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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
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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