L24Biol261 PopGen Evolution2014

8/9/2019 L24Biol261 PopGen Evolution2014

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R. Lewontin The Genetic Basis of Evolutionary Change.Columbia University Press 1970.

Darwin called attention to the actual variation among actualorganismsas the most essential and illuminating fact of nature.the Darwinian view that evolution begins with the conversionof variation between individuals into variation between

populations, the divergence of populations partly or whollyisolated from one another.


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During a drought in 1976-78 on the island of Daphne Major in the Galapagos

Archipelago, the population size of finches (Geospiza fortis) decreased (Graph A) due

to the decline in seed supply(Graph B), during which time the size and hardnessof

the seeds increased (Graph C), as a result of which the average size of finches

increased (Graph D), because larger birds can handle such seeds more efficiently.

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Random mating means that mating generally occurswithout a correlation with a genotype in particular, orphenotypic variation in general.

Non Random mating:(1)If mating between relatives happensmore frequentlythan

expected by chance, then the offspring are inbred.

(2)Alternatively if less frequently than by chance, there isoutbreeding.

(3) If individuals mate with phenotypically similar individuals,there is positive assortative mating- inbreeding

(4)Alternately if individuals mate with phenotypically unlikepartners, there is negative assortative mating (or disassortative

mating ).

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Self pollinating plants present the extreme case of

inbreeding. The loss of heterozygosity is easy to track.

Homozygotes

always

produce

homozygous

offspring.

Half of the

offspring of

heterozygotes are

homozygous, the

heterozygousindividuals

frequency

decreases by 1/2

each inbred

generation.

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In a large population there is a small chance that changes in allelefrequencies will be affected by stochastic demographic effects,some individuals die and others fail to breed for unknown reasonsor chance effects.

In a small population, chance inbreeding among a small numberof closely related individuals in a subpopulation, may causerandom changes in allele frequency. The smaller the population,the more likely individuals are related by descent due to

inbreeding.

Chance effects in a small populationcombined with inevitableinbreeding causes allele frequencies to drift towards 1.0 (fixation)or 0.0 (elimination).

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Genetic Drift The figure above indicates the divergence inhypothetical populations that started with( 50%) A alleles and(50%) a. Eventually one or the other allele goes to fixation in each

subpopulation by chance.

Evolution in small populations: genetic drift - randomdemographic effects and unavoidable inbreeding in smallpopulations

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Random genetic drift . The graph on the left is a simulation of drift with 16 individuals, in 107

populations that are initially heterozygous. The messier graph on the right is a real - life experiment

with 16 individuals that were heterozygous for the bw/bw75allele that shows the same pattern

expected for a small population that is drifting in allele frequency.

[Data in part (A) from P. Buri. 1956. Evolution 10: 367. Graphs from D. L. Hartl and A. G. Clark.

1989. Principles of Population Genetics. Sunderland, MA: Sinauer Associates]

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genetic drifteliminates mostbut not all mutations,regardlessof their selection coefficients. The time to fixation depends onchance, population size and selection. A longer time means a

greater chance of being lost

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SyntheticNeo-Darwinismadmitsthatfounder effects, drift,migration,although mostimportantlypositiveselectioninfluence changes in allelefrequencies between

populations, which if largeenough can lead to speciation,diversification of 1 ancestralspecies.

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Analogy would lead me one step further, namely, to thebelief that all animals and plants havedescended from oneprototype( Darwin 1859)

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Related sequences

(RNA pol II in L.Bromham

Reading the DNAstory

) show a

nested hierarchy

of substitutionsunder strong

stabilizing

selectionrelated todivergence times:

Human- rat- 4bp

0AA chg

Human- fly-9bp0AA chg

HumanFungus-11bp 1AA

chg

Human -plant-20bp6AA chg

Human-E coli-23bp8AA chg

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5*/.0A major fraction of humangenetic variation is found insmall populations, or there islittle genetic evidence for races.

Both protein polymorphismsand DNA markers concur that85-95% of variation was within

groups, 7-10% of variation wasbetween continental groups

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Multiregional model:The traditional paleontological model has Homo erectus

migrating out of Africa 1-2 million years ago, colonizing Europe and Asia andmore or less simultaneously evolving into Homo sapiens, because of relativelyhigh migration and intermixing.

Out of Africa model:after H. erectushad colonized Europe and Asia, modernH. sapiensevolved in Africa, and began migrating to Europe and Asia,

50-100,000 years ago.


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Within-species, mitochondrial and non-

recombining Y chromosome regions as

well as low recombination autosomalhaplotypes can be used to make a gene-

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L24Biol261 PopGen Evolution2014

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Transcript of L24Biol261 PopGen Evolution2014