Week1 LECTURE1 Nesse 2010(1)

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Unformatted text preview: vironments. The role of natural selection in shaping living organisms has been empirically conrmed beyond dispute. Selection is by no means the only factor, however. Mutations are inevitable; DNA is damaged by radiation and toxins, and replication is not perfect. Other random events are also important; genetic drift can push neutral or even deleterious alleles to high frequency, whereas a storm might eliminate all individuals with a useful mutation. Population bottlenecks, inbreeding, and migrations also shape gene frequencies, which in turn in uence the distribution of phenotypes. Natural selection and these other evolutionary mechanisms change species, and, equally important, keep them the same via stabilizing selection that disfavors individuals with extreme traits (2, 3). These core principles are, however, only the roots of a rapidly growing network of explanations based on evolution. One main branch is phylogeny. Long-established methods for analyzing relationships within and among species are now being augmented by new methods that use molecular genetic data to test hypotheses about the relationships among populations and species and about the large-scale history of life itself (4). The other main branch is the study of adaptation. The unity of all life was only one of Darwin's greatest discoveries; the other was his explanation for why organisms have traits that are so well adapted to the challenges they face. No plan is involved; natural selection tends to increase the frequencies of alleles of individuals that survive and reproduce better than others in speci c environments (5). Sewall Wright (6) envisioned this process as a landscape of hills and valleys, where the hills represent peaks This paper results from the Arthur M. Sackler Colloquium of the National Academy of Sciences, “Evolution in Health and Medicine” held April 2–3, 2009, at the National Academy of Sciences in Washington, DC. The complete program and audio les of most presentations are available on the NAS web site at www.nasonline.org/Sackler_Evolution_Health_Medicine. Author contributions: R.M.N., C.T.B., P.T.E., D.R.G., D.N., R.L.P., M.G.T., S.C.S., and D.V. designed research; R.M.N., C.T.B., P.T.E., P.G., D.R.G., D.N., G.S.O., R.L.P., M.D.S., M.G.T., S.C.S., and D.V. performed research; R.M.N. analyzed data; and R.M.N., C.T.B., P.T.E., J.S.F., P.G., D.R.G., D.N., G.S.O., R.L.P., M.D.S., M.G.T., S.C.S., and D.V. wrote the paper. The authors declare no con ict of interest. This article is a PNAS Direct Submission. 1 To whom correspondence should be addressed. E-mail: [email protected] www.pnas.org/cgi/doi/10.1073/pnas.0906224106 of tness and the valleys regions of reduced tness. Selection tends to move traits up nearby slopes toward tness hilltops, but nearby higher peaks can be dif cult to reach because the transition requires moving through “valleys” of decreased tness (6). Tinbergen (7) and Mayr (8) provided an important clari cation of the difference between proximate questions about mechanisms and evolutionary questions about origins and functions. They helped biologists recognize that every trait of every organism needs two separate and complementary kinds of explanation, proximate explanations of how mechanisms work, and evolutionary explanations (sometimes called “ultimate explanations”) about how they got to be the way they are. For instance, the proximate explanation of the adrenal gland includes its anatomy, tissues, chemical constituents, and the developmental processes that assemble them. Separate, and equally important, is an evolutionary explanation: the phylogeny of the adrenal gland and how it has conferred a selective advantage. Notice that each kind of question has two subquestions. A complete biological explanation requires answers to what are now known as Tinbergen’s four questions: What is the mechanism? How did the mechanism develop? How has it given a selective advantage? What is its phylogeny? Many advances in evolutionary biology have emerged from asking evolutionary questions about traits important to medicine and public health, and the answers provide advances for medicine; the bene ts ow in both directions. Rates of aging are heritable, so why has not selection eliminated or at least greatly slowed aging? The strength of selection is weaker at older ages, so deleterious mutations can accumulate, and genes that give advantages in youth will be selected for even if they have pleiotropic deleterious effects later in life (9, 10). Populations with mostly females can have many more offspring than those with an equal sex ratio, so why are not sex ratios more often female biased? Because parents maximize their reproductive success by making offspring of whichever sex is less common, notwithstanding the penalty to group success, as R. A. Fisher (11) recognized long ago. Why is reproduction sexual at all, given that nonsexual reproduction is twice as productive? This is a fascinating problem, only partly solved; most proposed solutions attribute it to the advantages of having genetica...
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