Lecture 13 Population Structure Nonrandom Mating Core Concepts 1 Breeding behavior and population structure can reduce effective population size 2 Nonrandom mating distorts Hardy Weinberg equilibrium but does not itself change allele frequency 3 Inbreeding usually reduces the fitness of offspring and numerous mechanisms have evolved to prevent it 4 Nonrandom mating usually increases the effect of genetic drift Last of the 5 violations of Hardy Weinberg Who you mate with is a function of your genotype This allows for changes of allele frequencies When not all individuals reproduce or there is a skewed sex ratio the effective number of individuals in the population from an evolutionary standpoint is less than the actual total population Ne number effective Nf number females Nm number males Ne 4 Nm x Nf Nm Nf Nm 10 Nf 100 Nm 100 Nf 100 Ne 4 100 x 100 Ne 4 10 x 100 100 100 Ne 200 10 100 Ne 36 Nm 1 Nf 100 Ne 4 1 x 100 1 100 Ne 4 Notice how small these populations actually are How do we get this type of breeding structures Population Structures Polyandry many males Each female has multiple male mates e g termites jacana Polygyny many females Single male has multiple female mates e g lions the harem groups more common in vertebrates Cooperative Mating Social raising of offspring give up some of their individual reproductive success to help family raise others offspring e g Florida Scrubjay Age Specific Fecundity Larger older females have more eggs e g Fish Unequal mating success Unequal fecundity By reducing the number of individuals in the effective population the effect of drift become larger Nonrandom mating When organisms are more or less likely to mate with others of their genotypes than by chance e g positive and negative assortative mating Who you mate with is a function of your genotype Either Like with like like with unlike or assortative mating Types of Assortative Mating Selfing The most extreme form of nonrandom mating The only one that does not change allele frequencies p q It does change genotype frequencies p2 q2 2pq LECTURE HANDOUT 13 5 25 Boxes The heterozygotes disappear and the population evolves into two Inbreeding The most common type F how strongly inbred a population is measure of inbreeding 0 at HW equilibrium random mating 0 5 complete selfing resulting in all homozygotes The probability that two alleles in a population are IBD Identical By Descent Genetic drift mimics the effects of long term inbreeding because it increases the probability of identity by descent LECTURE HANDOUT 13 Punnet Square 1st page Hf HO 1 F Populations that are inbreed will reduce heterozygosity POWERPOINT SLIDE Calculating F inbreeding coefficient Half sibling mating probability of IBD Multiplied bc depend on one another Grandmother has two alleles Alternatives are added Total 1 8 The consequence of inbreeding is that the IBD F becomes higher and higher as the same allele is repeated within the population and may reach 1 Inbreeding Drift We have seen how inbreeding affects drift but it can also work in the reserve direction LECTURE HANDOUT Circles front page Inbreeding reduces the population size affecting drift Drift in subdivided groups behaves as if the population is inbreed regardless of whether there was nonrandom mating in the population In small subpopulations small colonies of mice in a barn alleles can be identical by descent under random mating and thus larger population mimics the effects of inbreeding Thus F is also a measure of population subdivision and propensity to genetic drift Inbreeding temporarily increases phenotypic variation But over the long term decreases genetic variation by reducing effective population size making genetic drift stronger Ways to reduce Ne increase drift more powerful than natural selection Unequal mating success Population subdivision Inbreeding mimics drift and reduced population size Small population sizes drive high inbreeding coefficients Thus then the population drifts faster than it would normally Outcomes 1 Increases and then decreases phenotypic variants Vp Due to number of increased heterozygotes As you remove alleles variation decreases 2 Inbreeding Depression Deleterious alleles that are recessive are more common dominant would be eliminated by selection quickly The average fitness is lowered w bar Preventing Inbreeding sex biased groups male leaves Mate Choice not mating with relatives Dispersal leave natal group Self Incompatibility for hermaphroditic organisms if they try to self pollinate growth is prevented Dioecy separate organisms male and female groups Monoecy both male and female parts but they are separated to prevent self pollination flowering at different times or at different locations on the plants Asynchrony when male parts and female parts are not receptive released at the same time So why do it Reproductive Assurance you may not be able to find a mate Some offspring is better than no offspring regardless of inbreeding depression Saving energy on reproductive structures activities courtship dances acquiring territory ect Lecture 14 Multiple Loci Core Concepts 1 The mathematics of multiple loci is the same as single loci but with more combinations 2 When many loci and alleles affect a trait there will be hidden variation 3 There are so many possible combinations of alleles with multiple loci that variation becomes continuous and is best described statistically Lecture Topic Population Genetics Loci 1 This lecture Quantitative Genetics 1 Polygenic Variation when there is more than one locus multiple loci 1 loci Pleiotrophy when one locus affects more than one trait Locus Loci one spot on the genome that has a particular function Genotype Examples or Aa Bb AB ab A1A2 B1B1 Separating by locus separating by parent When a trait is determined by a small number of genes alleles the phenotypes fall into a small number of discrete categories With a large number of underlying genes the variation becomes continuous Hidden Variation combinations appear A small population even if all the alleles are there not all the possible genetic Has nothing to do with recessively you just don t have those genotypes Even if you have all the genotypes you will only see the 5 phenotypes The intermediate phenotypes have the most possible combinations The extreme phenotypes have the least possible combination Thus these are the most likely to be absent in the population Curve look like Bell curve a normal distribution As you move to more loci