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Weak forces in Weak forces in EvolutionEvolution

Dan Graur

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Changes in allele freque

ncies are

Changes in allele freque

ncies are

importantimportant..Changes in g

enotype frequencies are

Changes in genotype freq

uencies are not not

so importantso important..

1.1.

2.2.

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MatingMating

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Deviation from randomness:Deviation from randomness:By genetic similarity:By genetic similarity:

Assortative matingAssortative matingDisassortative matingDisassortative mating

By genetic relatedness:By genetic relatedness:InbreedingInbreedingOutbreedingOutbreeding

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disassortativeassortative

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Human assortative mating: Rule #1

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Strong assortative mating by skin color in the US

Percentage of non-hispanic whites: 80%Percentage of blacks: 13%Expected percentage of interracial marriages: 10.4%Observed percentage of interracial marriages: 0.9%

Ratio of = 2:1

white female/black male black female/white male

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Assortative mating Assortative mating is very strong in is very strong in humans even with humans even with respect to respect to non-non-geneticgenetic traits. traits.

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Degrees of inbreeding found in nature:Degrees of inbreeding found in nature:

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Inbreeding is Inbreeding is usually usually forbidden… forbidden…

An exception!An exception!

C vii

P xii C v

P ix

P = PtolemyC = Cleopatra

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The fishThe fish Rivulus Rivulus marmoratusmarmoratus exhibits the exhibits the most extreme form of most extreme form of inbreeding: inbreeding: SelfingSelfing

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Random mating

equilibr

ium allele

frequencies

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Assortative mating

excess

homozygotes

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Disassortative

mating

excess heterozygo

tes

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Even if extreme extreme deviations deviations from random from random matingmating occur in all generations, allele allele frequencies frequencies remain remain constantconstant.

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Mating pattern Mating pattern is is notnot an important an important

evolutionary force evolutionary force

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Clinical effects of inbreeding

P.T. Barnum + Tom Thumb

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Clinical effects of inbreeding

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MigrationMigration

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Migration will cause changes in the allele frequencies of each of the two subpopulations.

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However, because of gene flow, the two subpopulations are, in fact, one population, in which allele frequencies do no change.

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Genotype frequencies will change in a similar fashion to that in disassortative mating.

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Migration Migration is is notnot an important an important

evolutionary force evolutionary force

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MutationMutationMutation:

A transmissible change in the genetic material

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Mutations are the Mutations are the ultimate source of ultimate source of variation. Only variation. Only mutations can mutations can create gcreate genetic enetic novelnovelty. ty.

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Mutations arise all Mutations arise all the time. Per the time. Per definition, the definition, the initial frequency initial frequency of a mutation in a of a mutation in a diploid population diploid population is 1/2N.is 1/2N.****N = population sizeN = population size

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The human population on The human population on October 31, 2011 was October 31, 2011 was estimated to be 7 billion estimated to be 7 billion people. The number of people. The number of alleles at an autosomal alleles at an autosomal locus is, therefore, 14 locus is, therefore, 14 billion. A mutation arising billion. A mutation arising today in the human today in the human population will have an population will have an initial frequency of about 7 initial frequency of about 7 × 10× 101111……

……resulting in a change in resulting in a change in allele frequencies fromallele frequencies from 00 toto 0.00000000007.0.00000000007.

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Mutation is the Mutation is the ultimate source of ultimate source of variability, but it is variability, but it is notnot an important an important evolutionary force. evolutionary force.

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For a mutation to For a mutation to become significant, become significant, it must increase its it must increase its frequency, so that frequency, so that it becomes fixedit becomes fixed** in in the population. the population.

**frequency of allele = 1.0frequency of allele = 1.0

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Two factors can lead Two factors can lead to the fixation of a to the fixation of a new mutation:new mutation:

SelectionSelectionRRaandndoom gm geenneettiic c

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