Mechanisms of Evolution

Isolation and Selection15.2 pages 407-410

Isolation

Geographic Isolation: occurs whenever a physical barrier divides a population.

Example: River forms over time. Species of tree frog can’t cross the river.

Isolation

Reproductive Isolation: occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring. (genetic or behavioral)

Example: one type of tree frog mates in fall, another in spring.

Speciation vs. Selection

1. Speciation results in different species.

2. Selection creates different variations of the same species.

Dwarf

Floppy

Two fur colors, both Dwarf

Stabilizing Selection

…favors average individuals in a population.

Example: Husky – too big, sink in snow; too light, not strong enough.

Directional Selection

…favors one of the extreme variations of a trait.

Example: Woodpecker – big beak, catch bugs; small/medium, can’t.

Disruptive Selection

…favors individuals with either extreme of a trait’s variation.

Example: Clams – white clams safe on exposed rocks; black clams safe on rocks in water; grey/shiny clams eaten.