30 March 20152108Pop_Genetics.ppt1 Population Genetics Selection at the genetic level.

30 March 2015 2108Pop_Genetics.ppt 1

Population Genetics

Selection at the genetic level


Observations

• Observations• Diversity

• Adaptation

• Variation

• High reproductive rates = overproduction• “struggle for existence”


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


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


Variation• Sources of variation• Point mutation: Change in one base pair in DNA

directs production of modified protein, or no product


Variation• Sources of variation• Chromosomal mutation (mice on Madeira)

• Rearrangement of genes (loci) on chromosomes• Alters expression of some genes

• Gene duplication


Variation

• Sources of variation• Sex and Recombination

• Crossing-over, independent assortment, fertilization

• Alters expression

• Produces new combinations in genotypes

• Produces new combinations of phenotypes


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.


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


Population Genetics• If we know allele frequencies, we can

calculate genotypic and phenotypic frequencies.


Population Genetics

• Appears that recessive allele/phenotype is disappearing!• Puzzled early geneticists

~ 1900-1908

• Have allele frequencies changed? • NO Change !


Population Genetics

• Hardy-Weinberg Principle/equilibrium• G. H. Hardy (1877-

1947) • English mathematician

• Wilhelm Weinberg (1862-1937)

• German physician & geneticist

http://en.wikipedia.org/wiki/File:[email protected]

http://upload.wikimedia.org/wikipedia/en/d/df/Wilhelm_Weinberg.jpg


Population Genetics

• Hardy-Weinberg Principle/equilibrium• Allele frequencies remain unchanged generation to

generation


Population Genetics

• Hardy-Weinberg Principle/equilibrium• Mathematical model

• Assumptions/conditions• Large population

• Random interbreeding

• No selection

• No mutation

• No gene flow (migration among populations)


Population Genetics

• Violation of Hardy-Weinberg equilibrium• Small, NOT Large, population

• “Genetic drift”


Population Genetics• Violation of Hardy-

Weinberg equilibrium• Small population

• “Bottleneck” effect

• Founder effect


Population Genetics• Violations of Hardy-Weinberg equilibrium

NON-Random breeding• Sexual selection & female choice

Pea-fowl

• Male dominance, combatBig-horn sheep


• Mutation

• Introduces/ adds new alleles



Population Genetics

• Violations of Hardy-Weinberg equilibrium

• Gene flow• Migration among

populations;

• Interbreeding with members of other populations.



• Gene flow (migration among populations)

• Movement of pollen from one population to another


Population Genetics

• Violations of Hardy-Weinberg equilibrium• Movement of pollen from one population to another

Population Genetics


• Violations of Hardy-Weinberg equilibrium

YES, selectionRelative fitness

Balancing selection, “heterozygote advantage”

Frequency-dependent selection


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.

30 March 20152108Pop_Genetics.ppt1 Population Genetics Selection at the genetic level.

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Transcript of 30 March 20152108Pop_Genetics.ppt1 Population Genetics Selection at the genetic level.