PopGen4: Assortative mating

Transcription

1 opgen4: Assortative mating Introduction Although random mating is the most important system of mating in many natural populations, non-random mating can also be an important mating system in some populations. Non-random mating results in changes in the genotype frequencies in the population, i.e., how the alleles are put together into genotypes, but it does NOT change the allele frequencies themselves. Since genotype frequencies will be affected, non-random mating results in a deviation from Hardy-Weinberg equilibrium. For the purpose of this course we will divide mating systems into four broad categories. 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 The topic of these notes is ASSORTATIVE MATING, the non-random mating system where mates are chosen by their phenotypes. ositive assortative mating Under a system of OSITIVE ASSORTATIVE MATING individuals choose mates that are phenotypically like themselves. Note that under random mating there will always be some fraction of the population that mate with phenotypically similar individuals. So when we say positive assortment, we mean that the mating of phenotypically similar individuals is occurring at a frequency that is greater than expected by chance. We can look to ourselves, humans, for an example of positive assortative mating. It has been shown that there is significant assortment according to phenotypes such as height, IQ score and race. There is assortment for certain socioeconomic traits. Interestingly, in the US, one of the highest correlations is found between the numbers of rooms in the homes of the parents of married couples. To consider the effect of positive assortment on genotype frequencies, let: Genotype AA Aa aa Frequency 1 3 Note: We are NOT assuming HW frequencies here Even though we do not assume HW equilibrium, we still compute p and q as before: p = 1 + (1/) q = 3 + (1/)

2 Now consider that the frequency of positive assortment in a population is equal to α: α = the frequency of AA x AA, Aa x Aa and aa x aa in addition to random mating (1 - α) is the random mating fraction and Remember that the three types of mating under positive assortment yield the following: AA x AA = 100% AA Aa x Aa = (1/4)AA + (1/)Aa + (1/4)aa aa x aa = 100% aa The frequencies of AA, Aa, and aa in the next generation ( 1 and 3 1 ) are as follows: ( 1 α ) p + ( ( 1/ 4) ) ' 1 = α Freq of AA under random mating Freq of AA under positive assortment has two sources: 100% from AAxAA,and 1/4 from AaxAa ( 1 α ) + α( ( 1/ ) ) = pq ' Freq of Aa under random mating Freq of Aa under positive assortment has one source: 1/ from AaxAa matings ( 1 α ) q + ( ( 1/ 4) ) ' 3 = α random mating component positive assortment component Note that when α > 1, the frequencies will no longer sum to 1. To get the population frequencies you need to standardize by the sum We can look at the effect on heterozygosity under this system of mating (plot A below). Remember that we have assumed a very simplistic scenario as heterozygotes can be distinguished from homozygotes, so no AA Aa mating will occur for this locus. The second plot below (B) shows the effect of dominance [formula not shown].

3 A. Effect of complete (α = 1) and partial (α = 0.75) positive assortative mating on heterozygosity B. Effect of positive assortative mating (α = 1) on heterozygosity under complete dominance Frequency of heterozygotes α = 0.75 α = 1.0 p = q = 0.5 Frequency of heterozygotes [Formula not shown] p = q = 0.5 α = dominance generation generation When there is no dominance effect the frequency of heterozygote genotypes in the population declines very rapidly. The frequency will rapidly go to zero when the effectiveness of positive assortment is 100% (α = 1). When positive assortment is not complete (e.g., α = 0.75) The population heterozygosity declines quickly until is reaches an equilibrium value (in the case above equilibrium is about 0.1). Dominance slows the rate to equilibrium considerably by the effect of dominance. In the example above the frequency of heterozygotes will eventually, for all practical purposes, go to zero; = after 198 generations. In example A above p = q = 0.5. Genotype frequencies at generation zero and after 0 generations are as follows. Genotype frequencies Generation AA Aa aa (α = 0.75) Check for yourself; before and after 0 generations p = q = 0.5

4 ositive assortative mating and the process of speciation We do not cover the topic of speciation in this course. However, it is worth noting the potentially important role that positive assortative mating could have in the process of speciation. There are many hypotheses about the causes of speciation. One hypothesized mechanism is called REINFORCEMENT. Under reinforcement, natural selection acts to favour the process of positive assortative mating. The idea is based on the premise that mating between individuals from populations that have diverged will result in offspring with reduced fitness; hence, selection will favour positive assortment because reproductive effort will not be wasted on producing less-fit hybrid offspring. Under these circumstances assortative mating could lead to increased reproductive isolation and eventually the final stages of the speciation process. The type of selection that would lead to reinforcement is called DISRUTIVE SELECTION; a selective pressure for divergence into two populations into ecologically distinct types. Until now I have relied on your intuitive sense of what a species is. A discussion of species concepts is well beyond the scope of this course, so for the purpose of our discussion we will assume a species is an independent evolutionary lineage, and we are interested in what role positive assortative mating might play in the process of splitting one interbreeding population into two independent evolutionary lineages. The consensus opinion is that the process of reinforcement is probably rare. However, recent examples, many coming from lakes filled with closely related fish species, is leading many biologists to think that reinforcement is worth a closer look. ied flycatcher colour polymorphism A good example of positive assortative mating and reinforcement of a species barrier was presented by Sætre et al. (1998) for species of flycatcher. The power of this example comes from studying mate choice in two populations (or close species) where the two ranges overlap (SYMATRY), and comparing the patterns with those occurring in regions where the two populations occur singly (ALLOATRY). The pied flycatcher (Ficedula hypoleuca) is typically black and white over much of its distribution. However, in parts of Central and Eastern Europe the pied flycatcher is sympatric with another species, the collared flycatcher (F. albicollis), and in the areas of sympatry, the males are a dull brown rather than black and whites. Allopatric type Sympatric type Adapted from Butlin and Tregenza 1998 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

5 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. Females 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. Mate preferences of female flycatchers Adapted from Sætre et al. (1998) It is likely that the genetic basis of colour polymorphism in ied flycatchers is more complicated than our single locus model above. Moreover the flycatcher case involves interbreeding between two populations and our model is based on a single random mating population. Nevertheless our model does provide some sense of positive assortative mating for change in genotype frequencies in a population, and the ied flycatcher provides an example of the relevance of such non-random mating systems to evolution. 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 gametic phase disequilibrium 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

6 Negative assortative mating NEGATIVE ASSORTATIVE MATING is the avoidance of mating with phenotypically similar individuals. This is also called DISASSORTATIVE MATING. Of course there will be a certain frequency of mating among dissimilar individuals in a random mating population; so we are interested in the cases where the frequency of disassortment is greater than expectations under random mating. Negative assortative mating is an important breeding pattern in plants. In fact plants have evolved a diverse collection of assortative systems. Systems of self incompatibility mechanisms can be divided into two categories: (i) gametophytic and (ii) sporophytic. GAMETOHYTIC incompatibility derives from failure of pollen with an allele in common with the maternal plant to fertilize that plant; i.e., it is based on allelic incompatibility. SOROHYTIC incompatibility works at the level of the genotype; diploid plants must have different genotypes for successful fertilization. Drosophila show an unusual type of negative assortative mating called RARE MALE ADVANTAGE. Here, the female Drosophila prefers to mate with males with the rarest phenotypes. It should be obvious that negative assortative mating will result in an excess of heterozygotes. 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. Final note: Although assortative mating on its own does not affect allele frequencies, when it is combined with natural selection they can have a significant affect on the rate of change in the allele frequencies at the locus subject to assortative mating.