Q: A high proportion of the cats on Key West have extra toes (polydactyly). The most likely explanation is: a) b) c) d) High rate of mutation Founder effect Bottleneck effect Cats with extra toes are better at catching mice e) Extra toes are sexually appealing to female cats f) ? How will this population evolve in the future? Evolutionary Mechanisms Biological evolution: change in genetic composition of a population over time • How can the gene pool of a population be characterized quantitatively? • What happens to the gene pool of a sexually reproducing population over generations? • What mechanisms cause evolutionary change? – Model systems to study evolutionary mechanisms Quantifying genetic variation in sexually reproducing populations Only locus X is shown, with three alleles (X1, X2 , and X3 ) The gene pool is the sum of all alleles Genetic structure is the frequency of the different genotypes in the population. Fig. 21.03 Allele frequencies Cystic fibrosis is a recessive genetic disease. Among Northern Europeans, the incidence of CF is 1 per 2500 live births. Q1: What is the frequency of the CF allele in the Northern European population? Q2: What proportion of the population are carriers of the CF allele? Random Mating In Generation II, the allele frequencies are: p= q= For a population in equilibrium: F(AA) = F(Aa) = F(aa) = Q3 - equilibrium In both populations shown below, p = 0.6 and q = 0.4; which population(s) are in Hardy-Weinberg equilibrium? Population A 36 red (CRCR), 48 roan (CRCr), and 16 white (CrCr). a. Population A b. Population B Population B 32 red (CRCR), 56 roan (CRCr), 12 white (CrCr). c. Both A and B d. Neither A nor B Hardy-Weinberg (H-W) Equilibrium • Assumptions. • If the H-W assumptions are met, then allele frequencies will not change from one generation to the next. HIV infection • Is there genetic variation among HIV virus particles in an infected individual? • Is there significant mortality in the virus population of an infected individual? • Does genetic variation make a difference in survival and reproduction of HIV virus? HIV prevalence, 2009 http://wwwnc.cdc.gov/travel/yellowbook/2012/chapter-3-infectious-diseases-related-totravel/hiv-and-aids.htm Numbers of people living with HIV/AIDS WHO/UNAIDS HIV infection time course Q4: 3TC resistant viruses Patient No. 1 a. Patient No. 2 b. Patient No. 3 c. Arose by mutations induced by 3TC Arose from a small pool of mutant viruses already resistant to 3TC Arose by gradual adaptation of viruses to 3TC Weeks Figure 22.13 Evolution of Drug Resistance in HIV Campbell & Reece 7th ed. p. 448 Why do anti-HIV drugs become ineffective? • Structure of HIV reverse transcriptase & resistance mutations • Blue = AZT resistance • Lt. Blue = ddI, ddC, 3TC • Violet = both AZT + ddI Huang et al., 1998, Science 282:1669 Origin of Genetic Variation: Mutation • Point mutations • Insertions/Deletions • Inversions/Translocations Q6: How many times did SIV make the jump to human hosts to become HIV? a. Once b. Twice c. 3 times d. 4 times e. 5 or more Q7: What anti-HIV therapies are informed by the theory of natural selection? A. Multiple-drug cocktails B. Drug treatment immediately after exposure C. Stopping drug treatment when resistance emerges D. All of the above. E. None of the above. HIV infects T cells via CD4 and CCR5 cell surface receptors Frequency of CCR5-delta32 allele in different human populations • Northern Europe • Central Asia • Asia, Africa 10% 2% 0% Why is the CCR5-delta 32 allele so frequent among Northern Europeans? Propose at least two alternative hypotheses. What percentage of people in each region are expected to be resistant to HIV infection?