Encyclopedia of Evolution.pdf - Online Reading Center

natural selection
Fact 1. Populations can grow exponentially. Because
reproductive groups of individuals can multiply, populations
grow in curves, not in lines (see population). In a population
of bacteria, one bacterium can produce two, but one
million bacteria can produce two million bacteria during cell
division. This allows populations to grow explosively. This
will occur, of course, only in an unlimited environment. As
Darwin wrote:
There is no exception to the rule that every organic
being naturally increases at so high a rate, that, if
not destroyed, the earth would be covered by the
progeny of a single pair. Even slow-breeding man
has doubled in twenty-five years, and at this rate,
in less than a thousand years, there would literally
not be standing-room for his progeny. Linnaeus has
calculated that if an annual plant produced only
two seeds—and there is no plant so unproductive
as this—and their seedlings next year produced two,
and so on, then in twenty years there should be a
million plants.
Darwin did some calculations for the Origin of Species (incorrectly,
as it turned out) on the reproductive rate and population
growth of elephants in an unlimited environment:
The elephant is reckoned the slowest breeder of all
known animals, and I have taken some pains to estimate
its probable minimum rate of natural increase;
it will be safest to assume that it begins breeding
when thirty years old, and goes on breeding till
ninety years old, bringing forth six young in the
interval, and surviving till one hundred years old; if
this be so, after a period of from 740 to 750 years
there would be nearly nineteen million elephants
alive, descended from the first pair.
Fact 2. Populations sometimes grow, and sometimes
become smaller, but tend to remain stable. Therefore something
must be preventing population growth, either by
repressing reproduction or by causing death. Darwin wrote:
Lighten any check, mitigate the destruction ever
so little, and the number of the species will almost
instantaneously increase to any amount.
Darwin insisted on evidence, and on experimentation
where possible. He dug and cleared a small plot of ground
at his estate and laboriously marked each seedling that germinated.
The total was 357. Of these, 295 seedlings died,
mainly because slugs and insects ate them. He also counted
the number of bird nests on his property in the spring of
1854 and again in the spring of 1855 and estimated that
during the intervening winter nearly four-fifths of the birds
had died.
Fact 3. Resources are limited. Ultimately, if reproduction
continued unabated, “the world could not contain” the offspring,
as Darwin wrote. However, resources are limited by
much more than simply room in the world. Organisms need
resources and ecosystem services that provide food, water,
shelter, and oxygen, and process their wastes. Each kind of
organism is adapted to live in only a limited range of conditions;
therefore, for a polar bear, it does not much matter
how big the world is, but only how big the arctic is, and
for species that are limited to a single island or mountain or
swamp, the world is pretty small. The disparity between limited
resources and exponential population growth in human
societies had been famously pointed out by Malthus (see
Malthus, Thomas), and Darwin simply applied it to nature:
“It is the doctrine of Malthus applied with manifold force to
the whole animal and vegetable kingdoms …”
These three facts lead to Inference 1, which is the struggle
for existence. In nearly all natural populations, there are
not enough resources for all of the individuals to have all of
the resources they need. Today scientists say that competition
occurs in nearly all natural populations nearly all of the
time. Sometimes competition is strong enough that scientists
can actually observe the struggle. In a jungle, vines grow
over trees, and trees crowd together, as if they are fighting
for access to the light. In a desert, the bushes are evenly
spaced across the landscape, because there is not enough
water for any bush to grow close to another bush. Predators
fight over a kill, and birds chase one another out of the
choice territories. But even when it cannot be seen, competition
can be inferred. Competition is the unavoidable result
of exponential population growth and limited resources. As
Darwin wrote:
Nothing is easier than to admit in words the truth of
the universal struggle for life, or more difficult—at
least I have found it so—than constantly to bear this
conclusion in mind … We behold the face of nature
bright with gladness, we often see superabundance
of food; we do not see or we forget, that the birds
that are idly singing round us live on insects or
seeds, and are thus constantly destroying life; or we
forget how largely these songsters, or their eggs, or
their nestlings, are destroyed by birds or beasts of
prey; we do not always bear in mind, that, though
food may now be superabundant, it is not so at all
seasons of each recurring year.
Darwin’s theory of evolution by means of natural selection
severely shook the complacency of European science,
which had comfortably assumed the divine creation and
long-term stability of species. His invention of ecology had
at least as great an affect on scientific and popular thought.
Many people thought of the natural world as God’s vast and
orderly garden, full of life. Darwin showed the natural world
to be violent and full of death. To his readers, Darwin had
shaken their view of the world—and they had not even gotten
past Chapter 3 (The Struggle for Existence) of the Origin!
The poet Alfred, Lord Tennyson, reflected the sadness not
only of the death of individuals but the extinction of species
in his poem In Memoriam, which is the origin of the famous
phrase “red in tooth and claw.” (Tennyson published the