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
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
P1
P2
P3
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:
p = P1 + (1/2)P2
The formulas for the next generation:
&
q = P3 + (1/2)P2
P1' = (1 − α ) p 2 + α (P1 + (1 / 4)P2 )





Freq of AA under
random mating
Freq of AA under positive
assortment has two sources:
100% from AAxAA, and
1/4 from AaxAa
α = AA x AA, Aa x Aa and aa x aa
P2' = (1 − α )2 pq +

and
Freq of Aa under
random mating
(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
α ((1 / 2)P2 )

Freq of Aa under positive
assortment has one source:
1/2 from AaxAa matings
P3' = (1 − α )q 2 + α (P3 + (1 / 4)P2 )


 
random mating
component
positive assortment
component
Positive assortative mating
α > 0 = frequencies will no longer sum to 1.
For population frequencies: standardize by the sum P1 + P2 + P3 .
For example:
'
Frequency of Aa = P2
'
i
∑P
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
Frequency of heterozygotes
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
0
1
3
5
7
9
11
13
15
17
19
generation
4
Positive assortative mating
Frequency of heterozygotes
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
0
1
3
5
7
9
11
13
15
17
19
21
generation
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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Positive assortative mating and speciation
Example: positive assortment in species 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
Collared Flycatcher
(F. albicollis)
Pied Flycatcher
(F. hypoleuca)
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.
9