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Mechanisms of Evolution
Darwin Presents His Case
Darwin returned
to England in
 He did not
present his
findings to the
community until
25 years later
Darwin Presents His Case
While visiting
the Galapagos
Darwin had
observations of
many different
species that
were found no
where else on
Darwin Presents His Case
Each of these island
species look very
similar to species
found on the mainland
of South America.
Yet, the island species
were clearly different
from the mainland
species and different
from each other.
Darwin Presents His Case
Darwin began filling notebooks
with his ideas about species
diversity and the process that
would later be called evolution.
Darwin was stunned by his
findings which led to the delay
in publishing his thoughts.
All of Darwin’s ideas
challenged the fundamental
scientific beliefs of his day.
Darwin Presents His Case
In 1858, Darwin
received a short essay
from Alfred Russell
Wallace, a fellow
This essay
summarized thoughts
on evolutionary
change and gave
Darwin the incentive
to publish his own
Darwin Presents His Case
In 1859, Darwin
published the
results of his work,
On the Origin of
Darwin Presents His Case
In his book, he proposed a
mechanism for evolution called
natural selection.
He presented evidence that evolution
has been taking place for millions of
years and continues in all living
Darwin's work was considered as
brilliant by many, while others
strongly opposed his message.
What did Darwin really say?
What have we learned since Darwin?
Darwin explained
evolution as a change
in the traits (alleles) of
a population over time.
Darwin also proposed
that all organisms
developed from
previously existing
Mechanisms of Evolution
Organisms are a very stable
There are many processes
that keep the organism
maintained and properly
functioning (homeostasis).
Likewise, populations tend
to have stable proportions
of traits unless one or more
of the following processes
acts on the variation
contained in the population.
Mechanisms of Evolution
Natural Selection: process by
which individuals that are
better suited for their
environment survive and
reproduce more successfully
(survival of the fittest).
 Fitness: The ability of an
individual to survive and
reproduce in its specific
 Fitness is the result of
 Adaptations are inherited
characteristics that increase
an organisms chance for
The Process of Natural Selection
Organisms in a population
may vary:
 Each organism has
unique DNA.
 This variation comes from
mutations and crossingover during meiosis.
 These variations can be
passed down to offspring.
The Process of Natural Selection
Organisms have the
potential to
reproduce quickly:
 Populations tend
to grow
 More offspring
are produced than
can survive.
The Process of Natural Selection
The ecosystem can
only hold a limited
number of organisms:
Resources are limited;
there is a carrying
capacity for each
The Process of Natural Selection
The organisms with the
most favorable
characteristics survive
to reproduce:
“Nature” selects the most
fit organisms in a
population for the
particular environment.
Favorable characteristics
become more widespread
in the population.
Mechanisms of Evolution
Genetic Drift:
Change in the traits
in a population due
to chance.
Genetic drift can
cause big losses of
genetic variation
for small
Mechanisms of Evolution
Genetic Drift:
Founder effect: A founder effect
occurs when a new colony is
started by a few members of the
original population. This small
population size means that the
colony may have:
 reduced genetic variation from
the original population.
 a non-random sample of the
genes in the original
Mechanisms of Evolution
Genetic Drift:
Genetic Bottle-Neck:
Population bottlenecks occur
when a population's size is
reduced for at least one
 Because genetic drift acts
more quickly to reduce
genetic variation in small
populations, undergoing a
bottleneck can reduce a
population's genetic
variation by a lot, even if
the bottleneck doesn't last
for very many generations.
Mechanisms of Evolution
3. Gene Flow: migration of individuals from
one population to another, introducing new
Mechanisms of Evolution
Non-Random Mating:
the choosing of mates
because of certain traits
which results in
amplification of those
traits (alleles).
Speciation: the
formation of a new
 A species is a group
of organisms in a
population that have
the potential to
New species evolve do
to a sub-population
geographically isolated
from the rest of the
population, then
reproductively isolated
through one of the
barriers listed on the
next slides.
Habitat Isolation:
Two species that occupy
different habitats within
the same area may
encounter each other
rarely, if at all, even
though they are not
isolated by obvious
physical barriers, such
as mountain ranges.
Habitat isolation:
Two species of garter
snakes occur in the
same geographic
areas, but one lives
mainly in water while
the other is primarily
Temporal Isolation:
species that breed
during different
times of day,
different seasons, or
different years
cannot mix their
Temporal Isolation:
In North America, the
geographic ranges of the
eastern spotted skunk and the
western spotted skunk overlap,
but the western spotted skunk
mates in late winter and the
eastern spotted skunk mates in
late summer.
Behavioral Isolation:
courtship rituals that
attract mates and other
behaviors unique to a
species are effective
reproductive barriers,
even between closely
related species.
 Such behavioral
rituals enable mate
recognition—a way to
identify potential
mates of the same
Behavioral Isolation:
Blue-footed boobies,
inhabitants of the
Galapagos, mate only
after a courtship
display unique to their
Part of the “script”
calls for the male to
high-step, a behavior
that calls the female’s
attention to his bright
blue feet.
4. Mechanical Isolation:
mating is attempted,
but morphological
differences prevent its
successful completion.
Mechanical Isolation:
The shells of two species of
snails spiral in different
directions: Moving inward
to the center, one spirals in
a counter-clockwise
direction, the other in a
clockwise direction.
As a result, the snails’
genital openings are not
aligned, and mating
cannot be completed
5. Gametic isolation- sperm of
one species may not be able to
fertilize the eggs of another
species. For instance, sperm
may not be able to survive in
the reproductive tract of
females of the other species,
or biochemical mechanisms
may prevent the sperm from
penetrating the membrane
surrounding the other species’
Gametic Isolation:
Gametic isolation separates
certain closely related
species of aquatic animals,
such as sea urchins.
Sea urchins release their
sperm into the surrounding
water, where they fuse and
form zygotes.
Gametes of different species,
such as the red and purple
urchins shown here, are
unable to fuse because
proteins on the surfaces of
the eggs and sperm cannot
bind to each other
6. Hybrid
Sterility: even if hybrids
are vigorous, they may be
If the chromosomes of the two
parent species differ in number
or structure, meiosis in the
hybrids may fail to produce
normal gametes.
Since the infertile hybrids
cannot produce offspring when
they mate with either parent
species, genes cannot flow freely
between the species .
Hybrid Sterility:
The hybrid offspring of a donkey and
a horse is a mule, which is robust but
The Pace of Evolution
Although Darwin and
Wallace, the
originators of the
theory of evolution
through natural
selection, thought the
pace of evolution was
gradual (occurring
over millions of years),
many scientist think
the pace varies.
The Pace of Evolution
Equilibrium: This is an
interpretation of the
fossil record showing
rapid speciation
followed by long
periods of equilibrium
with little evolution.