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Darwin’s Theory of Evolution
• Evolution, is change over time, OR is the
process by which modern organisms have
descended from ancient organisms.
• A scientific theory is a well-supported
testable explanation of phenomena that have
occurred in the natural world.
Evolution is a Theory – Just like
• Evolution is a well
explanation of
phenomena that
have occurred in
the natural world
• A theory in science
is a well tested
hypothesis, not just
a guess
Evolution is when organisms change over time.
So, modern organisms descended from ancient
Charles Darwin: born in England in 1809
Sailed around the world 1831-1836
How do you think Darwin came up with
his theory?
Voyage of the Beagle
Darwin’s Journey
Based on the excerpts you read from
Darwin’s journal, The Voyage of the
Beagle, what factors may have contributed
to Darwin’s theory of evolution?
What did Darwin’s
Travels reveal
• The diversity of living
species was far greater
than anyone had previously
– Diversity = Many different
kinds of species
• These observations led him
to develop the theory of
1. Patterns of Diversity
• Darwin visited Argentina and Australia which had
similar grassland ecosystems.
– those grasslands were inhabited by very different
– neither Argentina nor Australia was home to the sorts of
animals that lived in European grasslands.
Examples of Diversity
• He saw rabbits in England but not in
Australia even though the environment
was similar
2. Living Organisms and Fossils
• Darwin collected the preserved remains of
ancient organisms, called fossils.
• Some of those fossils resembled organisms
that were still alive today.
Living Organisms and Fossils
• Others looked completely unlike any creature he
had ever seen.
• As Darwin studied fossils, new questions arose.
– Why had so many of these species disappeared?
– How were they related to living species?
Types of fossils
Trace fossils
Frozen or amber
Petrified fossils
The Galapagos Island
• A group of islands of the northwest coast of
South America
• The islands are close together but have
very different climates
• Darwin observed that the characteristics of
many plants and animals varied among the
different islands.
• Examples from the Islands:
Land tortoises
Galapagos finches
More examples on the Galapagos Islands
Blue footed booby
Marine Iguanas
4. The Journey Home
• Darwin Observed that
– Many islands close together had
different climates.
– Characteristics of many plants and
animals varied greatly among the
Darwin finally published his ideas in 1859
in his book “The Origin of Species” that
summarized all his findings from his trip
around the world.
Theory of acquired characteristics
Lamarck said organisms acquired traits by using
their bodies in new ways
These new characteristics were passed to
Lamarck was totally wrong!
Evidence of Evolution
1. Fossil Record
2. Geographic Distribution of Living
3. Homologous Body structures
4. Similarities in Embryology
1. The Fossil
Fossil Record provides
evidence that living
things have evolved
Fossils show the history
of life on earth and
how different groups
of organisms have
changed over time
Relative vs.
Relative Dating
Can determine a fossil’s
relative age
Performed by estimating
fossil age compared
with that of other fossils
Drawbacks – provides no
information about age
in years
2. 2. Geographic Distribution of Living
Similar animals in different locations were
the product of different lines of descent
Homologous body structures
Structures that have different
mature forms but develop
from the same embryonic
Wings and legs all descended
from fish fins.
e.g. Wing of bat, human arm,
leg of turtle
Homologous Body Structures
Vestigial Organs
– traces of homologous organs in other
– Organ that serves no useful function
e.g. Appendix in man
Similarities in Embryology
In their early stages of development, many
animals look similar, providing evidence that
they shared a common ancestry.
Embryological development
Artificial Selection
Nature provides variation, humans select variations
that are useful.
Example - a farmer breeds only his best livestock.
1. Evolution by Natural Selection
What do you think natural selection mean? (“Nature
chooses”, chooses what? Best adaptations for
The Struggle for Existence-members of each
species have to compete for food, shelter, other
life necessities in order to survive.
Survival of the Fittest-Some individuals are better
suited for the environment. Organisms with most
favorable adaptation will survive.
Natural Selection
Natural selection is the process by which individual organisms with
favorable traits are more likely to survive and reproduce.
Fitness is the ability of an individual to survive and reproduce
in its specific environment.
Adaptation: an inherited characteristic that increases an
organism’s chance of survival.
2. Components of Natural
• Not all individuals will
be able to reproduce.
• Due to environmental
issues, illness, etc…
Birds eat green beetles, not
brown ones.
What’s the end
Components of Natural Selection
The brown
trait has a
The brown beetles
that are left will
mate and have
brown offspring.
This is called
• Finally, the brown trait
(which is more
advantageous) allows
the beetle to survive in
order to reproduce.
• Eventually, all beetles in
this population will be
• This PHENOTYPE has
been SELECTED over
the green phenotype.
Image courtesy of
3. Descent with Modification
• Descent with Modification- Natural selection produces
organisms that have different structures, establish
different niches or occupy different habitats.
– This causes today’s species to look different from their ancestors.
Common Descent- were derived from common ancestors
Summary of Darwin’s Theory
1. Organisms differ; variation is inherited.
2. Organisms produce more offspring than survive.
3. Organisms compete for resources.
4. Organisms with advantages survive to pass
those advantages to their children.
5. Species alive today are descended with
modifications from common ancestors.
Evolution of Populations
Occurs when there
is a change in
relative frequency
of alleles
• Evolution –
change over time
Individuals better adapted to the
environment are able to survive and
Group of individuals of same species
that interbreed.
Common group of all genes present in a
Variation and Gene Pool
Combined genetic
information of all
Allele frequency is
number of times
alleles occur in the
Evolution: Any change in the
relative frequency of
alleles in a population
Single-Gene vs. Polygenic Traits
Single-Gene trait: any trait controlled by one gene (example:
tongue rolling)
Natural selection on single-gene traits can lead to change in
allele frequencies and thus evolution. Distinct phenotype
Polygenic traits: any trait controlled by two or more genes
(example: height in human) Many Phenotypes
Natural selection can affect the distribution of
phenotypes in any of three ways:
• Directional selection- Individuals at one end of
the curve have higher fitness than those in the
middle. (Example: seed size and birds beak size)
• Stabilizing selection-Individuals near the
center of the curve have higher fitness than
those at either ends of the curve. (Example:
weight of human infants at birth).
• Disruptive selection-Individuals at the upper
and lower ends of the curve have higher
fitness than those near the middle. (Eample:
seed size and bird beak size).
Natural Selection on Polygenic Traits
• Shifts to
middle range
• Shifts to
2 extremes
• Shifts to
1 extreme
Genetic Variation
Gene Flow
Movement of genes
from one population to
Sexual reproduction
causes new
combinations of genes.
Changes in DNA
Sources of genetic Variation in Populations
processes can
lead to this:
Mutations change in DNA
Gene Shuffling –
from sexual
Genetic Drift
• Suppose that some organism left behind a
few more offspring than other organisms.
• The ones that are left are the “lucky” ones.
But their genes may be no more
advantageous than anyone else’s.
• Entirely random.
• Doesn’t produce adaptations, only a
mixing of the gene pool.
Genetic Drift changes populations…….
• Random change in allele frequency
causes an allele to become
Founder Effect:
genetic drift due to the
migration of a small subgroup of a population.
(example: fruit flies migrating from mainland to
different Hawaiian islands.)
Hawaiian Honeycreepers
An example of adaptive radiation –
these species all diverged from a
common ancestor (founder species)
Conditions needed for Genetic Equilibrium
Speciation is the formation of new species
• As new species evolve, population become
reproductively isolated.
1. Reproductive Isolation – Members of two
population cannot interbreed & produce
fertile offspring.
2. Behavioral Isolation - capable of
breeding but have differences in
courtship rituals (EX. Meadowlarks)
3. Geographical Isolation – Separated by
geographic barrier like rivers ,mountains,
or bodies of water (ex: squirrel)
4. Temporal Isolation – Two or more
species reproduce at different times.
Table 23.1a