Mechanisms for Evolution

Mechanisms for Evolution• Natural selection

– traits that improve survival or reproduction accumulate in the population• ADAPTIVE change

• Genetic drift– frequency of traits changes

in a population due to chance events• RANDOM change

Natural Selection• Selection acts on any trait that affects

survival or reproduction– predation selection (speed, camouflage, defenses)

– physiological selection (disease resistance, protection from injury)

– sexual selection (attractiveness, fertility)

Modes of Selection

Directional: Darker mice are favored because they live in dark rocks.Disruptive: Intermediate color at a disadvantage. Mice live in both light and dark rocks.Stabilizing: Intermediate color is advantaged, extremes are not favored.

Genetic Drift • Chance events change frequency of

traits in a population– not adaptation to environmental

conditions– Founder effect– Bottleneck

 Genetic drift (can greatly affect small populations)

CRCR

CRCW

CRCR

CWCW CRCR

CRCW

CRCW

CRCWCRCR

CRCR

Only 5 of10 plantsleaveoffspring

CWCW CRCR

CRCW

CRCR CWCW

CRCW

CWCW CRCR

CRCW CRCW

Only 2 of10 plantsleaveoffspring

CRCR

CRCR CRCR

CRCRCRCR

CRCR

CRCR

CRCR

CRCRCRCR

Generation 2p = 0.5q = 0.5

Generation 3p = 1.0q = 0.0

Generation 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3

Founder effect (Genetic drift)

• A new population is started by a small group of individuals or are isolated from other population– just by chance some rare traits may

be at high frequency; others may be missing

– skews the gene pool of new population

– less genetic diversity

albino deer Seneca Army Depot

Example: Distribution of blood types• Distribution of the O type blood allele in native

populations of the world reflects original settlement

Bottleneck effect (genetic drift)

• A large population drastically reduced by a disaster– famine, natural disaster, loss of habitat…– loss of variation by chance event• narrows the gene pool

Example: Cheetahs • All cheetahs share a small number of

alleles– less than 1% diversity– as if all cheetahs are

identical twins• 2 bottlenecks– 10,000 years ago• Ice Age

– last 100 years• poaching & loss of habitat

Question???• What do you think is the definition of

a species?

• Pair up and write down your definition of a species and how do we get new species?