Population Genetics 4: Assortative mating

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1 opulation Genetics 4: Assortative mating Mating system Random ositive assortment Negative assortment Inbreeding Mate choice is independent of both phenotype and genotype Mate choice is based on similarity of phenotype Mate choice is based on dissimilarity of phenotype 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

2 ositive assortative mating ositive 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 ositive 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 1 3 Note: We are NOT assuming HW frequencies here ositive assortative mating Some background material: p = 1 + (1/) & q = 3 + (1/) 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/)Aa + (1/4)aa aa x aa = 100% aa

3 ositive assortative mating Some background material: The formulas for the next generation: p = 1 + (1/) & q = 3 + (1/) ( 1 α ) p + ( ( 1/ 4) ) 1 = α 1 + 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 and (1 - α) is the random mating fraction ( 1 α ) + α( ( 1/ ) ) = pq Freq of random Aa mating under Freq of Aa under positive assortment has one source: 1/ from AaxAa matings AA x AA = 100% AA Aa x Aa = (1/4)AA + (1/)Aa + (1/4)aa aa x aa = 100% aa ( 1 α) q + ( ( 1/ 4) ) 3 = α 3 + random mating component positive assortment component ositive assortative mating α > 0 = frequencies will no longer sum to 1. For population frequencies: standardize by the sum For example: Frequency of Aa = i i 3

4 ositive assortative mating Example: p = q = 0.5 and α = 0.75 Genotype frequencies Generation AA Aa aa (α = 0.75) Check for yourself; before and after 0 generations p = q = 0.5 ositive assortment: 1. genotype frequencies change. allele frequencies do NOT change ositive assortative mating A. Effect of complete (α = 1) and partial (α = 0.75) positive assortative mating on heterozygosity Frequency of heterozygotes α = 0.75 α = 1.0 p = q = generation 4

5 ositive assortative mating B. Effect of positive assortative mating (α = 1) on heterozygosity under complete dominance Frequency of heterozygotes [Formula not shown] p = q = 0.5 α = dominance generation ositive 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

6 ositive assortative mating and speciation Example: positive assortment in species of flycatcher (Saetre et al. 1998) ied flycatcher colour polymorphism Allopatric type Sympatric type Adapted from Butlin and Tregenza 1998 ositive assortative mating and speciation In Central and Eastern Europe, where the ied flycatcher is sympatric with the collared flycatcher, the two species exhibit distinct colour differences ied Flycatcher (F. hypoleuca) Collared Flycatcher (F. albicollis) Sympatry Allopatry Allopatry 6

7 ositive 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. hylogenetics 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. ied 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. ositive assortative mating ositive 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

8 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

9 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