1 Bio1B Evolution 4 Last lecture: • More history - Darwin+Mendel => the neodarwinian synthesis • Mechanisms of evolution: – Evolution in populations - population genetics – Allele, genotype and phenotype frequencies – Predicting genotype freq’s: Hardy (Castle) Weinberg Equilibrium • Application: Null model for evolution • Predicting heterozygote frequencies for recessive traits Today Evolutionary processes • Sampling effects => “genetic drift” – Relevance in evolution - loss of variation, bottlenecks • Mutation as the ultimate source of variation; effects on fitness • Migration (gene flow) - spreading mutations, clines • Selection – fitness – Forms and consequences
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Bio1B Evolution 4Last lecture:• More history - Darwin+Mendel => the neodarwinian synthesis• Mechanisms of evolution:
– Evolution in populations - population genetics– Allele, genotype and phenotype frequencies– Predicting genotype freq’s: Hardy (Castle) Weinberg Equilibrium
• Application: Null model for evolution• Predicting heterozygote frequencies for recessive traits
TodayEvolutionary processes• Sampling effects => “genetic drift”
– Relevance in evolution - loss of variation, bottlenecks• Mutation as the ultimate source of variation; effects on fitness• Migration (gene flow) - spreading mutations, clines• Selection
– fitness– Forms and consequences
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Hardy-Weinberg genotype frequencies as a function ofallele frequencies at a locus with two alleles
Futuyma, 2nd Ed.
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Effect of small population size - “genetic drift”
• Sampling gametes => zygotes• Small population have greater sampling error =>
larger fluctuations in allele frequency=> reduced variation within populations
Fig. 23.8
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Population bottlenecks
• Habitat loss or over-harvesting• Colonization of new areas (eg.
islands; humans “Out of Africa”⇒Loss of genetic diversity⇒Rapid change in allele
frequencies => divergence
Fig. 23.9 Fig. 23.10
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Mutations - forms
Fig. 17.22
HbbA HbbS
Fig 18.8
Regulation Coding
Changes in:Coding sequenceGene regulationGene copy numberChromosome number &structure
Exons -> mRNA -> aa
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Agouti melanisticmutations:Recessive [regulatory& coding]
Mc1r MelanisticmutationsDominant[structural]
So what? … Mc1r & melanoma!
X
Some key genes in melanin production pathway
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Migration (asgene flow)
• Gene flow = movement of genesamong populations
• Arises from net movement birth-> reproduction or gametedispersal -> zygotes
• Spreads new mutations;maintains variation
• Opposes effects of genetic driftor local selection
• Spread of genes from GMOcrops is a concern
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Genetic fitness
• Selection acts throughthe phenotype
• Fitness = Survival andreproduction relative toother phenotypes orgenotypes in thepopulation
• Relative fitness can beenvironment dependent
Sorry Arnie….
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Forms of selection (Fig. 23.13)
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Relative fitnesses:
Without malaria:
HbbAA > HbbAS > HbbSS
anaemia
With malaria:
HbbAS > HbbAA > HbbSS
Example of Heterozygote advantagesickle-cell anaemia
Moreresistantto malaria Note - fitness of HbbAS depends on
environment (+ malaria)
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Association between melanicphenotype and Mc1r allele in rock
pocket mice from Pinacates lava flow
Hopi Hoekstra inthe field..
Geographic cline
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Experimentalevidence forrapid evolutiondue to selection
Eg. guppy color -field experiments;text pp460)
• Others - microbialevolution etc etc
Fig. 22.13
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Genomic signatures of recent selection
New mutation arisesthat increases fitness
Under directional selectionincreases to p = 1; dragslinked sites with it
Results in a region of lowvariation relative to others
Variation
Variable site
high high highlowStorz 2005
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Genomicsignatures of
selection;localized
reductions indiversity
Sutter et al. 2007Science 316:112
What’swith mycrazy dog?
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