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.
Theory vs theory
Theory
• Hypothesis supported
repeatedly by data
• Makes testable
predictions
theory
• Layman’s and TV use
of the word
• Confused with
hypothesis in Science
Examples of Theory
•
•
•
•
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Cell Theory
Big Bang Theory
Atomic Theory
Theory of Gravity
Theory of Evolution
Evolution
• Has itself "evolved" or changed over time.
• Illustrates “Science as a Process”.
• Students should be able to give the main points
of several views.
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.
Lamarck
• 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.
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 kids is
the “winner” in evolution.
Requirements
• In order for Natural Selection to work,
you must have:
– Variations within a population.
– Long periods of time (according to Darwin).
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
•
•
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•
•
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Direct observation of evolutionary changes.
Fossils
Homology
Convergent Evolution
Biogeography
Molecular
Direct Observations
•
•
•
•
Color patterns in guppies
Drug resistant HIV
Beak size in Birds
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.
Comments
1. Fossilization is a rare event.
2. Only hard parts fossilize well.
3. Problem in finding fossils.
4. Interpretation.
5. Missing Links.
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
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.
• Problem - related species have similar
DNA sequences.
Evolution Viewpoint
• Related species share a common
ancestral DNA. The closer the
relationship, the more similar the DNA
sequences should be.
Summary
• Darwin's ideas now a "Theory”.
• Predictions of a Theory are tested by
experiments and observations.
• Be familiar with the pre-Darwin views of
evolution.
• Know Darwin’s “observations” and
“inferences”.
• Be able to discuss the various evidences of
Darwinian evolution.