Mechanisms for Evolution Natural selection traits that improve

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Mechanisms for Evolution • Natural selection – traits that improve survival or reproduction accumulate

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 –

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.

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

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) C WC W CR CR CR

Genetic drift (can greatly affect small populations) C WC W CR CR CR CW Only 5 of 10 plants leave offspring CR CR C WC W CR CR CR CW Generation 1 p (frequency of CR) = 0. 7 q (frequency of CW) = 0. 3 Only 2 of 10 plants leave offspring CR CR CR CR CW Generation 2 p = 0. 5 q = 0. 5 CR CR Generation 3 p = 1. 0 q = 0. 0

Founder effect (Genetic drift) • A new population is started by a small group

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

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 –

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

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?

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?