Encyclopedia of Evolution.pdf - Online Reading Center

evolutionary medicine
variable in the binding region of the H protein are the ones
that the investigators predict will be next year’s big flu, and
these are the strains against which the flu vaccine is produced.
This is a clear application of evolution to medicine.
Evolutionary analyses, such as cladistics, have helped
researchers to understand the origin of the strains of diseases
such as influenza. If the phylogeny of influenza viruses is constructed
using the nucleoprotein (not the same as neuraminidase),
the branches clearly correspond to those of the major
hosts of the virus (horses; humans; swine; ducks and fowl). If
the phylogeny of the influenza viruses is constructed using the
H protein, each major branch is a mixture of viruses of different
hosts, except for the horse influenza viruses, which have
remained distinct. The meaning of this result is that the influenza
viruses exchange genes for the H protein (see horizontal
gene transfer). In particular, human viruses and pig viruses
can both infect ducks, and while inside the ducks the viruses
exchange segments of nucleic acid that code for H genes, but
not those that code for nucleoprotein genes. The cladistic
reconstruction of the evolution of influenza viruses has allowed
public health investigators to discover where new strains of
influenza come from: from regions of high human population
density where people are in contact with both pigs and ducks.
This is the reason that most new strains of influenza evolve
in China. This evolutionary insight has allowed public health
researchers to know where to look for new strains of influenza.
This is another clear application of evolution to medicine.
The major role of evolution in pathogens has been the
evolution of bacteria that resist antibiotics (see resistance,
evolution of). Without an evolutionary viewpoint, physicians
thought that the best thing to do was to use antibiotics
as much as possible. The result of the overuse of antibiotics
was the evolution of bacteria that resisted them. Most physicians
and public health researchers now understand the
lesson of evolution: Antibiotics must be used only when necessary,
to prevent the evolution of resistant strains. This is yet
another clear application of evolution to medicine.
Recently, zoologist Bryan Grenfell and colleagues compared
the phylogenies and the infection (epidemiological)
patterns of pathogens. They found a correspondence between
the patterns by which pathogens evolve and the patterns by
which they spread:
• Some pathogens cause brief infections but there is crossimmunity
between the strains. For example, infection
by one strain of measles makes the host immune to other
strains as well. In such pathogens, many strains can coexist
in the host population.
• Some pathogens cause brief infections but there is little
cross-immunity between the strains. For example, infection
by one strain of influenza does not make the host immune
to other strains. In such pathogens, there is a rapid turnover
of strains, with few strains coexisting at any one time.
• Some pathogens cause the host immune system to respond
in a way that weakens the host such that the host can be
infected by other strains. For example, infection by one
strain of dengue fever puts the host at greater risk of infection
by another strain. In such pathogens, a small number
of major strains coexist.
• Some pathogens such as HIV cause a persistent infection.
In such pathogens, many strains evolve within the individual
host (see AIDS, evolution of).
Therefore evolution is essential not only to the practice of
medicine against infectious diseases, but also to the explanation
of infection patterns.
Pathogens and hosts coevolve (see coevolution). A
pathogen can be successful either by spreading rapidly from
one host to another, or by letting the host live. In the first
case, particularly common in diseases such as cholera that
are spread without direct personal contact, pathogens remain
virulent. In the second case, particularly common in diseases
such as smallpox that spread directly from one person to
another, pathogens evolve into milder forms. A host is most
successful when it resists the pathogen. Evolutionary biologist
Paul Ewald pointed out that humans can tip the competitive
balance away from virulent diseases by restricting the transmission
of virulent strains, for example through the control
of water pollution. He called this the “domestication” of the
pathogens.
Relationship between fetus and mother. The placenta
allows the mother to nourish the fetus (see mammals, evolution
of). The placenta allows food and oxygen from the
mother’s blood to enter the fetus, and wastes such as carbon
dioxide from the fetus to enter the mother’s blood, without
a direct contact of the blood. Direct contact would allow the
mother’s immune system to attack the parasitic fetus. In Rhesus
(Rh) factor incompatibility, this happens anyway.
Some aspects of pregnancy are not fully explained by the
assumption of a complete harmony between the interests of the
mother and of the fetus. The placenta (which contains much
tissue of embryonic rather than maternal origin) produces
Igf2 (insulin-like growth factor) that raises the blood pressure
and blood sugar levels of the mother. This benefits the fetus,
but may endanger the health of the mother, in particular her
ability to have further offspring. Having more children will
enhance the evolutionary fitness of the mother but may not be
in the interests of the fetus. This has also been interpreted as
a conflict of interest between mother and father, in which the
father’s fitness is promoted by the mother providing so many
resources to the fetus that the mother’s own future reproduction
or even survival is impaired (see selfish genetic elements).
Although the paternal alleles may work against the
interests of the mother, they do not usually produce immediate
harm upon her. In the case of gestational trophoblast disease,
however, this is not the case. The enhanced expression of some
paternal alleles in the embryo causes the embryo to become
cancerous; this can endanger the mother’s life.
Most of the time, the relationship between fetus and
mother is mutualistic rather than a conflict of interest. But
even here, some symptoms that are often interpreted as disease
may actually arise from evolution. Morning sickness
can be interpreted as an excessive aversion to any foods that
could possibly be spoiled or toxic—a response that benefits
both the fetus and the mother.
The immune system. Allergies are also called hypersensitivities
because they occur when the immune system (espe