CHAPTER 22
DESCENT WITH MODIFICATION:
A DARWINIAN VIEW OF LIFE
QUESTION?
How did the diversity of life originate?
Through the process of Evolution.
EVOLUTION
The processes that have transformed life on earth from its beginnings
to today's diversity.
Evolution is the most pervasive principle in biology.
PRE-DARWINIAN VIEWS
1. Greeks
2. Fixed Species
3. Catastophism
4. Hutton and Lyell
5. Lamarck
GREEK PHILOSOPHERS
1. Plato - Organisms are already perfectly adapted to their
environments.
2. Aristotle - Organisms arranged on a “scale of life” from simple to
complex.
RESULT
No evolution.
Life is already perfect and doesn’t need to change.
All the rungs on life's "ladder" are already occupied.
FIXED SPECIES CONCEPT
The creator had designed each and every species for a particular
purpose.
RESULT
No evolution.
Created the viewpoint that all species could be identified and named
(Taxonomy).
A major factor in the Linnaeus classification system.
CATASTROPHISM
Georges Cuvier
(1769-1832).
Attempted to relate
fossils to current life.
THEORY
Fossils were the remains of species lost due to catastrophe.
No new species originated; species could only be lost over time.
Result - No evolution.
JAMES HUTTON
1795 - Gradualism
Profound change is the cumulative product of slow, but continuous
processes.
RESULT
Changes on the earth were gradual, not catastrophic.
CHARLES LYELL
1797 - 1875.
Incorporated Hutton’s
gradualism into a theory
called Uniformitarianism.
UNIFORMITARIANISM
Geological processes have operated at the same rate over the Earth’s
history.
RESULT
The Earth must be VERY old. (much older than 6000 years of the fixed
species concept).
Idea that slow and subtle processes can cause substantial change.
JEAN BAPTISTE LAMARCK
Published theory
in 1809.
Theory - Life
changed from
simple to complex
over time.
LAMARK
Fossils were the remains of past life forms.
Evolution did occur.
MECHANISMS
1. Use and Disuse  Body parts used to survive become larger and stronger.
 Body parts not used to survive deteriorate.
MECHANISMS
2. Acquired Characteristics
 Modifications acquired by use/disuse were passed on to offspring.
PROBLEM
No knowledge of genetics.
Acquired traits are not transmitted to offspring.
LAMARCK’S CREDITS
Did suggest correctly the role of fossils in evolution.
Did suggest that adaptation to the environment is a primary product
of evolution.
THOMAS MALTHUS
Essay on human population growth in
1798.
Disease, famine, homelessness, and war
are inescapable because human
populations grow faster than food
supplies.
Darwin read Malthus.
CHARLES DARWIN
Father of the
modern theory of
evolution.
Theory Descent with
Modification.
DARWIN'S BACKGROUND
Trained as a Naturalist (after trying religion and medicine).
VOYAGE OF THE BEAGLE
RESULT
Darwin's training and travel opportunities allowed him to formulate
and support his ideas on Natural Selection.
GALAPAGOS FINCHES
ALFRED WALLACE - 1858
Paper on Natural
Selection identical to
Darwin's ideas.
(Although he didn’t
call it natural
selection)
RESULT - JULY 1, 1858
Dual presentation of the Wallace-Darwin ideas to the Linnaean
Society of London.
DARWIN - 1859
Publication of
"The Origin of
Species”
COMMENT
Darwin best remembered for the theory because of his overwhelming
evidence and because he published.
DARWINIAN VIEW
History of life is like a tree with branches over time from a common
source.
Current diversity of life is caused by the forks from common ancestors.
EXAMPLE
“THE ORIGIN OF SPECIES”
Documented the occurrence of evolution.
Suggested that the mechanism for evolution was Natural Selection.
OBSERVATIONS:
Observation 1 – Members of a
population often vary greatly in their
traits.
.
OBSERVATION 2
Traits are inherited from parents to
offspring.
OBSERVATION 3
All species are
capable of
producing more
offspring than
their environment
can support.
OBSERVATION 4
Owing to lack of food or other resources,
many of these offspring do not survive.
INFERENCE 1
Individuals whose inherited traits give them a higher
probability of surviving and reproducing in a given
environment tend to leave more offspring than other
individuals.
INFERENCE 2
This unequal ability of individuals to survive and
reproduce will lead to the accumulation of
favorable traits in the population over
generations.
NATURE
Determines which characteristics are favorable.
Determines who survives.
Result - “Natural Selection”
NATURAL SELECTION IN ACTION
ARTIFICIAL SELECTION
When man determines the characteristics that
survive and reproduce.
Result - the various breeds of animals and plants
we’ve developed.
EX - MUSTARD PLANT
Original
Cultivars
EVOLUTION SUCCESS MEASURED BY
Survival
Reproduction
Whoever lives long enough and has offspring is
the “winner”
REQUIREMENTS
In order for Natural Selection to work, you
must have:
Variations within a population.
Long periods of time.
SUBTLETIES OF NATURAL SELECTION
1. Populations are the units of Evolution.
2. Only inherited characteristics can evolve.
COMMENT
Acquired characteristics may allow a species to evolve
"outside" of Natural Selection.
Ex: culture, learning
EVIDENCES FOR EVOLUTION
Direct observation of evolutionary
changes.
Fossils
Homology
Convergent Evolution
Biogeography
Molecular Biology
DIRECT OBSERVATIONS
Color patterns in guppies
Drug resistant HIV
Beak size in Birds
PitX1 gene
Others
COLOR PATTERN IN GUPPIES
Field Experiment (see text for full
details)
Changed the selection pressure on
male guppies
Result – color pattern change in 15
generations (22 months).
HIV DRUG RESISTANCE
Drug resistance strains selected for by treatments
Result – resistant strains became 100% dominant in 4-5 weeks.
BEAK SIZE
Field Study – measured the beak size of all birds in a population over
several years.
Result – drought and food competition changed beak size.
FOSSILS
Relics or impressions of organisms from the past.
Problem:
 Show changes over time from simple to complex.
 Many fossils don't have descendants.
EVOLUTION VIEWPOINT
Life has changed
over time.
Many species failed
to survive and
became extinct.
HOMOLOGY
Homologous
Structures Common "building
plan” with
divergent functions.
Mammal forelimbs
PROBLEMS
Vestigial Organs Rudimentary
structures of
marginal, if any,
use.
Whale Legs
HUMAN EXAMPLE
EVOLUTION VIEWPOINT
Remodeling of ancestral structures as their functions or adaptations
changed.
HOMOLOGY IN EMBRYOS
Problem - closely related organisms go through similar stages in their
embryonic development.
Ex: Gill pouches in vertebrates
EVOLUTION VIEWPOINT
Ontogeny is a replay of Phylogeny.
(Development reflects descent)
Development reflects inheritance of common control genes.
CONVERGENT EVOLUTION
Unrelated organisms show similar adaptations.
Cause – lived in a similar environment with similar selection pressures.
BIOGEOGRAPHY
The geographical distribution of species.
Problem:
 Species mixtures on islands
 Marsupials in Australia
EVOLUTION VIEWPOINT
Biogeographical patterns reflect descent from the ancestors that
colonized that area.
MOLECULAR BIOLOGY
Study of Evolution at the DNA or protein levels.
EVOLUTION VIEWPOINT
Related species share a common ancestrial DNA. The closer the
relationship, the more similar the DNA sequences should be.
SUMMARY
Know Darwin’s “observations” and “inferences”.
Be able to discuss the various evidences of Darwinian evolution.