Encyclopedia of Evolution.pdf - Online Reading Center

• Individuals within a population differ genetically (see population
genetics). Though in most cases all members of
a population have the same genes on their chromosomes,
individuals differ in the alleles for the genes that they possess
(see Mendelian genetics). Everyone is aware of
the astounding and obvious diversity within and between
human populations; and yet humans have less genetic
diversity than most other species. There is no species in
which all the genetic diversity, in all its populations, could
be contained within a single pair of individuals gathering
into an ark. This genetic diversity is the raw material upon
which natural selection and other evolutionary processes
work. Evolution, therefore, occurs within populations,
because populations contain genetic diversity.
Growth of Populations
Populations grow exponentially. In bacteria, one individual
divides to become two, each of these two divides again, producing
four. Each generation, the population size can double.
After 10 generations, there would be 2 10 (which is 1024)
bacteria (the 10 is the exponent). The graph of population
growth over time is therefore a curve, not a straight line. In
species that reproduce sexually, the calculations are more
complex but still follow an exponential pattern. The human
mind, accustomed to think in linear terms, can be surprised
by population growth. If a nation’s human population doubling
time is 20 years, then a simple linear calculation of the
resources that it will need 20 years from now, based on previous
years, will fall far short of what it will actually need for a
population that is twice as large as it is now.
Some populations grow faster than others:
• Populations with a high fertility rate grow faster than
those with a lower fertility rate. Fertility rate is the average
number of offspring per female. Populations of rabbits,
in which females typically have numerous offspring more
than once a season, grow faster than human populations.
In many poor countries, the fertility rate is much higher (in
some cases, up to eight children per family) than in richer
countries. A fertility rate of about two results in a population
in which births and deaths balance one another—a
population that remains the same size.
• Populations with a low mortality rate grow faster than those
with a higher infant mortality rate. In many populations,
most deaths occur among young individuals. In human
populations, the death of individuals during their first year
is called infant mortality. A plant may produce a thousand
seeds but only one may grow; the 99.9 percent mortality
rate offsets the high fertility rate. Although humans have a
lower fertility rate than most other species, they also have a
low mortality rate. Even the highest infant mortality rates
among human populations are less than 15 percent.
• Populations with a low age at first reproduction grow
faster than those with a higher age at first reproduction.
Rabbit populations grow faster than human populations
not only because rabbits have high fertility rates but also
because rabbits begin to reproduce during their first year
of life.
population
Exponential growth cannot continue indefinitely. Among
the factors that cause population growth to slow down, or
even stop, or cause populations to shrink, are:
Density-independent factors. These are factors that
would occur with about the same intensity regardless of how
large or dense the population is. Weather events (such as
floods or droughts) are usually density-independent.
Density-dependent factors. These factors occur with
greater intensity in populations that are larger or denser. These
include:
• Disease. Parasites can spread more rapidly when organisms
are crowded together with one another, with their wastes,
and with their trash.
• Competition. At low density, there is enough room
(and therefore enough resources) for all the individuals.
However, at high density, resources (such as light,
water, and soil space for plants, or food items for animals)
can become scarce for the average individual. As
a result of competition among individuals, some obtain
more resources than others. For example, in a population
of plants, there may be a few large ones, which produce
most of the seeds, and many small ones that do not
produce at all. In an animal population, a dominant male
may be responsible for most of the reproduction. In all
these cases, not all of the individuals will reproduce as
much as they could. Competition can take either subtle or
violent forms.
The carrying capacity of a population is the number
of individuals for which there are adequate resources
for survival and reproduction. In practice, it is usually
impossible to calculate the carrying capacity of a population,
because the individuals can adjust their resource use.
At high density, plants adjust their growth and animals
adjust their behavior in such a way that resources are used
more efficiently. This is perhaps most evident with human
beings. The American population uses about one-third of
the world’s resources. If all human populations used as
much energy and materials as Americans, the human carrying
capacity of the world would be about 800 million. The
current world population, about six and a half billion, far
exceeds this because most people in the world use far fewer
resources, and use them more efficiently, than Americans.
Scientists therefore do not know what the world carrying
capacity for humans is, or whether the human population
has exceeded it.
The fact that populations grow exponentially but
resources do not, an insight first developed by Thomas Malthus
(see Malthus, Thomas), was the foundation upon
which the theory of natural selection was built (see Darwin,
Charles; Wallace, Alfred Russel). Malthus used
the principles of population to explain human misery; Darwin
and Wallace used it to explain the creativity of evolution.
Further Reading
Brown, Lester R., Gary Gardner, and Brian Halweil. Beyond Malthus:
Nineteen Dimensions of the Population Challenge. New
York: Norton, 1999.