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Descent with Modification
Darwinian View of Life
Chapter 22
Why is Evolution So
Controversial?
Often evolution (and science in general) is pitted
against religion in “winner take all” stance
Modern debate mostly involves the teaching of
evolution rather than the theory itself
And that means mixing science, religion and politics, etc.--Never an easy combination!
Plenty of misconceptions and confusion surround
evolution, adding to controversy
3 Key Observations About Life
1. Organisms are suited for life in their
environments.
2. Organisms share many characteristics of life
(unity).
3. There is a rich diversity of life.
These observations led Charles Darwin to develop a
scientific explanation for these observations.
Endless Forms Most Beautiful
•
•
Study of biology upended in 1859 when Darwin
published The Origin of Species
Darwin proposed: Descent with modification
•
•
Species are descendants of ancestral species which were
different from present day species
Evolution: change in genetic composition of
population from generation to generation
Summary of Other Ideas
•
•
Many scientists before and during the time of Darwin
theorized on what was the cause of the unity and
diversity of living organisms
Some ideas supported Darwin’s theories while some did
not
Intellectual Context of Darwin’s Ideas
1809
Lamarck publishes his
hypothesis of evolution.
1798
Malthus publishes
“Essay on the Principle
of Population.”
1812
Cuvier publishes his extensive
studies of vertebrate fossils.
1795
Hutton proposes
his principle of
gradualism.
1830
Lyell publishes
Principles of Geology.
1858
While studying species in
the Malay Archipelago,
Wallace (shown in 1848)
sends Darwin his hypothesis
of natural selection.
1790
1870
1809
Charles Darwin
is born.
183136
Darwin travels around
the world on HMS
Beagle.
The Galápagos Islands
1859
On the Origin of
Species is published.
1844
Darwin writes his
essay on descent
with modification.
Not in Darwin’s “Camp”
•
Aristotle viewed species as unchanging
•
•
Linneaus: Developed taxonomy and the binomial classification system
•
•
Arranged them on a scala naturae, ladder of permanent “rungs”
Believed in a divine placement of organisms in this classification, not necessarily
changing either
Cuvier (geologist): Founded of modern paleontology (study of fossils)
•
•
Saw that fossils are deposited in strata (layers) of rocks with older layers
containing fossils of species that may be extinct now
Believed in catastrophism, the sudden loss of local species due to disasters
Lamarck’s Hypothesis of Evolution
• Other scientists were thinking about life forms evolving over time
• Lamarck hypothesized:
•
Use and disuse of body parts could change organisms over their lifespan
•
These changes could be passed to offspring by Inheritance of Acquired
Characteristics
Lamarck’s Hypothesis of Evolution
• Lamarckism unsupported by much of genetic
evidence
• BUT emerging field of epigenetics may prove to
support some of Larmarck’s ideas!
Scientists in Darwin’s “Camp”
•
Hutton and Lyell believed:
•
•
•
Earth’s surface changes from cumulative, SLOW continuous actions still
operating today (uniformitarianism)
Strongly influenced Darwin’s thinking
•
He reasoned the Earth must be much older than previously believed
•
Also reasoned biological organisms may also have slow, continuous change
Malthus (economist):
•
Limited resources shape population sizes
Voyage of the HMS Beagle
Darwin in 1840,
after his return
from the
voyage
HMS Beagle in port
Great
Britain
EUROPE
NORTH
AMERICA
ATLANTIC
OCEAN
The
Galápagos
Islands
AFRICA
PACIFIC
OCEAN
Pinta
Genovesa
Santiago
Fernandina
Isabela
0
20
40
Kilometers
SOUTH
AMERICA
Equator
Daphne
Islands
Pinzón
Santa Santa
Cruz
Fe
Florenza
Equator
Chile
PACIFIC
OCEAN
San
Cristobal
Española
Andes Mtns.
Marchena
PACIFIC
OCEAN
Brazil
Argentina
Cape Horn
Malay Archipelago
AUSTRALIA
Cape of
Good Hope
Tasmania
New
Zealand
Darwin’s Focus on Adaptation
• During the voyage, Darwin observed many adaptations:
• Inherited characteristics of organisms which increase their
survival and reproduction in different environments
• Darwin saw how adaptation to environment and the
origin of species is closely related
• Theorized: Could new species arise from gradual
accumulation of adaptations to different environments?
Darwin’s Focus on Adaptation
How do adaptations arise? Largely dependent
on:
Natural selection: Process in which individuals
with certain inherited traits tend to survive and
reproduce at higher rates than other individuals
because of those traits
Example of Speciation in Galapagos Finches
(a) Cactus-eater
(b) Insect-eater
(c) Seed-eater
Artificial Selection
• Humans have modified other species by selective breeding for
1000’s of years, a process called artificial selection
The Reason Why You Shouldn’t Procrastinate
• In 1844, Darwin wrote an essay on natural selection--but
did not publicize it
• In June 1858, Wallace sent Darwin a paper to review with
an almost identical theory of natural selection
• Darwin quickly finished The Origin of Species by Means of
Natural Selection and published it
•
Wallace graciously stepped away so now we call it “Darwinism”
and not “Wallacism”!
Observations of Darwin
• Observation #1: Members of a population often vary
in their inherited traits
Observations of Darwin
Observation #2: All species can produce more offspring
than the environment can support
• Many offspring fail to survive and reproduce
Important Points About Natural Selection
• Note that individuals do not evolve; but populations
evolve over time
• Natural selection can only increase or decrease
heritable traits that vary in a population
• If everyone is genetically identical, no basis for selection
• Adaptations vary with different environments
• Single adaptation not preferred for all environments
Direct Observations of Evolutionary Change
• The bacterium Staphylococcus aureus is commonly
found on people
• One strain, methicillin-resistant S. aureus (MRSA) is a
dangerous pathogen
• S. aureus became resistant to penicillin in 1945 and
resistant to methicillin in 1961
Direct Observations of Evolutionary Change
• Methicillin works by inhibiting a protein used by
bacteria in their cell walls
• But MRSA bacteria use a different protein in their cell walls
• When exposed to methicillin, MRSA strains are more
likely to survive and reproduce than nonresistant S.
aureus strains
• MRSA strains are now resistant to many antibiotics
Homology
• Homology is similarity resulting from common
ancestry
• Homologous structures are anatomical
resemblances that represent variations on a
structural theme present in a common ancestor
Mammalian Forelimbs: Homologous Structures
Humerus
Radius
Ulna
Carpals
Metacarpals
Phalanges
Human
Cat
Whale
Bat
Embryonic Homologies
Pharyngeal
pouches
Post-anal
tail
Chick embryo (LM)
Human embryo
• Comparative embryology reveals anatomical homologies
not visible in adult organisms
Vestigial Structures
• Vestigial structures are remnants of features that served
important functions in the organism’s ancestors
• Common human vestigial structures:
•
Appendix
•
Male nipples
•
Wisdom teeth
•
Body hair and goosebumps
•
Ear muscles
Evolutionary Tree
Branch point
Lungfishes
Digitbearing
limbs
Amnion
Amniotes
Mammals
2
Lizards
and snakes
3
4
Homologous
characteristic
Crocodiles
Ostriches
6
Feathers
Hawks and
other birds
Birds
5
Tetrapods
Amphibians
1
Different Cause of Resemblance:
Convergent Evolution
• Convergent evolution is the evolution of similar, or
analogous, features in distantly related groups
• Analogous traits arise when groups independently adapt to
similar environments in similar ways
• Convergent evolution does not provide information about
ancestry
Examples of Convergent Evolution
Homology vs. Convergent Evolution
• Both are evidence of evolution
• Analogous features (convergent evolution) share
similar function but not common ancestry
• Homologous features (homology) share common
ancestry but not necessarily similar function