The Theory of Evolution

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Transcript The Theory of Evolution

One of the most respected evolutionary biologists has defined biological
evolution as follows:
"In the broadest sense, evolution is merely change, and so is allpervasive; galaxies, languages, and political systems all evolve.
Biological evolution ... is change in the properties of populations of
organisms that transcend the lifetime of a single individual. The
ontogeny of an individual is not considered evolution; individual
organisms do not evolve. The changes in populations that are
considered evolutionary are those that are inheritable via the genetic
material from one generation to the next. Biological evolution may be
slight or substantial; it embraces everything from slight changes in the
proportion of different alleles within a population (such as those
determining blood types) to the successive alterations that led from the
earliest protoorganism to snails, bees, giraffes, and dandelions."
- Douglas J. Futuyma in Evolutionary Biology, Sinauer Associates 1986
Evolution and Populations
It is important to note that
biological evolution refers to
populations and not to
individuals and that the
changes must be passed on
to the next generation.
 In practice this means that
evolution is a process that
results in heritable changes in
a population spread over
many generations.
Competing Theories
Lamarck’s Theory of Evolution
Proposed in 1809, Lamarck
observed that characteristics of
living things could change over
For example, if muscles are used,
they grow stronger. These are
called “acquired” characteristics.
Competing Theories
Lamarck’s Theory of Evolution
Lamark proposed that organisms can pass on these acquired
characteristics to their offspring.
For example: Ancestors of giraffes had short necks. Lamark’s
theory goes that since they had to stretch to get food, their necks
became longer and they could pass this trait to their offspring.
Competing Theories
Darwin and Wallace’s theory of Natural Selection would
have explained the giraffe’s long necks in the following
• An original group exhibited variation in
neck length.
• Longer necks were more successful in
the given environment.
• Longer necked giraffes were then able
to reproduce more often than shorter
necked giraffes and thus pass on their
long neck genes to the next
• After many generations, the population
has a general increase in neck length.
Competing Theories
Even though the giraffe's neck
is extremely long, it has only
seven neck vertebrae, the
same number that people and
most other mammals have.
Definition of Evolution:
The accumulation of
changes in the heritable
characteristics of a
Evolution describes the
changes in the various
genes of a species over
These changes are
the result of:
 Mutations
 Natural selection
 Chance
Voyage of the Beagle, 1831-1836
Succession of Types
Why should extinct armadillo-like species and
armadillos be found on the same continent?
The changes imply ancestry!
“This wonderful relationship in the same continent
between the dead and the living will, I do not doubt,
throw more light on the appearance of organic beings
on our earth, and their disappearance from it, than any
other class of facts.”
--The Voyage of the Beagle, ch. 8
Basic Tenets of the Theory of Evolution:
Populations tend to produce more
offspring than the environment can
This leads to a struggle for existence in which
some individuals survive and some die.
Carrying Capacity
2. Natural Selection
 The over-production of
offspring leads to
competition and
survival of the
individuals best suited
to that particular
The better adapted
individuals tend to survive
and reproduce more than the
less well-adapted individuals.
3. The variation in individuals in a population
is controlled by their genes and is therefore
The better adapted individuals pass on their
characteristics to more offspring than the less
well adapted.
The results of natural selection therefore
As one generation follows another, the
characteristics of the species gradually
How Does Variation Arise?
In the DNA:
-Sexual Reproduction
In the Environment:
-Genetic Drift
Variation is non-directional (random)
Sexual Reproduction
 Role:
Promotes variation in a species
through meiosis and fertilization which is
essential to natural selection.
Changes in the base pair
sequence of DNA.
Most are neutral or harmful to
the organism, but some have
positive effects.
Note: In an organism that sexually
reproduces, the mutation must
occur in a gamete (sex cell) if it is
to be passed on to offspring, and
thus contribute to the evolution of
that organism.
A mutation has caused the garden moss
rose to produce a flower of a different color.
The selection process is directional:
It is dictated by the
environment (not random).
This leads to differential survival:
 The result is that the
individuals best adapted to
a specific environment will
survive to reproduce.
What are some things you think
these organisms can do better
than others?
Differential Survival:
 If
change (either
gradually or
suddenly), the “best
adapted” may also
 This
process can
lead to changes in
the species
Genetic Drift:
Sometimes the
frequency of a
gene in a
population is
determined by
chance and not by
whether it gives the
individual an
advantage in the
This is known as
genetic drift.
It is important to know at
this point that evolution
doesn’t work on the level
of individuals or speciesit works on the level of
Thus, individuals do
not evolve,
populations do.
Explain why evolution
works on populations
and not individuals.
When two groups of
a species are in
environments they
cannot interbreed.
If the selection
pressure is different,
they will eventually
become different
species: aka
The eastern meadowlark
(Sturnella magna) and the
western meadowlark (Sturnella
neglecta) have very similar
body shapes and coloration.
Their ranges overlap in the
middle of the country.
Considered different species:
 Songs are different
 Behavioral differences that
prevent interbreeding.
In the summer of 1995, at least 15 iguanas survived Hurricane Marilyn on a raft of
uprooted trees. They rode the high seas for a month before colonizing the
Caribbean island, Anguilla. These few individuals were perhaps the first of their
species, Iguana iguana, to reach the island.
Artificial Speciation
Allopatric speciation of squirrels in the Grand Canyon
Harris’s antelope
squirrel (south rim)
antelope squirrel
(north rim)
Ex. Many of the finches in the Galapagos
Why are finches different on different
Islands have different
kinds of seeds available
which select for different
types of beaks to eat
 In time the birds become
so genetically/physically
different they cannot
Environments constantly change and if a
species cannot adapt, they may go extinct.
Ex. Dinosaurs
Couldn’t adapt to
colder climate
Their place was
taken by mammals
(homeothermicwarm blooded)
Not all dinosaurs
went extinct. Some
also evolved to
become warmblooded birds.
There have been 5 major extinction events during
the history of life on Earth.
Some 99.9% of all species
that have ever lived on Earth
are now extinct.
The CretaceousTertiary Extinction
(65.5 MYA) is
probably the most well
known, as it killed off
all non-avian
dinosaurs. 75% of all
species went extinct.
There have been 5 major extinction events
during the history of life on Earth.
This wasn’t even the largest though. That honor goes to
the Permian-Triassic Extinction (251 MYA) which caused
the extinction of 96% of all marine species and 70% of
all land species.