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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?