Mating system Random Mate choice is independent of both phenotype and - - PDF document

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Population Genetics 4: Assortative mating

Mating system Random Mate choice is independent of both phenotype and genotype Positive assortment Mate choice is based on similarity of phenotype Negative assortment Mate choice is based on dissimilarity of phenotype Inbreeding Mating with relatives at a rate greater than expected by chance

Assortative mating: non-random mating system where mates are chosen according to their phenotypes

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Positive assortative mating

Positive assortative mating: non-random mating system where mates are chosen based on similarity of phenotypes

• Some fraction will mate with similar individuals under random mating
• Positive assortment = greater than chance expectations
• humans: lots of positive assortment (IQ, race, etc.)

As always, we examine the effect at the population level.

Genotype AA Aa aa Frequency P1 P2 P3 Note: We are NOT assuming HW frequencies here p = P1 + (1/2)P2 & q = P3 + (1/2)P2 α = AA x AA, Aa x Aa and aa x aa and (1 - α) is the random mating fraction

AA x AA = 100% AA Aa x Aa = (1/4)AA + (1/2)Aa + (1/4)aa aa x aa = 100% aa Some background material:

Positive assortative mating

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p = P1 + (1/2)P2 & q = P3 + (1/2)P2 α = AA x AA, Aa x Aa and aa x aa and (1 - α) is the random mating fraction

AA x AA = 100% AA Aa x Aa = (1/4)AA + (1/2)Aa + (1/4)aa aa x aa = 100% aa Some background material:

Positive assortative mating

The formulas for the next generation:

( ) ( ) ( )

           

AaxAa from 1/4 and AAxAA, from 100% : sources two has assortment positive under AA

• f

Freq 2 1 mating random under AA

• f

Freq 2 ' 1

P 4 / 1 P 1 P + + − = α α p

( ) ( ) ( )

         

matings AaxAa from 1/2 : source

• ne

has assortment positive under Aa

• f

Freq 2 mating random under Aa

• f

Freq ' 2

P 2 / 1 2 1 P α α + − = pq

( ) ( ) ( )

           

component assortment positive 2 3 component mating random 2 ' 3

P 4 / 1 P 1 P + + − = α α q Positive assortative mating

α > 0 = frequencies will no longer sum to 1. For population frequencies: standardize by the sum P1

’ + P2 ’ + P3 ’.

For example:

∑

=

i ' i ' 2

P P Aa

• f

Frequency

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Positive assortative mating

Genotype frequencies Generation AA Aa aa 0.250 0.5 0.250 20 (α = 0.75) 0.396 0.208 0.396 Example: p = q = 0.5 and α = 0.75 Check for yourself; before and after 20 generations p = q = 0.5 Positive assortment:

• 1. genotype frequencies change
• 2. allele frequencies do NOT change

0.1 0.2 0.3 0.4 0.5 0.6 1 3 5 7 9 11 13 15 17 19

• A. Effect of complete (α = 1) and partial

(α = 0.75) positive assortative mating on heterozygosity generation Frequency of heterozygotes α = 0.75 α = 1.0 p = q = 0.5

Positive assortative mating

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0.1 0.2 0.3 0.4 0.5 0.6 1 3 5 7 9 11 13 15 17 19 21

generation Frequency of heterozygotes [Formula not shown] α = 1.0 + dominance p = q = 0.5

• B. Effect of positive assortative mating

(α = 1) on heterozygosity under complete dominance

Positive assortative mating Positive assortative mating and speciation

Reinforcement: natural selection for positive assortment

• invoked where divergent populations overlap (Sympatry)
• why?

− avoid matings between individuals from divergent populations − avoid wasting reproduction on producing “less-fit” hybrids − lead to increased reproductive isolation

• consensus opinion: reinforcement is probably rare
• disruptive selection: selection pressure for divergence of two populations into

ecologically distinct types

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Allopatric type Sympatric type Adapted from Butlin and Tregenza 1998

Pied flycatcher colour polymorphism

Example: positive assortment in species of flycatcher (Saetre et al. 1998)

Positive assortative mating and speciation

In Central and Eastern Europe, where the Pied flycatcher is sympatric with the collared flycatcher, the two species exhibit distinct colour differences

Pied Flycatcher (F. hypoleuca) Collared Flycatcher (F. albicollis) Allopatry Allopatry Sympatry

Positive assortative mating and speciation

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Sætre et al. (1998) Four points:

• 1. Between species matings are more rare than expected, and hybrids have reduced fitness
• 2. Phylogenetics indicated that plumage polymorphism is derived.
• 3. Female of sympatric populations/species prefer males that have the sympatric colouring rather than the

allopatric colouring (positive assortment).

• 4. Pied females exhibit the opposite preference (for dull brown males) than is exhibited in most other

populations; in most populations the preference is for striking black and white males.

Positive assortative mating and speciation

Adapted from Sætre et al. (1998)

Mate preferences of female flycatchers

Positive assortative mating

Positive assortment keynotes:

• Increases homozygosity, thereby preventing HW equilibrium
• Does not affect allele frequencies
• Affects only those genes related to the phenotype by which mates are chosen. The
• ther loci can be in HW equilibrium
• Results in LD because it prevents equilibrium of allele frequencies between the locus

subject to assortment and other loci in the genome

• Dominance dilutes the effect of positive assortment

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Negative assortative mating

Negative assortative mating: non-random mating system where mates are chosen based on dissimilarity of phenotypes

• also called disassortative mating
• negative assortment = greater than chance expectations
• Drosophila: “rare-male advantage”
• common in plants as “self-incompatibility”
• gametophytic: allelic incompatibility
• sporophytic: genotypic incompatibility

Negative assortative mating

Negative assortment keynotes:

• Yields an excess of heterozygotes, as compared with HW equilibrium
• Does not affect allele frequencies (An exception is the rare male advantage

phenomenon in Drosophila, because of greater reproductive success of rare males. Under “normal” cases of negative assortative mating, all males have equal mating success)

• Loci not subject to negative assortative mating can be in HW equilibrium
• Dominance dilutes the effect of negative assortment
• Increases the rate to equilibrium of alleles among loci because linkage phases are

disrupted by recombination in double homozygotes.

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Final note: Assortative mating combined with natural selection can have a significant affect on the rate of change in the allele frequencies at the locus subject to assortative mating.