Population Genetics 4:

Assortative

Mating system

Random 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 Mating with relatives at a rate greater than expected by chance

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

1 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 P 1 P 2 P 3 Note: We are NOT assuming HW frequencies here

Positive assortative mating

Some background material:

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

2 Positive assortative mating

Some background material: The formulas for the next generation:

' 2 P1 = (1−α )p + α(P1 + (1/ 4)P2 ) p = P1 + (1/2)P2 & q = P3 + (1/2)P2   Freq of AA under Freq of AA under positive random mating assortment has two sources: 100% from AAxAA, and 1/4 from AaxAa

α = AA x AA, Aa x Aa and aa x aa P' = 1−α 2 pq + α 1/ 2 P and 2 () (( )2 ) Freq of Aa under Freq of Aa under positive random mating assortment has one source: (1 - α) is the random mating fraction 1/2 from AaxAa

AA x AA = 100% AA ' 2 P3 = (1−α)q +α(P3 + (1/ 4)P2 ) Aa x Aa = (1/4)AA + (1/2)Aa + (1/4)aa   random mating positive assortment aa x aa = 100% aa component component

Positive assortative mating

α > 0 = frequencies will no longer sum to 1.

’ ’ ’ For population frequencies: standardize by the sum P1 + P2 + P3 .

P ' For example: Frequency of Aa = 2 P ' ∑ i i

3 Positive assortative mating

Example: p = q = 0.5 and α = 0.75 Genotype frequencies

Generation AA Aa aa 0 0.250 0.5 0.250 20 (α = 0.75) 0.396 0.208 0.396

Check for yourself; before and after 20 generations p = q = 0.5

Positive assortment: 1. genotype frequencies change 2. allele frequencies do NOT change

Positive assortative mating

A. Effect of complete (α = 1) and partial (α = 0.75) positive assortative mating on heterozygosity

0.6 p = q = 0.5 0.5

0.4

0.3 α = 0.75

0.2

0.1 α = 1.0 Frequency of heterozygotes

0 1 3 5 7 9 11 13 15 17 19 generation

4 Positive assortative mating

B. Effect of positive assortative mating (α = 1) on heterozygosity under complete dominance

[Formula not shown] 0.6 p = q = 0.5 0.5

0.4

0.3

0.2 α = 1.0 + dominance

0.1 Frequency of heterozygotes 0 1 3 5 7 9 11 13 15 17 19 21 generation

Positive assortative mating and

Reinforcement: for positive assortment

• invoked where divergent populations overlap ()

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

• consensus opinion: reinforcement is probably rare

• : selection pressure for divergence of two populations into ecologically distinct types

5 Positive assortative mating and speciation

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

Pied flycatcher colour polymorphism

Allopatric type Sympatric type

Adapted from Butlin and Tregenza 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 Collared Flycatcher (F. hypoleuca) (F. albicollis)

Sympatry

Allopatry Allopatry

6 Positive assortative mating and speciation

Mate preferences of female flycatchers

Adapted from Sætre et al. (1998)

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

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 other 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

7 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.

8 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.

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