Mechanisms of Evolution

• 1) Micro-evolution– change within a species

• 2) Macro-evolution– change between species

Micro-evolution

• Hardy-Weinberg (2 mathmaticians)– “Frequency of alleles in a

population stays the same generation after generation unless acted upon by some outside force.”

• So, what kind of things can change the frequency of alleles in a population?

Hardy-Weinberg (Cont)

• All alleles contribute by males (p+q)• All alleles contributed by females

(p+q)• Together males and females mate• (p+q)(p+q) = 1

– all combinations =100%

• p2 + 2pq + q2 = 1 (Also show Punnett)

Hardy-Weinberg (Cont)

• The p = Allele F = Free Earlobes• The q = Allele a = Attached earlobes• Since q (or a) is the only phenotype

with a known genotype (aa), then we can calculate the frequencies of alleles in a population

• aa -> a2 -> a = (a2)1/2

• F = 1-a (since both a and F = 100%)

Hardy-Weinberg

• For a Non-Evolving population, these conditions must be met:

•1) No mutations•2) No migration•3) No Genetic Drift-Population Large•4) Random Mating•5) No Natural Selection

• If a population changes (gene frequency) or evolves, it must be due to some factor that violates one of the conditions stated previously.

Gene Flow: Alleles introduced to a gene

pool– 1) Mutation– 2) Migration (immigration/emigration)

• Animals leave area for another– contribute genes to neighbor’s gene

pool• Plants have seeds dispersed by high

winds

• Immigration/Emigration– Very Common– 2nd only to Natural Selection as

cause of evolution

Conflicting Forces

• Gene flow - increase similarity between populations

• Natural Selection - decrease similarity between populations

• Possible Outcome:– gradient of variation from one

population to another– Increase distance = increase variation

Genetic Drift: Evolution by chance

• 1) Bottleneck Effect: Most of the population is destroyed by a natural event (volcano, earthquake, etc.) and a few individuals survive to reproduce

• 2) Founder Effect: Few people leave an area to colonize new area

– Good chance not carry representative sample of all genes present in original population

– Environment is different•different selection pressures•Therefore, evolve in new direction

– In practice, difficult to tell how much of genetic differences between old and new populations is due to:•1) Founders effect•2) Different selection pressures in two environments

– Founder Effect is great•1) Population of plants that

populate an island from a single seed

•2) Animals which have descended from one original wild female

–domestic hamsters

Mating Preferences

• Females consistently choose one trait over another– Exert selection pressure for one gene

•tails of peacocks

–Female preferences is a form of Natural Selection•One genotype has reproductive advantage over another