g., beak size in African seedcrackers)
Other evolutionary mechanisms include genetic drift (random changes in small populations), gene flow (migration), and mutation (source of new variation).
Evidence for Evolution
Evidence comes from: the fossil record, comparative anatomy (homologous structures, vestigial structures), comparative embryology, biogeography (species distribution patterns), and molecular biology (shared DNA sequences, cytochrome c similarities).
Industrial melanism in peppered moths is an example of:
As industrial pollution darkened tree bark, dark moths were camouflaged and light moths fell to predators — selection shifted the population toward the dark phenotype (directional).
Correct answer: Directional selection
Which of the following is NOT a mechanism of evolution?
Lamarck's idea that acquired traits can be inherited (e.g., a giraffe stretching its neck) was disproven. Evolution requires heritable genetic variation, not traits acquired during a lifetime.
Correct answer: Lamarckian inheritance of acquired traits
Explain how antibiotic resistance in bacteria demonstrates natural selection in action.
This is textbook directional selection — the environment selects for a pre-existing heritable trait.
Correct answer: Before antibiotics, rare bacteria with resistance mutations exist in the population. Antibiotic use kills susceptible bacteria, leaving resistant ones to survive and reproduce. Their offspring inherit the resistance. Over time, the population becomes predominantly resistant — not because bacteria 'learn' or 'adapt' but because selection favoured pre-existing genetic variants.