7.2 Natural Selection
- Syllabus
- 2025
- Topic
- 7.2
- Level
- —
Learning objectives
7.2A—Describe the importance of phenotypic variation in a populationDescribe the importance of phenotypic variation in a population.• Natural selection acts on phenotypic variations in populations.• Environments change and apply selective pressures to populations.• Some phenotypic variations can increase or decrease the fitness of an organism in particular environments.7.2B—Explain how variation in molecules within cells connects to the fitness of an organismExplain how variation in molecules within cells connects to the fitness of an organism.• Variation in the number and types of molecules within cells can provide populations a greater ability to survive and reproduce in different environments.
Why Phenotypic Variation Matters for Selection
Natural selection acts on phenotypic variation: observable or functional differences among individuals in a population. Without relevant variation, a selective pressure cannot favor one phenotype over another.
Population contains phenotypic variants → the environment applies a selective pressure → variants differ in survival or reproduction → fitness differs → the population's trait frequencies can change across generations.
Environments change, so a phenotype that increases fitness under one set of biotic or abiotic conditions may have no advantage—or may decrease fitness—under another. Variation therefore gives a population multiple possible responses to changing pressures.
Natural selection acts directly on phenotypes, not on a hidden genotype in isolation. It does not make every difference adaptive; only variations that affect survival or reproductive success in the current environment influence fitness.
From Molecular Variation to Organism Fitness
Variation in the type or number of molecules inside cells can change how cells function. Those functional differences can alter an organism's ability to survive and reproduce in a particular environment.
Molecular type or amount differs → cellular process changes → organism performance or phenotype changes → survival or reproduction changes under specific conditions → fitness differs.
Consider enzyme variants that function differently at different temperatures. If one variant maintains a needed cellular reaction more effectively under the current temperature, organisms carrying it may perform and reproduce better. If temperature changes, the relative advantage can also change.
A population containing different molecular variants is more likely to include individuals whose cellular machinery functions well under a range of environments, increasing the population's capacity to survive and reproduce when conditions differ.
More of a molecule is not automatically better, and one molecular type is not universally superior. Fitness depends on the molecule's effect on function in the particular environment.