Types of Natural Selection

Stabilizing Selection

• Favors average individual in population.

• Reduces variation in population.

• Ex: average height of humans.

Directional Selection

• Favors one extreme variation of a trait.• Can lead to rapid evolution.

• Ex: Food shortage causes small seeds to disappear. Small birds die out and big birds become dominant.

Disruptive Selection

• Favors both extreme variations.

• Leads to evolution of 2 separate species.

• Ex: two extremes in bird beaks: short and fat (eats hard seeds) or long and thin (eats worms from earth)

Speciation• Evolution of a new species.

• Members of similar population can no longer interbreed and produce fertile offspring.

Causes of Speciation

Geographic Isolation• Physical barrier divides population

Reproductive Isolation• Differ genetically or behaviorally.

Different species of bowerbird construct elaborate bowers and decorate them with different colors in order to woo females.

The Satin bowerbird (left) builds a channel between upright sticks, and decorates with bright blue objects.

MacGregor’s Bowerbird (right) builds a tall tower of sticks and decorates with bits of charcoal.

(evolution.berkeley.edu/evosite/evo101)

Polyploidy

• Having multiple sets of a chromosome pair.

Patterns of Evolution

Divergent Evolution

• Species become increasingly different.

Type of divergent is…• Adaptive

Radiation- one species evolves in to many species to fit a number of different habitats

• “Darwin’s Finches”

Convergent Evolution

• Distantly related organisms evolve similar traits.• Unrelated species occupy similar environments in

different parts of the world.• Ex: Anteater family (Armadillo) in N.America and

Anteater family (Pangolin) in Africa. • They share no genetic information yet become

more similar due to same environment, climate, etc.