Unit 7 AP Biology Notes: How Selection Shapes Populations (Natural vs. Artificial)

Natural selection

A mechanism of evolution in which individuals with certain heritable traits survive and reproduce more successfully in a given environment, causing allele frequencies in the population to change over generations.

Population

A group of individuals of the same species living in the same area that can interbreed.

Evolution

A change in the genetic makeup of a population over time, often described as changes in allele frequencies across generations.

Variation

Differences in traits among individuals in a population (e.g., size, coloration, enzyme activity, behavior) that selection can act on.

Heritability

The extent to which trait variation is genetic and can be passed from parents to offspring; required for natural selection to cause evolutionary change.

Differential survival and reproduction

Nonrandom differences in survival and/or number of viable offspring produced by individuals with different heritable traits in a particular environment.

Allele frequency

How common an allele is in a population; evolution is tracked as changes in allele frequencies over generations.

Phenotype

Observable or measurable traits of an organism; natural selection acts directly on phenotypes.

Genotype

An organism’s genetic makeup (its alleles); changes in genotype/allele frequencies across generations reflect evolution.

Fitness (evolutionary fitness)

Reproductive success—how many surviving, reproducing offspring an individual contributes to the next generation (not simply strength or health).

Adaptation

A heritable trait that increases fitness in a specific environment, becoming more common via natural selection.

Selection pressure

Any environmental factor that affects survival or reproduction (e.g., predators, toxins, pathogens, climate, limited food).

Mutation

A random source of new genetic variation; mutations do not arise because an organism “needs” them.

Recombination

Shuffling of alleles during meiosis and sexual reproduction that generates new combinations of existing genetic variation.

Directional selection

Natural selection that favors one extreme phenotype, shifting the population’s average trait value (often after an environmental change).

Stabilizing selection

Natural selection that favors intermediate phenotypes and selects against extremes, reducing variation (often in stable environments).

Disruptive selection

Natural selection that favors both extreme phenotypes over the intermediate, increasing variation and potentially contributing to divergence.

Sexual selection

Selection based on differences in mating success; traits that increase mating success can spread even if they reduce survival.

Genetic drift

Random changes in allele frequencies due to chance events, especially strong in small populations (not driven by trait advantages).

Artificial selection

Evolution driven by humans intentionally choosing which individuals reproduce based on desired heritable traits, changing allele frequencies over generations.

Selective breeding

A human-directed process where only individuals with preferred phenotypes are allowed to reproduce, often producing rapid evolutionary change if the traits are heritable.

Genetic diversity

The variety of alleles/genotypes in a population; reduced diversity can limit future adaptability and increase vulnerability to disease or environmental change.

Trade-off (in selection)

A cost associated with improving a trait; selecting for one trait (e.g., rapid growth) can worsen another (e.g., health or survival).

Acclimation

A non-heritable change within an individual’s lifetime due to environmental conditions; unlike adaptation, it does not change allele frequencies in the population.

Teleological language (in evolution)

Goal-directed wording (e.g., “in order to,” “so that”) that incorrectly implies evolution has intent; natural selection has no foresight or goals.