AP Biology 7.1-7.2: Natural and Artificial Selection

Evolution by natural selection

Evolution by natural selection is the change in a population over generations caused by individuals with heritable traits that improve survival or reproduction leaving more offspring.

What three conditions are required for natural selection to produce evolutionary change?

Individuals must vary, some of the variation must be heritable, and organisms must produce more offspring than available resources can support. These conditions create competition and differential reproductive success.

Why does competition occur within populations?

Organisms generally produce more offspring than can survive, while resources such as food, water, and shelter are limited. Individuals therefore compete for resources needed for survival and reproduction.

What does fitness mean in the context of evolution?

Fitness is an organism's relative contribution of viable, reproducing offspring to the next generation. A trait increases fitness if it raises the likelihood of survival and reproduction in a particular environment.

Why does natural selection act on individuals but cause populations to evolve?

Individuals with particular phenotypes may survive or reproduce more successfully, but their inherited alleles become more common in the population. Evolution is therefore measured as a population-level change across generations.

What is an adaptation?

An adaptation is a heritable trait that increases an organism's survival or reproductive success in its current environment. Examples include webbed feet for swimming and thick fur for cold conditions.

Why is a trait not universally advantageous?

The fitness value of a trait depends on environmental conditions. For example, large leaves may be favored in a moist environment but small leaves may be favored when water becomes scarce.

What is the difference between genetic and environmental variation?

Genetic variation results from differences in DNA and can be inherited, whereas environmental variation results from conditions such as nutrition and is generally not transmitted genetically. Natural selection can cause lasting evolutionary change only when selected variation has a heritable basis.

How can a population become resistant to an antibiotic without the antibiotic causing the resistance mutation?

Resistance mutations arise randomly before antibiotic exposure and may initially be rare. The antibiotic kills susceptible bacteria, allowing resistant individuals to survive and reproduce so the resistance allele increases in frequency.

Does natural selection create traits because organisms need them?

No. Natural selection acts on heritable variation already present in a population; environmental change alters which existing phenotypes reproduce most successfully. Mutations do not arise because an organism needs a particular trait.

What did Darwin and Alfred Russel Wallace independently propose?

They independently developed natural selection as the mechanism explaining how populations change and become adapted to their environments.

How did Galápagos finches support Darwin's reasoning about natural selection?

Finch species on different islands resembled a mainland ancestor but differed in beak shapes suited to different foods. This pattern suggested descent from a common ancestor followed by adaptation to local resources.

How does the Grants' study of Galápagos finches demonstrate natural selection?

During a period with many small, soft seeds and few large, hard seeds, small-billed finches survived and reproduced more successfully, and the population's average bill size decreased in later generations. When seed availability changed, the directional trend also changed.

What is directional selection?

Directional selection favors one extreme phenotype, causing the population's average phenotype to shift toward that extreme over generations. For example, selection for smaller bills when small seeds are abundant can decrease the population's average bill size.

What is stabilizing selection?

Stabilizing selection favors intermediate phenotypes and selects against both extremes. It tends to reduce variation around the population's average phenotype.

What is disruptive selection?

Disruptive selection favors both extreme phenotypes over the intermediate phenotype. It can increase variation and may contribute to populations diverging if the groups experience different selection pressures.

How do directional, stabilizing, and disruptive selection differ?

Directional selection favors one extreme, stabilizing selection favors the intermediate phenotype, and disruptive selection favors both extremes while selecting against the intermediate phenotype.

What is artificial selection?

Artificial selection is the selective breeding of organisms by humans to increase the frequency of desired heritable traits in future generations.

How does artificial selection differ from natural selection?

In artificial selection, humans choose which organisms reproduce based on desired traits. In natural selection, environmental conditions determine which heritable traits increase survival or reproductive success.

How can artificial selection produce substantial changes in organisms?

When humans repeatedly breed individuals with a desired heritable trait, the alleles associated with that trait become more common over generations. Continued selection can produce populations that differ greatly from their ancestors.

What are examples of artificial selection?

Examples include breeding dogs for particular behaviors or body forms, developing crop varieties with greater yield, and selecting livestock for increased milk or meat production.

What is one potential consequence of artificial selection?

Artificial selection can reduce genetic variation or increase harmful traits linked to desired characteristics, especially when breeding relies on a small number of closely related individuals.

Why can a trait that is adaptive today become disadvantageous later?

Environmental conditions and competing organisms can change, altering the relationship between a phenotype and fitness. Natural selection may therefore favor different variants in a later environment.