How Populations Evolve Ch. 23

• Individuals are selected but populations evolve

• i.e. English Peppered Moth• Populations (not individual organisms) are

smallest units that can evolve• Population = group of individuals of same

species at same place and time• Species = individuals that can reproduce

and produce fertile offspring

English Peppered MothExample

Before industrial revolution After industrial revolution

English Peppered Moth Example

Population Genetics

•Genome = total genes for individual (or species)•Gene pool = total genes of population•Population Genetics = Mendel + Darwin (Genetics) + (natural selection)•Microevolution = change in allele frequency (same as population genetics)•Hardy-Weinberg Lawmathematical concepts to represent alleles in populationBaseline for comparison

Conditions for H-W

• Very large population size • Isolation from other populations

(no immigration or emigration)• No mutations• Random Mating (all have equal chance)• No Natural Selection

All must be true for no change in allele frequencies from generation to generation

Agents of Microevolution(If one condition of H-W not true)

• Genetic drift (random changes in small populations)

• Gene Flow (immigration and emigration)

• Mutation

• Nonrandom Mating (selecting for traits)

• Natural selection

Genetic drift

• Founder Effect: a few leave the larger population to start a new colony and thereby change the allele frequencies

i.e. 1814 British colony founded on an islandOne individual was a carrier for retinitis

pigmentosa which causes blindness Harmful recessive (aa)By the 1960’s 4 people had disease, 9

others carriers

Genetic drift

• Bottleneck Effect: some survive and some don’t by chance (not because they are more adapted)

i.e. overhunting of northern elephant seals

1890’s hunters decreased population to 20 individuals, now even though population has increased to 30,000, no genetic variation

Genetic Drift: allele frequency changes in smallpopulations

The migrationof peoplethroughout the world has increasedgene flow.

Modes of selection

• Stabilizing selection: middle or intermediate phenotypes selected for, extreme phenotypes selected against

i.e. best adapted is “average”• Directional selection: favors one of the extremes

over the average and other extreme

i.e. favors rare individuals• Diversifying selection (disruptive selection): both

extremes are favored over average

Modes of selection