Transcript Natural Selection

The Evolution of Populations
Darwin’s Theory of Natural Selection
1) Organisms reproduce others of their kind
2) More offspring are produced in nature than can
survive to reproduce
3) There are variations among offspring within a
population
4) Some variations are favorable and allow
individuals to survive better
5) Organisms that survive better tend to reproduce
more frequently
6) Over long periods of time, favorable adaptations
make up most of the population
Natural Selection
Mechanisms
Normal Distribution – Bell Curve
Almost everything you can think of fits this curve!
Directional Selection
Breeders always select for the fastest greyhounds
Selects for: One extreme trait
Selects against: The most common and other extreme
Disruptive Selection
Selects For: The two extremes
Selects Against: The most
common
Stabilizing Selection
• Tall trees
– Knocked
over by wind
• Short trees
– Not enough
sunlight
• Medium trees
– Perfect!?
Selects For: The most common trait
Selects Against: Both extremes
Review
Natural Selection is not “goal oriented”
 Only acts on traits that currently exist and are best suited for
reproductive success in a particular environment
Example: Tigers will not become blue just because they might want to
I wish, but he
exists only
in my dreams.
He’s not blue,
but he’s better
than nothing!