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How Populations Evolve Ch. 23
• Individuals are selected but populations
• 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 Moth
English Peppered Moth Example
Before industrial revolution After industrial revolution
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 Law
mathematical concepts to represent alleles in population
Baseline 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
• 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 island
One 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
Genetic Drift: allele frequency changes in small
The migration
of people
throughout the
world has
gene 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