Hello welcome aboard the HMS Beagle, where we explore foundational papers on
evolutionary biology.  Why do we age? When we look at the intricate evolutionary
mechanisms that guide the growth and
development of organisms, it's remarkable
that natural selection still hasn't
eliminated the process of aging to
simply maintain what has already been
formed. In evolutionary biology, fitness
refers to the number of offspring an
organism has in its lifetime. Natural
selection favors traits that increase
fitness. Early evolutionary biologists
investigating aging believed that it was
a mechanism designed by natural
selection in order to eliminate the
weak and worn out members of a population.
However, it wasn't clear how natural
selection could favor individuals that
age faster than others as it was obvious
that a long-lived individual would
simply have more opportunities to have
offspring than a short-lived one. So, in
the eyes of natural selection, aging
would obviously be an unfavorable trait.
But if aging isn't favored by evolution,
why is it so common? To answer this
question we'll look at the antagonistic
pleiotropy theory of aging introduced by
George C. Williams. In his 1957 paper,
Williams discusses two key concepts to
explain the evolution of aging. The first
concept is the reproductive probability
distribution. The idea behind this
distribution is quite simple. As an
individual approaches sexual maturity,
its probability of reproduction
increases, reaching a peak sometime after
full sexual maturity. However, as the
individual continues to age, the
cumulative probability of its death also
increases due to exposure to predation,
disease and other death factors. So even
in the absence of biological aging the
probability of reproduction begins to
decrease shortly after it reaches its
peak. Another way to look at the
reproductive probability distribution is
to draw the graph for the total
remaining probability of reproduction.
When an individual is born it's total
reproductive probability
is at a maximum. As the individual ages,
however, its remaining reproductive
probability decreases. In the end, our
curve for the total remaining
probability of reproduction looks kind
of like this, with a maximum at age zero.
This means that if a mutation has a
negative effect on fitness very early in
life,
this affects almost the entire curve.
Whereas if a mutation has a negative
effect on fitness later in life, this
only affects a portion of the curve. The
second concept Williams explains is
called pleiotropy. In short pleiotropy
is when one gene influences
multiple unrelated traits. Pleiotropy is
a universally recognized concept. In fact
most genes are pleiotropic. To explain
the evolution of aging Williams
introduces a special type of pleiotropy,
where a single gene has opposite effects
on fitness at different ages. An example
for this kind of pleiotropy would be
a gene that provides a benefit to the
organism at an early age but has a
detrimental effect on the organism later
in life. When there's a conflict of
interest between a benefit early in life
and a detrimental effect later in life,
natural selection favors a benefit early
in life, simply because it has a larger
effect on the organisms overall fitness.
The rate of aging of an organism
evolves through two opposing forces of
selection. The first force of selection
favors increased fitness at an early age
at the cost of faster aging. The opposing
force of selection acts to reduce the
rate of aging, since biological aging is
still an unfavorable trait.
Williams argues that the rate of aging
of any species depends on the delicate
balance between these two opposing
forces.
Simply put, biological aging is an
unfavorable side effect of otherwise
favourable genes that provide a benefit
to the organism early in life. Since
Williams' paper exciting observations
supporting his theory have been
made. For instance while estrogen is
critical for human reproduction it's
also associated with an increased
occurrence of breast cancer and
endometrial cancer. Another example is
the pacific salmon where if reproduction
is prevented in laboratory conditions,
lifespan may be more than doubled. That
was "Pleiotropy, Natural Selection and the Evolution of Senescence"
by George C. Williams published in December 1957 in the Journal, Evolution.
You can find the link
to the full paper in the description
below. Please reach out if you have any
questions, suggestions or corrections and
I hope you enjoyed learning more about
evolution today.
