7.2 Natural Selection

Natural selection can't 'see' genes directly. It acts on phenotypes, the observable traits that genes help produce, such as color, size, behavior or resistance to a toxin. When a phenotype affects survival or reproduction and it's heritable, selection on the phenotype changes the allele frequencies behind it.

Without variation there's nothing to choose between. If every individual were identical, a change in the environment would affect them all equally and the population couldn't shift toward a better-suited form. That's why the sources of variation from Units 5 and 6, meaning mutation, crossing over, independent assortment and random fertilization, matter so much for evolution.

A selective pressure is any factor in the environment that makes some phenotypes more successful than others: a predator, a disease, a pesticide, a new food source, a climate shift. Environments change, so selective pressures change too.

Classic examples:

• DDT resistance in insects: when the pesticide DDT was sprayed widely in the 1940s and 1950s, a few insects already carried alleles that let them survive it (for example by breaking DDT down faster or by having nerve-cell proteins less affected by it). They survived and reproduced while most others died, so within a few years many insect populations were mostly resistant.
• Peppered moths in England: as soot darkened tree bark during the Industrial Revolution, dark moths were better camouflaged from birds and became common. After air pollution laws cleaned the air in the 20th century, light moths became common again.
• Flowering time and climate change: after a multi-year drought in southern California in the early 2000s, field mustard plants grown from post-drought seeds flowered earlier than plants from pre-drought seeds grown under identical conditions. Plants that flowered and set seed before the soil dried out had higher fitness.

The same phenotype can increase fitness in one environment and decrease it in another. The sickle-cell allele shows this clearly. Two copies cause sickle-cell disease and lower fitness. But one copy gives protection against malaria, so where malaria is common, heterozygotes have higher fitness than people with two normal alleles, and the allele stays common (more on this heterozygote advantage in 8.7). Where there's no malaria, the allele is only harmful.

Variation isn't only in outward traits. Differences in the number and types of molecules inside cells can let a population survive and reproduce in a wider range of environments. Examples: different versions of an enzyme that work best at different temperatures, different forms of hemoglobin that hold oxygen more or less tightly (useful at high altitude), or extra copies of a detoxifying enzyme gene that make an insect more resistant to a pesticide. A population carrying several versions of a molecule is better prepared if conditions change.