ASSORTATIVE MATING ASSORTATIVE DATING

ASSORTATIVE MATING ASSORTATIVE DATING

Evolutionary Mechanisms

HARDY-WEINBERG EQUILIBRIUM

(1) random mating

(2) equal number of males and females

(3) the population is infinitely large

(4) there is no migrationin or out

(5) natural selection, mutation, & geneticdrift are not acting on the population

(5) meiosis is fair

(6) all matings produce the same number of off-spring on average

(7) generations do no overlap

(8) there are no differences among genotypes in the probability of survival

HARDY-WEINBERG EQUILIBRIUM

NON-RANDOM MATING

If individuals (usually females) are choosy in their selection of mates the gene frequencies may become altered.

Darwin called this sexual selection.

Breeding territories, courtship displays, "pecking orders" can all lead to it.

In each case certain individuals do not get to make their proportionatecontribution to the next generation.

NON-RANDOM MATING

SEXUAL SELECTION

Drawbacks -- Differential contributions to the next generation.

Eventually, sexual selection will come up against opposing forces of viability selection. 

Traits that are too conspicuous will exposeindividuals to predation.

ASSORTATIVE MATING

Mating of individuals that are phenotypically similar.

Such individuals are more likely to carry the same alleles for genes determining morphology

Positive --

Individuals show a preference for their own phenotype (most common)

Reproductive isolation among between sympatric species may sometimes be viewed as a form of assortative mating.

Complete positive assortative mating willresult in speciation.

ASSORTATIVE MATING

Non-random mating is frequently the result of social factors.

Mating for other traits is essentially random.

SOCIAL FACTORS & EFFECTS

Assortative mating increases homozygosityat the expense of heterozygosity

No change in allele frequency, only genotype frequency

Matings between close relatives is a special case of assortative mating.

The closer the kinship, the more alleles shared

Predisposes to homozygosity.

Potentially harmful recessive alleles - invisible in the parents - become exposed to the forces of natural selection in the children.

Many species have mechanismswhich help them avoid inbreeding.

INBREEDING

AB CD

AC

AA

AD

Inbreeding increaseshomozygosity

INBREEDING

Negative --

Purposeful avoidance of mate with a similar phenotype

TYPES OF ASSORT. MATING

Assortative mating leads to nonrandom patterns of mating

The basis for assortative mating is not relatedness but phenotypic similarity or dissimilarity. 

Both processes sort existing variation, altering genotypic frequencies within populations. 

Inbreeding and assortative mating do not dramatically alter allele frequencies. 

Highly significant consequencesfor the evolution of populations.

ASSORTATIVE MATING VS. INBREEDING

CLASS RESULTS [2004]

Age 9 13

Stature 6 16

Hair Color 15 6

Eye Color 5 18

Education 19 4

Religion 18 5

Intensity 15 8

Economics 20 3

Residence 20 3

Hobbies 15 8

Music 19 4

Entertainment 12 11

same different

SYMMETRY

SYMMETRY

SYMMETRY

tracing the change in spousal resemblance over time,

analyzing the resemblance between the spouses of biologically related individuals

MEASURING ASSORTATIVE MATING

Dad

’s h

eig

ht

Mom’s height

Mechanisms that change gene frequencies:

natural selection

genetic drift

bottlenecks and founder effects

assortative mating

inbreeding

EVOLUTIONARY FACTORS