Natural Selection
Explain variation, heritability, overproduction, competition, differential survival, and adaptation. Distinguish individual survival from population-level evolution.
A lesson of Biology, in Evolution and Diversity of Life, about 10 minutes.
By the end of this lesson, you will be able to explain the mechanism of natural selection, describe the conditions required for it to occur, explain adaptation, and give examples of directional, stabilizing, and disruptive selection.
Darwin's Theory
Charles Darwin (1859, On the Origin of Species) proposed natural selection as the mechanism of evolutionary change. Unlike previous ideas, Darwin provided a plausible, testable mechanism.
The Four Conditions for Natural Selection
Natural selection occurs when all four conditions are met:
- Variation exists in traits within a population
- Inheritance: traits are heritable (passed from parents to offspring)
- Overproduction: more offspring are produced than can survive
- Differential fitness: some traits give individuals higher survival and reproductive success in a given environment
Result: favorable traits become more common in the population over generations.
Key Vocabulary
Fitness: reproductive success — not strength or health in themselves, but the ability to survive and reproduce in a given environment.
Adaptation: an inherited trait that increases fitness in a specific environment. Adaptations arise from natural selection acting on variation.
Selection pressure: any environmental factor that affects survival and reproduction.
Examples of Natural Selection
Antibiotic resistance: bacteria with mutations that confer resistance survive antibiotic treatment; sensitive bacteria die; resistant bacteria reproduce → population becomes resistant. This is natural selection in real time.
Peppered moths (industrial melanism): before industrialization, light moths were camouflaged on lichen-covered trees; dark moths were rare. After industrial soot darkened trees, dark moths were camouflaged and light moths became easy prey. Dark moths increased in frequency — selection favored melanism in polluted environments.
Beak variation in Darwin's finches: different islands had different food sources → birds with beak shapes suited to available food survived better → natural selection produced beak diversity.
Types of Natural Selection
Directional selection: selection favors one extreme phenotype → the average shifts in one direction. Example: antibiotic resistance; running speed in cheetahs.
Stabilizing selection: selection favors the intermediate phenotype → variance decreases. human birth weight (very small or very large infants have higher mortality).