KEY CONCEPT Darwin proposed natural selection as a mechanism for evolution.

Several key insights led to Darwin’s idea for natural selection. Darwin noticed a lot of variation in domesticated plants and animals. Artificial selection is the process by which humans select traits through breeding. neck feathers crop tail feathers

Natural selection is a mechanism by which individuals that have inherited beneficial adaptations produce more offspring on average than do other individuals. Heritability is the ability of a trait to be passed down. There is a struggle for survival due to overpopulation and limited resources. Darwin proposed that adaptations arose over many generations.

Struggle for Survival Populations would grow geometrically if resources were unlimited. Disease, predation, and limited food supply keep populations smaller. An entire family of small frogs found on the coast of South America is thought to have been wiped out by a deadly amphibian disease. The Darwin's frog and its cousin the Chile Darwin's frog, pictured, have not been spotted for months and scientists believe they are now extinct

Natural selection explains how evolution can occur. There are four main principles to the theory of natural selection. variation Fitness is the measure of survival ability and ability to produce more offspring. overproduction adaptation descent with modification DESCENT with MODIFICATION ADAPTATION OVERPRODUCTION VARIATION

Variation The inherited differences that are the basis for natural selection.

Overproduction Many offspring increases the chance for survival But – it results in competition for resources

Adaptation A variation that allows an individual to survive better than the individuals it competes against.

Descent with Modifications Heritability of adaptations. As long as the environmental conditions make a trait beneficial, each following generation will have more individuals with the trait.

Natural selection acts on existing variation. Natural selection can act only on traits that already exist. Structures take on new functions in addition to their original function. wrist bone five digits