Download Speciation vs. Selection

Mechanisms of Evolution
Isolation and Selection
15.2 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
Floppy
1. Speciation
results in
different
species.
Dwarf
2. Selection creates
different variations
of the same species.
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.