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Do humans still evolve? • Evidence for recent evolution in Homo sapiens • Factors that influence rates of evolution in Homo sapiens • Cultural evolution Mechanisms of Non-adaptive Evolution • Mutation Alteration of DNA during or after synthesis • Genetic Drift Loss of alleles due to random processes • Natural Selection Differential reproduction of heritable phenotypes Recent Evolution of Homo sapiens • Genetic evidence • Phenotypic evidence Genetic Legacy of Genghis Khan ● Many asian men share the same Y-chromosome ● The highest frequency occurs in Mongolia Why did human skin become so diverse? 1) Losing hair solved problem of overheating on warming savannas, but raised a new problem: exposure or UV radiation. 2) Darker skin, caused by production of melanin, prevents UV from reaching sensitive cells. UV radiation is a mixed blessing. Positive: -Vitamin D synthesis (this vitamin tells the gut to absorb calcium; indispensable for skeletal and immune systems) Negative: -Skin cancer (light-skinned people have 10x the risk of dying of skin cancer under equal solar radiation). -Causes breakdown of folic acid (a chemical needed for DNA synthesis). Jablonski & Chaplin 2000. The evolution of skin coloration. J. Human Evolution 39: 57-106 Zone 1 Zone 2 Zone 3 [Source: Jablonski & Chaplin 2000, J. Human Evolution 39: 57-106] Less UV Zone 1 Lighter skin Skin reflectance = Mean UV (-0.11) + 73 Zone 2 Zone 3 [Source: Jablonski & Chaplin 2000, J. Human Evolution 39: 57-106] Repeated Evolution of Light Skin in Northern Climates • Genes that cause light skin in European and Asian people are different • Convergent evolution of skin color occurred after emigration of humans from Africa. • Suggests light skin arose via natural selection and not genetic drift. [Source: Norton et al. 2006, Genetic Evidence for the Convergent Evolution of Light Skin in Europeans and East Asians. Molecular Biology & Evolution] Genetic Drift of Facial Features Australopithecus Homo [Source: Ackerman & Cheverud 2004, PNAS 101: 17946-17951] About 90% of current variation in head shape exists within racial groups (the opposite of skin color). Evolution of Lactose Tolerance Ancient cultures shifted to a pastural lifestyle. E. Africa E. Africa W. Africa China Thailand Finland Sweden % of Population Lactose Digesters in Human Populations 100 Herders 80 60 40 Farmers 20 0 Soft selection Fail to reproduce Reproduce successfully Hard selection Fail to reproduce Reproduce successfully Phenotypic value Relative cutoff Absolute cutoff Time Phenotypic value Cultural Evolution Cultural evolution is the change of a society and its values over time. Humans have natural social tendencies, but social behaviors can also be transmitted through nongenetic mechanisms. In humans, many phenotypes are maintained through learning in a social environment. “Homo sapiens is an ordinary species of animal except that a very large portion of the individual’s program is carried by the environment, an environment rich in conspecifics.” Dan Janzen University of Pennsylvania Humans have an innate ability to add. Humans have an innate ability to subtract. Our innate ability is pretty limited. Learned Concepts of Mathematics • • • • • • Zero Infinity Irrational numbers (e.g., Pi) Imaginary numbers Logarithms Calculus Other social species also transfer behavior through learning. Do humans still evolve? • Strong evidence exists for recent evolution in Homo sapiens. • Evolution by natural selection has slowed for most traits; genetic drift could be more important now. • Cultural evolution shapes human behavior tremendously.