Evolution Lecture 10: Population Genetics:Mutation, Migration and Drift

1. Evolution Lecture 10: Population Genetics:Mutation, Migration and Drift

2. Mutation Mutation adds variation to population It is, by itself not a powerful force Imagine we have the following freq:a=0.9 and A=0.1 Say, A is mutated to a at a rate of 1 copy/10,000 generations Back mutations rarely happen Observe

4. Mutation and rate of change? Allele frequency change occurs slowly!! Not a big deal by itself! This example is at a quick rate of mutation

5. Over long periods, mutation can change allele frequencies

6. Mutation and selection Mutation is a potent evolutionary force when tied to selection Lenski (1994) took 12 populations of cells and grew them on nutrient poor media (selective environment). He then took subsamples of each population daily for 1500 days and grew them in fresh media for 10,000 generations Samples were frozen (still living) at regular intervals. This was so relative fitness of ancestors and descendants could be compared. He also measured cell size. Individuals grown in harsh environments produced mutations that allowed it to reproduce quicker The time from the appearance of a mutation to the fixation of that was so quick we can almost not see it on a graph

8. Mutation-selection balance The rate at which deleterious alleles are being eliminated is equal to the rate at which new copies are made q=sqrt(u/s), where q is the equilibrium frequency, u is the mutation rate and s is the selection coefficient. Ranges from 0-1. This tells us the degree of selection against the mutation. If selection coefficient is small and and mutation rate is high, then the equilibrium frequency of that allele will be high.

9. Mutation-Selection Example Spinal muscular atrophy is a neurodegenerative disease and is caused by deletions in the gene telSMN on chromosome 5. Second most common autosomal recessive allele It has a freq of 0.01 in Caucasian population and has a selection coefficient of 0.9. You would expect this allele to become extinct, however, it occurs at 1/100 If we substitute allele freq. for q and selection coefficient for s and solve for u, we get a number that is 9.0 x 10-5 mutations per telSMN allele per generation When we examine 340 individuals, it was found that 7 of the parents did not have this mutation (brand new mutation) This rate is 1.1 x 10-4 Very close to estimate

10. Is Cystic fibrosis maintained by mutation-selection balance? Most common genetic disease LOF of CFTR gene. This is a cell surface protein that is expressed in the lungs and prevents bacterial (Pseudomonas) infection People of European ancestry seem to have this at a frequency of 0.02. Using the equation in Box 5.10, we find that the mutation rate creating the new allele would have to be very high (4 x 10-4) with a selection coefficient of 1.0 to maintain an allele frequency of 0.02. However, the real mutation rate is 6.7 x 10-7. Therefore, the frequency at 0.02 cannot be maintained by a steady supply of mutations Is it possible, the allele is being maintained by overdominance…heterozygote superiority?

11. Pseudomonas

12. Cystic fibrosis and het. superiority? It is possible that heterozygous individuals cystic fibrosis are resistant to typhoid fever? The CFTR protein is also found in the gut. Typhoid bacteria (Salmonella) exploit this protein to cross the gut and increase infection If you look at normal CFTR in homozyg., het., and homoz. with loss of both CFTR (F508) copies..you see

14. Selection for cyst fibrosis gene after a typhoid outbreak

15. Migration The movement of alleles between populations Migration can be caused by anything that moves alleles. Dispersal of animals, pollen on the wind etc.

16. Amounts of gene flow?

17. Migration can obviously change allele frequencies!!

18. Migration as a mechanisms of evolution Water snakes (Nerodia sipedon) in Lake eerie come in two color phases: banded and unbanded This is a two allele system Banded dominant to unbanded The mainland has really only banded The islands may have both

21. Lake Erie water snakes It was found that, when basking on islands, the unbanded snakes are more cryptic and thus remain hidden better. Why wouldn’t selection cause the unbanded pattern to go to fixation? Migration. Every year, banded snakes migrate from the mainland and introduce fresh banded alleles Therefore, migration offsets selection

23. Unopposed migration Migration may be opposed by selection If not, migration tends to homogenize populations If gene flow from the mainland to the island was not opposed by selection, than the island would be homogenized by banded color patterns Fst statistics predict the amount of allelic variation from 0-1. High numbers indicate high variation

24. Age of flower populations and diversity

25. Next time, genetic drift!!!