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

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 P 1 P 2 P 3 Note: We are NOT assuming HW frequencies here Positive assortative mating Some background material: p = P 1 + (1/2)P 2 & q = P 3 + (1/2)P 2 α = AA x AA, Aa x Aa and aa x aa and ( 1 - α ) is the random mating fraction 100% AA AA x AA = (1/4)AA + (1/2)Aa + (1/4)aa Aa x Aa = 100% aa aa x aa = 2

Positive assortative mating Some background material: The formulas for the next generation: P ' ( 1 α p ) 2 ( P ( 1 / 4 ) P ) = − + α +             1 1 2 p = P 1 + (1/2)P 2 & q = P 3 + (1/2)P 2 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 ) 2 pq ( ( 1 / 2 ) P ) = − α + α      2      2 and 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 matings 100% AA AA x AA = P ' ( 1 α q ) 2 ( P ( 1 / 4 ) P ) = − + α + (1/4)AA + (1/2)Aa + (1/4)aa      3    3     2 Aa x Aa = 100% aa random mating positive assortment aa x aa = component component Positive assortative mating α > 0 = frequencies will no longer sum to 1. For population frequencies: standardize by the sum P 1 ’ + P 2 ’ + P 3 ’ . ' P Frequency of Aa 2 For example: = 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 Frequency of heterozygotes 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 B. Effect of positive assortative mating ( α = 1) on heterozygosity under complete dominance [Formula not shown] Frequency of heterozygotes 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 5

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 Pied Flycatcher ( F. albicollis ) ( 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