Mechanisms of
Evolution
Evolution
occurs as a population’s
genes & their frequencies change over
time (due to mutations)
 Gene pool- all of the alleles of the
population’s genes
 Allelic frequency- % of any specific allele
in the gene pool
ex. % T or % t
 Evolution
Genetic Equilibrium
 If
a population’s frequency of alleles
remain the same over generations than it
is said to be in genetic equilibrium
 According to scientists this population is
not evolving because the phenotypes
would remain the same.
Types of mechanisms of evolution
 1.
Mutations- can be caused by
environmental factors like chemicals,
radiation, uv rays
-usually lead to death; but some
mutations may produce some useful
gene variation that can become a part of
the gene pool & get passed on
Types of mechanisms of evolution

2. Natural Selection-Individuals with certain
useful variations such as speed are more
likely to survive (survival of the fittest) in
their env. passing those variations on to the
next generation
 Overtime, offspring w/ certain variations
(adaptations) make up most of the population,
& may look entirely different from their
ancestors
Types of Natural Selection
 Stabilizing
Selection- favors average
individuals. This type of selection reduces
variation in a population
For example, plant height might be acted on by
stabilizing selection. A plant that is too short
may not be able to compete with other plants
for sunlight. However, extremely tall plants
may be more susceptible to wind damage.
Combined, these two selection pressures select
to maintain plants of medium height. The
number of plants of medium height will
increase while the numbers of short and tall
plants will decrease. Like goldilocks (one of the
items is “just right”.
Types of Natural Selection
 Directional
Selection- favors one of the
extreme variations of a trait & can lead to
the rapid evolution of a population
Using the familiar example of giraffe necks,
there was a selection pressure against
short necks, since individuals with short
necks could not reach as many leaves on
which to feed. As a result, the distribution
of neck length shifted to favor individuals with
long necks.
Types of Natural Selection
 Disruptive
Selection- favors both
extreme variations of a trait; resulting
eventually in no intermediate form of the
trait & leads to evolution of 2 new species
For example, imagine a plant of extremely
variable height that is pollinated by three different
pollinators, one that was attracted to short plants,
another that preferred plants of medium height
and a third that visited only the tallest plants. If
the pollinator that preferred plants of medium
height disappeared from an area, medium
height plants would be selected against and
the population would tend toward both short
and tall, but not medium height plants
Types of mechanisms of evolution

3. Genetic Drift- allele frequencies change by chance
events
Types of genetic drift
3A). Founder effect- certain individuals leave a
population and begin a new population in a different
location; usually the new pop. has less genetic variation
than the larger pop.
ex. Amish population- some have short arms w/ extra
fingers/toes (polydactylism) due to a recessive allele
(chance event) from 1 amish settler
Types of mechanisms of evolution
 3. Genetic Drift (continued)- allele
frequencies change by chance events
Types of Genetic Drift3B). Bottleneck effect- some chance
disaster (ex. Flood) destroys organisms
Bottleneck effect. From an original
peccary population with its own gene pool,
a few individuals sqeeze though a
"bottleneck".
In this case a few peccaries with the
genotype bb survive a catastrope. These
individuals become the founders
(originators) of a new peccary population.
As they reproduce, the new gene pool is
very different from that of the original
population.
Types of mechanisms of evolution
 4.
Gene Flow- transport of alleles by
migrating individuals from one population
to another population
ex. Migration into or out of a
population may be responsible for a
marked change in allele frequencies.
ex. Individual leaves a population, its
alleles are lost from the gene pool
Types of mechanisms of evolution

5. Non-random mating- in all populations
organisms usually select mates non-randomly
for traits that are easily seen (looks, personality,
etc.)
 This means that some organisms are more
desirable to mate with than others.
 Animal breeders do essentially the same thing
when they intentionally try to improve varieties or
create new ones by carefully making sure
mating is not random.
 Evolution is usually a consequence if any of
the 5 mentioned mechanisms occur.