30 march 20152108pop_genetics.ppt1 population genetics selection at the genetic level
TRANSCRIPT
30 March 2015 2108Pop_Genetics.ppt 2
Observations
• Observations• Diversity
• Adaptation
• Variation
• High reproductive rates = overproduction• “struggle for existence”
30 March 2015 2108Pop_Genetics.ppt 3
Variation
• Darwin did not know what causes variation.• Experiments with plants 3:1 phenotypic
ratio, but Darwin couldn’t/didn’t apply correct math.
• Mendel published on peas (1866)• Copy of Mendel’s paper found in Darwin’s library
years after his death.
• Darwin did read German; but . . .
• No evidence Darwin ever read Mendel’s paper, or understood it
30 March 2015 2108Pop_Genetics.ppt 4
Variation
• Mendel’s genetics rediscovered, 1900• Correns, Tschermak, DeVries
• Years of dispute over cause of evolution• Natural selection vs. genetics
• “Modern Synthesis” 1930’s• Genetics and natural selection reconciled
30 March 2015 2108Pop_Genetics.ppt 5
Variation• Sources of variation• Point mutation: Change in one base pair in DNA
directs production of modified protein, or no product
30 March 2015 2108Pop_Genetics.ppt 6
Variation• Sources of variation• Chromosomal mutation (mice on Madeira)
• Rearrangement of genes (loci) on chromosomes• Alters expression of some genes
• Gene duplication
30 March 2015 2108Pop_Genetics.ppt 7
Variation
• Sources of variation• Sex and Recombination
• Crossing-over, independent assortment, fertilization
• Alters expression
• Produces new combinations in genotypes
• Produces new combinations of phenotypes
30 March 2015 2108Pop_Genetics.ppt 8
Population Genetics
• Essential concept: Gene pool• “Collection” of all alleles of all individuals in
a population
• Within a gene pool, alleles have frequencies
• Allele frequency = proportion of an allele among all alleles of a given gene.
30 March 2015 2108Pop_Genetics.ppt 9
Population Genetics
• Gene pool• Allele frequency = proportion of an allele
among all alleles of a given gene. • In population of 500, = 1000 alleles at “eye color”
locus• 700/1000 are “red”
» f(A) 70%, or f(A) 0.70 = p
• 300/1000 are “sepia”
» f(a) 30%, or f(a) 0.30 = q
30 March 2015 2108Pop_Genetics.ppt 10
Population Genetics• If we know allele frequencies, we can
calculate genotypic and phenotypic frequencies.
30 March 2015 2108Pop_Genetics.ppt 11
Population Genetics
• Appears that recessive allele/phenotype is disappearing!• Puzzled early geneticists
~ 1900-1908
• Have allele frequencies changed? • NO Change !
30 March 2015 2108Pop_Genetics.ppt 12
Population Genetics
• Hardy-Weinberg Principle/equilibrium• G. H. Hardy (1877-
1947) • English mathematician
• Wilhelm Weinberg (1862-1937)
• German physician & geneticist
30 March 2015 2108Pop_Genetics.ppt 13
Population Genetics
• Hardy-Weinberg Principle/equilibrium• Allele frequencies remain unchanged generation to
generation
30 March 2015 2108Pop_Genetics.ppt 14
Population Genetics
• Hardy-Weinberg Principle/equilibrium• Mathematical model
• Assumptions/conditions• Large population
• Random interbreeding
• No selection
• No mutation
• No gene flow (migration among populations)
30 March 2015 2108Pop_Genetics.ppt 15
Population Genetics
• Violation of Hardy-Weinberg equilibrium• Small, NOT Large, population
• “Genetic drift”
30 March 2015 2108Pop_Genetics.ppt 16
Population Genetics• Violation of Hardy-
Weinberg equilibrium• Small population
• “Bottleneck” effect
• Founder effect
30 March 2015 2108Pop_Genetics.ppt 17
Population Genetics• Violations of Hardy-Weinberg equilibrium
NON-Random breeding• Sexual selection & female choice
Pea-fowl
• Male dominance, combatBig-horn sheep
Population Genetics• Violations of Hardy-Weinberg equilibrium
• Mutation
• Introduces/ adds new alleles
30 March 2015 2108Pop_Genetics.ppt 18
30 March 2015 2108Pop_Genetics.ppt 19
Population Genetics
• Violations of Hardy-Weinberg equilibrium
• Gene flow• Migration among
populations;
• Interbreeding with members of other populations.
30 March 2015 2108Pop_Genetics.ppt 20
Population Genetics• Violations of Hardy-Weinberg equilibrium
• Gene flow (migration among populations)
• Movement of pollen from one population to another
30 March 2015 2108Pop_Genetics.ppt 21
Population Genetics
• Violations of Hardy-Weinberg equilibrium• Movement of pollen from one population to another
Population Genetics
30 March 2015 2108Pop_Genetics.ppt 22
• Violations of Hardy-Weinberg equilibrium
YES, selectionRelative fitness
Balancing selection, “heterozygote advantage”
Frequency-dependent selection
30 March 2015 2108Pop_Genetics.ppt 23
Evolution & Population Genetics
• Limitations of evolution• Selection can only act on existing variations.
• Genes, alleles, phenotypes (anatomy and physiology) are limited by historical & developmental constraints.
• Adaptations are compromises.
• Better adaptations can be imagined, designed by engineers, but the genes or alleles don’t exist.
• Chance, natural selection, and environment interact.