“…natural selection is daily and hourly scrutinizing, throughout the world, the slightest variations; rejecting those that are bad,
preserving and adding up all that are good...”
-On the Origin of Species
Natural selection was Darwin’s great insight.
Natural selection explains why organisms are well suited—adapted—to their environment. Beneficial traits are preserved, while detrimental traits are “selected out.” Working on many traits over many generations, natural selection helps match populations to their environment.
Four conditions are necessary for natural selection:
Overproduction of individuals - Species produce many offspring, but only the tiniest fraction will survive.
Differing traits - Members of a species resemble one another, but a close look reveals that individuals differ in many ways.
Differential survival - Sometimes individuals with particular trait variations survive and reproduce better than individuals with other variations.
Heritability of traits - Many variations in traits are heritable—the traits are passed from parent to offspring through DNA.
Putting It Together
As long as:
- more individuals are produced than can survive, and
- differences in individuals’ traits influence their survival and are heritable,
then a population will evolve by natural selection.
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