EvoluTion And SoCiETy 589
Health and medicine
The direct and indirect applications of evolutionary biology are probably more
numerous, and more important, in medicine and public health than in any other
area [5]. Depending on the topic, evolutionary theory may provide new concep-
tual approaches to medically relevant research (e.g., the evolution of senescence),
principles that medical research and practice should take into account (e.g., natural
selection for drug resistance in pathogens), or methods for making inferences and
discoveries (e.g., phylogenetic methods for tracing the spread of pathogens). Some
education in evolutionary biology is essential for every medical researcher, because it
bears directly on almost every field of biological study, and is useful for every clini-
cal practitioner, because it deepens one’s understanding both of the human body
and its ills and of the organisms that cause harm [80]. Evolutionary Medicine, by S.
C. Stearns and R. Medzhitov, is the best introduction to this large subject [108].
As you approach this topic, bear in mind that owing to genetic differences (as
well as epigenetic and direct environmental effects), people vary in everything
from susceptibility to inherited or pathogenic diseases to their reactions to drugs
and other therapies. Moreover, many characteristics show genotype × environment
interactions (see Chapter 15); for example, genetic variation in N-acetyl transfer-
ases (enzymes that break down some environmental toxins) affects the risk of
developing cancer from smoking. Every physician should be aware that a particular
therapy may have to be modified, or may be unsuitable, for some patients.
The term “disease” includes many kinds of ills. Below we mention (1) evolution-
ary legacies that characterize the human species; (2) mismatches between mod-
ern environments and those that prevailed during most of human evolution; (3)
genetic diseases, caused by mutations—including both the ones we inherit and the
ones that arise within our cells (somatic mutations); and (4) interactions with other
organisms, including both our symbiotic microbiome and pathogens and parasites.EvoluTionARy lEGACiES Like almost all other eukaryotes, we have evolved
a limited life span and functional breakdown as we age (senescence). The theory
of life history evolution explains senescence mostly as the result of antagonistic
pleiotropy: genes that enhance fitness in younger age classes but reduce function
in older individuals (see Chapter 11). The theory predicts, and evidence shows,
that many genes, affecting many functions, have this effect, so there is no single
cause of senescence and ultimate death.
Many of our adaptations can go awry due to various stresses. For example,
there is evidence that fever is an adaptation to suppress or kill pathogens, but
extreme fever is dangerous; the same holds for allergic reactions such as sneezing
and inflammation. The sensation of pain is a necessary adaptation for withdraw-
ing from certain dangers, but it can become intolerable. Obesity, type II diabetes,
and addictive behaviors represent normal, necessary functions carried to abnor-
mal, harmful extremes. And our lives are subject to inherent genetic conflicts
(see Chapter 12). The level of expression of paternal alleles, which enhance fetal
extraction of nutrients through the placenta, is opposed by maternal alleles that
are selected to prevent the fetus from extracting too many nutrients and lowering
the mother’s fitness. Imbalance between the expression of these genes can result in
abnormalities, including Beckwith-Wiedemann and Prader-Willi syndromes.MiSMATCH WiTH ModERn EnviRonMEnTS Humans today inhabit environ-
ments that they have largely constructed. As we described in Chapter 21, the
agricultural revolution created conditions to which the human body was not
adapted, and it still isn’t. Agriculture resulted in dietary changes that caused
nutritional disorders and tooth decay; it increased exposure to new infectious22_EVOL4E_CH22.indd 589 3/22/17 1:49 PM