2-7- Forces of evolutiono Account for variability we see in the frequency of alleles 1. Mutation – change in DNA- Change in coding for proteino Example: sickle-cell anemia- Source of new genetic material 2. Gene Flow – Exchange of genetic material between populations- Example: island population has high O blood typeo After a small mix with another population, the frequency could drop dramatically 3. Genetic Drift - Small population size- 2 kinds:o Population bottleneck Some force of nature wipes out a good portion of the population- On one island, 1 in 12 people are colorblind- Hurricane killed most of population- King lived and was totally colorblind- Breeding ensues- Everyone is colorblindo Founder’s Effect Small group breaks off into new population- Reduced variation in population- Change in allele is from low population instead of large change 4. Recombination- Variation from sex cells 5. Natural Selection- Change in allele as the result of differential reproductive fitness- Allele confers an advantageo If an allele is in appreciable frequency (>1%) we look for a cause In Africa, >20% of the population have sickle-cell anemia- Map of sickle-cell frequency overlaps clearly with malaria- Falciparam malaria is carried by mosquitos- Plasmodium (that could be malaria) enters RBCso Multiplies using RBCs as hostso Continues acting on cells- RBCs have hemoglobin- HbA and HbS alleles are co-dominanto If you have one of each, enough of your hemoglobin is okay for sickle-cell not to occur- Deleterious allele (HbS) goes up in frequency as a result of another force(malaria)- Balanced polymorphismo Multiple alleleso Heterozygotic advantageo Polymorphism Most of our genes are constant (non-polymorphic) 28% have alleles- Type O people o Resistance to salmonella and shigellao Preferred by mosquitos- This can account for geographic differences Type A- Low frequency in smallpox areaso Smallpox uses molecular mimicry to trick cells Looks like A antogen so the body doesn’t pick up on its presence Lactose (in)tolerance- Chromosome #2- Autosomal dominanto Lactose, sugar in breast milko Lactase – enzyme that breaks it down from lactatingo Most mammalian species turn off lactase production around weaning
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