Darwin’s theory of natural selection:

• Population grow exponentially• Over population results in competition and

struggle for existence• In any population, there is unequal abilities

for individuals• Only the best fit survive and pass their traits• Evolution occurs when advantageous traits

accumulate in a population

Please see video

Figure 17.15

Figure 17.16

Figure 17.17

Condition for Hardy – Weinbergequation:

1. Large population2. No evolution: Population

is stabilized, in equilibrium

• No genetic drift

• No gene flow

• No mutation

• No non-random mating

• No natural selection

Cause of Evolution

• Genetic drift– Bottle neck effect– Founder’s effect

• Gene flow

• Mutation

• Non-random mating

• Natural selection

Habitat isolation:Two species of Garter snakeThamophis

Water

Terrestrial

Temporal isolation:Spotted skunk

S. putorius mates in winter

S. gracilismates in summer

Please see video

Behavioral isolation

Blue-footed boobies

Mechanical isolationMonkey flower Mimulus

Gametic isolation

Reduced hybrid viability

Salamander Ensatina

Reduced hybrid fertility

Hybrid breakdown

22 9

Types of Evolution

• Directional• Diversifying• Stabilizing• Heterozygotic

supriority• Minority advantage

• Divergent• Convergent• Parallel• Reciprocal• Adaptive radiation

Figure 18.6

Figure 18.2

http://www.faceresearch.org/demos/average

(Works when in slide show)

Heterozygotic superiority

Sickle cell anemia MalariaSS Normal Sss Disease RSs Normal R

1930. JBS Haldane

Australian Mole

North American Mole

The same evolutionary forces acted on humans