Descent with Modification: A
Darwinian View of Life
3 key observations
about life
1.
2.
3.
Striking ways
organisms are
suited to their
environment
Unity of living
things(so many
common features)
Rich diversity of life
DESCENT WITH
MODIFICATION
A CHANGE in the GENETIC
COMPOSITION of a
POPULATION from GENERATION
to GENERATION

1.
CAN BE VIEWED IN 2 WAYS
A PATTERN

OBSERVATIONS FROM NATURAL
WORLD
A PROCESS
2.

MECHANISMS THAT PRODUCED THE
OBSERVED PATTERNS


DID NOT BELIEVE ORGANISMS
CHANGED OVER TIME (some Greeks
did)
Viewed organisms as fixed: could be
arranged on ladder from least complex
 most complex


DEVELOPED
BINOMIAL
NOMENCLATURE
WITH A “NESTED”
CLASSIFICATION
SYSTEM


STUDY OF FOSSILS
GEORGES CUVIER (1769 – 1832)
OLDER THE STRATUM OF
SEDEMENTARY ROCK THE OLDER
& MORE DISSIMILAR THE FOSSILS
WERE TO CURRENT LIFE
 NEW SPECIES APPEARED, SOME
DISAPPEARED

CATASTOPHISM:
EVENTS IN THE
PAST OCCURRED
SUDDENLY &
WERE CAUSED BY
DIFFERENT
MECHANISMS
THAN SEEN NOW


Scottish Geologist
Explain geologic features by
gradual mechanisms still going on
now

Principle of
Uniformitarianism
 Mechanisms of
change are constant
over time



Hutton & Lyell’s ideas influenced
Darwin
Both indicated Earth had to be
much older than previously
believed
Darwin reasoned: slow & subtle
processes could produce biological
changes

1st to propose mechanism of
evolution
 Parts of body used the most
become larger, stronger; parts not
used deteriorate
 Inheritance of Acquired Traits

Example: A population of giraffes all had short
necks and were able to reach their food.
However, when they were forced to move to an
area with taller trees, they could no longer
reach their food. So they stretched and
stretched until they could reach. Then when
Mr. & Mrs. Giraffe had babies, their babies had
long necks because their parents had acquired
long necks.

Louis the Short-Necked Giraffe

EVOLUTION HAPPENS BECAUSE
ORGANISMS INATELY WANT TO
BECOME MORE COMPLEX


Plants & animals in temperate
zones of SA closely resembled
plants & animals of temperate
regions of Europe
Fossils in SA resembled living
organisms in SA

Took note of
many adaptations
 saw
adaptations & the
origin of a new
species as closely
related processes


14 different
species each
clearly
adapted to a
specific
niche
all similar to
finches on
mainland
SA
(common
ancestor)

Proposed that
natural selection
over long periods
of time could
cause an
ancestral species
to give rise to 2 or
more descendant
species

Natural Selection: a
process in which
individuals with
certain inherited
traits tend to survive
& reproduce at
higher rates than
other individuals
without those traits

Darwin provided massive amounts of
evidence that descent with modification
by natural selection explains :
 Unity of Life
 Diversity of Life
 Match between Organisms &
their Environment
NATURAL SELECTION

Differential success in
the reproduction of
different phenotypes
resulting from the
interaction of organisms
with their environment.
Evolution occurs when
this causes changes in
relative frequencies of
alleles in the gene pool.
ARTIFICIAL SELECTION

Selecting and breeding
individuals that possess
desired traits
NATURAL
SELECTION
ARTIFICIAL
SELECTION
OBSERVATION
1.
Population sizes
would increase
exponentially if all
individuals were
successful in
reproducing
OBSERVATION
2.
Populations tend to
remain stable in size
(other than seasonal
fluctuations)
OBSERVATION
3. Resources are
limited
INFERENCE
1. overpopulation
leads to
competition,
those with traits
that will help
them survive 
reproduce more
 passing along
favorable traits
OBSERVATIONS
4. There are
variations of
traits in
populations
5. Variation is
mostly heritable
INFERENCES
1.
2.
Individual with
variations that are
favorable will leave
more offspring than
one without those
traits
Unequal ability to
survive and
reproduce leads to a
change in a
population

Natural selection can happen rapidly and
observed directly


guppies, Drug-Resistant HIV
Also observed by researchers in the fields of
anatomy, embryology, molecular biology,
biogeography, and paleontology




homologous and vestigial structures
molecular homologies: same DNA, RNA; universal
genetic code
biogeographically distribution of organism
comparative data from biochem, molecular biology
and cell biology support the oldest known fossils are
prokaryotes

SIMILARITY RESULTING FROM
COMMON ANCESTORS

Similarities in early stages of
development suggest common ancestry
 At some point in development all
vertebrates have a tail

REMNANTS OF
FEATURES THAT
WERE
IMPORTANT IN
EVOLUTIONARY
ANCESTOR



All life forms use DNA & RNA
Genetic code is universal
Some human genes code for exact same
protein as bacterial gene


+/- have same function
Some organisms carry “vestigial genes”

Can organize organisms with shared
characteristics in nested groups
Deepest layer all living things share
 Each successive smaller group have their
own unique homologies


are hypotheses
that summarize
our current
understanding
of patterns of
descent

The independent evolution of similar
features in different lineages

are seen in
species that share
similar structures
but not common
ancestry
1.
2.
3.
past organisms differ from current ones
many species have become extinct
the evolutionary changes that have
occurred in various groups of
organisms


The GEOGRAPHIC DISTRIBUTION of
ORGANISMS
Influenced by:
 Continental Drift
 250 million yrs ago all land masses
joined  Pangea
 200 million yrs ago split began
 20 million yrs ago looked close to what
we have today

Understanding
continental drift
scientists look for
fossil evidence
explained by the
movement of
land masses
Direct Observations
1.
1.
2.
Anatomical Similarities
2.
1.
2.
3.
4.
Effect of invasive species
Drug-resistance in bacteria
Homologous & Vestigial Structures
Universal Genetic Code
Fossils
Biogeography