Investigations from last time. Inbreeding and neutral evolution Genes, alleles and heterozygosity

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1 Investigations from last time. Heterozygous advantage: See what happens if you set initial allele frequency to or 0. What happens and why? Why are these scenario called unstable equilibria? Heterozygous disadvantage: What happens when you set the initial allele frequency to the value of t/(s+t). For example, using s aa = -0. = t, s ab = 0, s bb = -0.5 = s Why is this also an unstable equilibrium? Tutorial work due on Friday, 30 January by 4:30pm in Wolfson House office Inbreeding and neutral evolution k.dasmahapatra@ucl.ac.uk PRVIOUSLY: Deterministic evolution via selection TODAY: )Inbreeding: does not cause evolution by itself 2)Neutral evolution or genetic drift: a random process Inbreeding occurs if offspring are produced by parents who are related to each other. Genes, alleles and heterozygosity Inbreeding depression is the reduced fitness in offspring produced by incestuous matings. Individual A Individual B What causes inbreeding depression? Homozygous at gene A X P B Y P A Y P A Z F Homozygous at 4 genes Heterozygous at 5 genes D Heterozygous at 2 genes O N G G R S Q Q

2 What causes inbreeding depression Inbreeding results in increased homozygosity Unrelated parents AB CD What causes inbreeding depression? Inbreeding increases individual homozygosity Gametes A B C D Increased homozygosity reduces fitness through Offspring AC BC BD AA CA CD The degree of inbreeding of an individual is measured by its inbreeding coefficient, F. Inbreeding coefficient (F) = probability that two alleles in an individual are identical by descent F varies between 0 (non-inbred) to (completely inbred) The degree of inbreeding of an individual is measured by its inbreeding coefficient, F. Inbreeding coefficient (F) = probability that two alleles in an individual are identical by descent F varies between 0 (non-inbred) to (completely inbred) AC AA CA CD Alleles at a locus may be identical (by state), but this does not necessarily mean they are identical by descent. Selfing brothersister mating Alleles in this individual are identical by descent F= 2 2

3 Inbreeding in human populations Inbreeding in human populations Inbreeding in human populations Inbreeding coefficient (F) = probability that two alleles in an individual are identical by descent OR Decrease in probability of heterozygosity at loci compared to the average individual. Kind of incest brother-sister father-daughter uncle-niece first cousins F Only ~3% increase in risk above normal 3

4 Any advantages to inbreeding? Many organisms habitually inbreed Self-pollination in plants, esp. weeds Fig wasps Parasites ffect of inbreeding on populations Consider alleles, A, and a with freqs p,q and inbreeding (IBD) at rate F: Genotype AA Aa aa Frequency p 2 + Fpq 2pq( - F) q 2 + Fpq (For a derivation of this, see Chapter 6 in Freeman & Herron s volutionary Analysis) Genetic drift Genetic drift: the statistical drift of allele frequencies between generations due to random sampling effects deterministic vs. stochastic Conclusions from simulations Genetic drift is random ffects of drift stronger in smaller populations Probability of fixation equals the initial allele frequency Drift reduces population heterozygosity Drift can modify effects of selection 4

5 Loss of heterozygosity by drift Ht = H0 H t = heterozygosity after t generations H 0 = initial heterozygosity N = effective population size 2N N usually much smaller than actual population size t XAMPL OF GNTIC DRIFT Asian bramble (Rubus alceifolius), introduced on Indian Ocean islands. Genetic variation studied by means of DNA fingerprint technique: "Amplified Fragment Length Polymorphisms" - AFLP for short. fluctuating population size skewed sex ratio reproductive skew age structure Réunion Vietnam Native range (Vietnam, right), versus an introduced population (the island of Réunion, left) (from L. Amsellem et al Mol. col. 9: Northern elephant seal No variation at 43 allozyme loci Only 2 mtdna haplotypes (23 haplotypes in SS) ffective population size of 30 individuals for 20 years Take home points Inbreeding coefficient, F, measures how inbred an individual is. Measures proportional loss of heterozygosity compared to uninbred individuals. Inbreeding usually reduces fitness by unmasking deleterious recessive alleles. F also measures heterozygote deficit in a population resulting from inbreeding. Genetic drift is the random fluctuation in allele frequencies due to sampling. Drift is faster in smaller populations. Drift also reduces population heterozygosity at a rate of: Ht = H0 2N t 5

6 Things to do Paul & Spencer (2008) Its ok, we re not cousins by blood: the cousin marriage controversy in historical perspective. PLOS Biology: 6: Futuyma (2005) volution. Chapter 9 (97-99, ) and Chapter 0. Freeman & Herron (2004) volutionary Analysis. Chapter 6 (sections on Genetic drift and Nonrandom mating). Play with genetic drift simulation: 6