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How Populations Grow
Section One
A population consists of all the
members of the same species
that live in the same place at the
same time.
A species is a group of
populations that are capable of
interbreeding and are
reproductively isolated from
other species.
The three key features of a
population are:
• Population size – Total number of
individuals
• Population density – The number of
individuals in a given area
• Population dispersion – How the
individuals are grouped: even, clumped,
random, etc.
The study of populations is
called demography.
Populations tend to grow as
long as their environment can
sustain the population size.
As populations grow, the number of
reproducing individuals and the number
offspring tend to lead to exponential
growth model.
Eventually, predation, disease,
habitat, and availability of
resources cause the
environment to reach a certain
limit that the habitat can
support. This size limit is called
carrying capacity.
As carrying capacity is reached, the death
rate begins to equal the birth rate and the
population is described with a logistic
model.
Some populations may grow
according to density
independent factors such as
weather and climate.
Changes in populations may be
caused by the population size.
We call the cause of these
changes density dependent
factors.
r – strategists are populations that
are usually small and are able to
reproduce rapidly when the
environment is favorable. Large die
offs occur when conditions change.
K-strategists are tend to be large
animals with slower reproduction,
parental care of young, and have a
need for stable environments.
These populations tend to live near
the limits of their carrying capacity.
How Populations Change
Section Two
Populations change thru time.
In following classical genetics,
biologists often assumed that
dominant genes would slowly
replace recessive genes in a
population.
After much mathematic
calculation and data collection
from populations, the HardyWeinberg Principle was
established.
The Hardy-Weinberg Principle
states that frequencies of alleles
in populations does not change
unless evolutionary forces act
on the population.
The five major forces that
change populations are:
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Mutation
Nonrandom Mating
Gene flow
Genetic drift
Natural Selection
Mutation rarely occurs in nature
and then, even more rarely, has
a noticeable effect on gene
pools.
Gene flow occurs mainly when
individuals migrate.
Immigrants bring genes into a
population and emigrates take
genes away.
Genetic drift occurs when a
small population is affected by a
disaster such as a flood,
disease, or volcano and then
recovers.
Nonrandom mating and
inbreeding as in zoos can
change the gene frequencies of
a population.
Natural selection enables
individuals who exhibit favorable
traits to survive and reproduce.
Thus, natural selection does
not change genotypes but
selects for phenotypes.
A favorable trait may be
controlled by a large number of
genes. This is a polygenic
trait.
Selection for a polygenic trait
leads to a normal distribution.
When selection moves toward an
extreme such as a larger grizzly bear, it
is called directional selection.
When selection moves away from the
extremes and toward the normal, it is called
stabilizing selection.