7.1 Introduction to Natural Selection

Darwin's idea

Charles Darwin proposed natural selection: individuals with favorable heritable traits tend to survive and reproduce more, passing those traits on. Over generations the population changes. Darwin called this descent with modification.

What it requires

Natural selection needs: variation among individuals, that the variation is heritable, overproduction of offspring (more than can survive), and differential reproductive success (some individuals leave more offspring than others). The environment 'selects' which traits are advantageous.

Key terms

Natural selection

Individuals with favorable heritable traits survive and reproduce more, changing populations over time.

Darwin

Proposed natural selection as the mechanism of evolution.

Heritable variation

Differences among individuals that can be passed to offspring — required for selection.

Differential reproductive success

Some individuals produce more surviving offspring than others.

Descent with modification

Darwin's phrase for how species change over generations from common ancestors.

Try an AP-style question

A student says, "Giraffes stretched their necks to reach leaves, and their offspring inherited longer necks." Why is this explanation incorrect?

1. Longer necks are always harmful to a giraffe's survival
2. Giraffes never actually used their necks to reach high leaves
3. Traits acquired during an individual's life are not passed on genetically
4. Stretching changes the neck genes, but only in the next generation