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Bio Final Lecture 12 Notes.docx

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Lecture 12: Human-Agricultural Coevolution  Summary  Coevolution: That reciprocal evolutionary response between species as a consequence of natural selection having been imposed on each other, by each other  One example: wolves and moose  Wolves prey on moose and impose selection, and the subsequent generation of moose due to the wolves’ selection, which selections for wolves with certain attribute.  It can also happen within a species between the sexes  One sex can impose changes in the other  sexual dimorphism and sexual antagonism  How do humans fit in?  Humans have imposed a selection on cows, is it reciprocal?  Natural Selection: Interaction between ecology and genetic  The ecology is what generates natural selection  The genetics provides the materials to be passed throughout the generations  BUT culture comes into play as well  Cattle Pastoralism  Unique to humans  Milk and Humans  Humans need milk, especially breast milk at infancy  The baby drinks Lactose, and needs Lactase into Glucose and Glactose  Lactase production normally declines in adulthood, with the exception of people of northern European origin  This decline leads to “lactose intolerance”  Lactose intolerance is the “Wild Type”  History  Origins in Europe, lactase production persists throughout adulthood  Allows digestion of milk  Has this had any effect on the cattle?  Hypotheses:  The substitutions are the causal variants (happens to be linked but not causal)  They were favoured by natural selection (were the adaptations favoured?)  Predictions  If substitutions in the lactase genes causes lactase persistence, then we can predict:  If other human populations with lactase persistence should have the same substitutions, or changes with similar consequences  But if it was the dairy agriculture that imposed evolution by natural selection on substitutions that cause lactase persistence, then we can predict:  Other human populations practicing dairy agriculture should have lactase persistent phenotypes  We should see genetic evidence of past selection at the lactase gene  Expanded Sampling of Lactase persistence  Many African cultures practice dairy farming  Do they show lactose tolerance?  Do adults have lactase persistence?  Results  Lactase Digestion  African cultures with dairy farming also have distinctive forms of the lactase gene  DNA sequences differences correlate significantly with the ability to digest lactose  Conclusions:  Strength inference that these molecular variants actually affect the phenotype  Independent evolution in different population supports driving force of natural selection  Chance vs. Determinism  Single events can be due to chance alone or to deterministic processes  Example: A single coin flip: could be a simple probability or could be a biased coin  Independent lines of evidence can confirm deterministic explanations by rejecting chance  e.g., a chance process predicts:  Heads 2x in a row: 0.5 * 0.5 = 0.25 = 25%  Heads 3x in a row: 0.5 * 0.5 * 0.5 = 0.125 = 12.5%  Heads 4x in a row: 0.5*0.5*0.5*0.5 = 0.0625 = 6.25%  Convergent Evolution  Independent evolution of the same trait in different groups  Groups can refer to populations or species [ex: bats and birds]  Allows more confident determination that selection is at work  Does convergent evolution use the same or different genes?  In this case, it appears to use the same genes  Summary: Lactase persistence evolution  Studies of Neolithic human remains (10,000 years BCE) [at the onset and before agriculture] suggest that they were unable to digest lactose as adults  Cattle domestication did not occur until approximately 7,500 to 9,000 BCE  Studies in African cultures also show mutations in lactase associated with lactase persistence into adulthood  Strong evidence of replicated association between genotype and phenotype  Independent derivation supports milk digestion as an example of convergent evolution toward adult digestion of milk  Predictions we haven’t touched upon yet  We should see some genetic evidence of this historical, natural selection affecting this Particular part of our genome  Can we detect an imprint of such coevolution in our genomes?  Turns out we can try to do that by applying the principles of population genetics :  To determine how natural selection can affect the genomes of an organism, we can:  We should see differences in the fate of those mutations that change amino acids (replacement = non-synonymous) versus those that do not (synonomous)  Lower genetic variability in regions of the genome that experience selection  More differentiation between populations at selected sites than the ‘rest of the genome’  Changes to the peptide sequence can alter the protein which can alter the fitness affects of the organism  Neutral: don’t affect fitness, expect to see similar properties in population  Detrimental: lower frequencies in the population of sequences that alter the amino acids, will be eliminated by natural selection  Beneficial: improve fitness, expect to observe more changes that alter the amino acid sequence  Predictions for DNA differences between species, rates of substitution  Dn = replacement Ds = synonymous  Neutral: Dn/Ds = 1 [no difference]  Detrimental: Dn/Ds <1 [fewer substitutions at replacement sites]  Beneficial: Dn/Ds >1 [more substitutions at replacement sites]  Effects of selective sweeps on nearby variation  Selective sweep  As beneficial mutations fix, they “drag” along with them
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