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Lecture 9 notes: History of Evolutionary Thought
GENERAL COMMENTS
1. What is evolution?
a. “Descent with modification.” (classical definition from Darwin’s time)
b. “Change in the genetic composition of a population from generation to
generation.” (modern definition; Darwin didn’t know about genes!)
2. Evolution is the unifying principal of all science, now ranked as principal, not theory,
equal to law.
3. Evolution is the best testable explanation of origins that we have; it offers a testable
explanation as to how the natural world came about.
4. Evolution dates from the publication of Origin of Species (1859 England, 1861 US)
by Ch. Darwin
a. Contributions of Origin:
1) Species change over time (evolve)
2) Natural selection is mechanism
3) Evidence. Prior to this time, much of science was based on flimsy evidence;
Ch. Darwin amassed a huge amount of data backing up his theories.
5. Evolution had been proposed before Ch. Darwin. For instance, Erasmus Darwin, Ch.
Darwin’s grandfather, wrote Zoonomia, which was a rather strange and lengthy poem
proposing evolution!
SCIENTIFIC THOUGHTS ON EVOLUTION PRIOR TO CH. DARWIN
1. Fixity of species. Most scientists prior to 1859 ascribed to a philosophy called
“natural theology,” which sought order in the chaos of Creation. Scientists thought
that species did not change with time; hence, were “fixed” from their point of
creation. This created internal conflict in Ch. Darwin—for many years, he knew that
organisms did change with time, did evolve into new species, but he didn’t want to
upset the scientific establishment and ruin his reputation.
2. Carl von Linné (1707-1778). Swedish naturalist, botanist. Latinized his name to
Carolus Linneaus
a. Wrote Systema Naturae in 1735, which went through 12 editions.
1) Attempted to name & classify all living things
2) Initiated binomial system used today, genus + specific epithet for all
organisms
3) Classified things from simple to complex, in evolutionary order, which is the
order suggested by the structure of organisms themselves; Linné still believed
in the fixity of species, thinking that God created everything w/o change
3. Georges Cuvier (1769-1832). Prof. comp. anatomy, Paris Museum of Natural Hist.—
a great anatomist, founded the modern science of paleontology
a. Noted fossils of extinct species laid down in strata, simple to complex.
b. Believed geological catastrophies, like the Biblical flood, caused mass extinctions
& explained layers of sedimentary rock in the geological strata. Since there were
numerous layers in sedimentary rock, there must have been numerous floods.
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c. Cuvier believed in the fixity of species; rejected evolution.
4. Not all progeny survive!
a. Rev. Thomas Robert Malthus (1766-1834) wrote An Essay on the Principle of
Population as it affects the future improvement of society, with remarks on the
speculations of Mr. Godwin, M. Condorcet and other writers—1798.
1) This booklet showed that human populations tend to grow exponentially
2) Since food is limited, only some progeny survive.
3) Applied concept to “the animal and vegetable kingdom.”
4) Gave Darwin idea for natural selection—only the fittest survive.
5. Geological gradualism
a. James Hutton (1726-1797) wrote The Theory of the Earth, 1795. This proposed
geological gradualism, meaning that current geological processes must be used to
explain past geological events.
b. Charles Lyell (1797-1875) wrote The Principles of Geology: being an Attempt to
Explain the Former Changes of the Earth’s Surface by Reference to Causes now
in Operation [Also known just as Principles of Geology]—1830-1833
1) Presented field data supporting Hutton, suggesting that the earth is at least
500,000 years old [we now know it’s about 5 billion years old]
2) Proposed uniformitarianism—geological processes proceeded at a steady,
uniform rate. Darwin read Lyell during voyage of Beagle. This was very
important to Ch. Darwin as it gave him the time necessary for evolution to
occur.
3) Lyell was rejected by much of established science at the time.
6. Early theories of evolution
a. Jean Baptiste Lamarck (1744-1829) wrote Zoological Philosophy, 1809
1) Arranged organisms in evolutionary tree, wrote that species were not fixed,
changed with time
2) Proposed mechanism for evolution, the inheritance of acquired characteristics.
What he proposed was that characteristics, developed or lost by the individual
through use or disuse, were passed on to progeny. The example common used,
but never written about by Lamarck, is giraffes’ necks lengthening as they
stretched, feeding in treetops, passing long-neck traits on to their progeny.
Lamarck’s proposed mechanism has been discounted as wrong because
acquired characteristics are not heritable.
DARWIN’S WORK
1. Charles Darwin (1809-1882): Wealthy background,
a. father was a domineering, rich physician, mother was a Wedgewood (also
wealthy).
b. Studied to be physician (couldn’t stand the sight of blood) then clergyman.
c. Loved natural history.
d. Initially believed in the fixity of species & natural philosophy.
e. Graduated w/ BA in 1831 from Cambridge.
f. Recommended for naturalist’s post aboard HMS Beagle.
2. Beagle’s voyage (1831-1836):
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a.
b.
c.
d.
3.
4.
5.
6.
Cartographic expedition, mainly to map coast of So. America.
Darwin bought a copy of Lyell’s Principles of Geology when in So. America.
He collected widely in S. America & Galapagos.
Noted animals in Galapagos were similar to S. American species but were unique
to Galapagos, thus must have changed over time (evolved).
1) This realization, plus Lyell’s book, shook Darwin’s faith in Church teachings
2) He had read Malthus’ book and began to think about natural selection as
mechanism for evolution.
3) Also saw how artificial selection (humans breeding pigeons and other animals
for desired traits) could produce wide variations in species over just a few
years—so, nature operating over hundreds of thousands of years (millions of
years, we now know) could certainly allow for the evolution of species.
After voyage:
a. Darwin worked on compiling his data.
b. He had become a famous naturalist during the voyage because of the specimens
sent to England during voyage.
c. Darwin worked slowly, reluctantly; he was sick and disturbed by the implications
of his work on the religious beliefs of the time, and worried that his beloved wife,
Emma Wedgewood, would not accept his believe in evolution.
d. Afraid he was going to die, he wrote a summary paper of his work in 1844.
1858- Alfred Russell Wallace wrote Darwin from the East Indies with a theory
identical to Darwins’.
a. Wallace, working on beetles, had come to the same conclusion about natural
selection that Darwin had and wanted Darwin’s thoughts/comments on it, and
wanted Darwin to send paper on to Lyell for publication.
b. Darwin saw that Wallace had trumped him.
c. Lyell and Hooker saved the day, and presented both Darwin’s 1844 essay and
Wallace’s paper to Linnaean Society of London on July 1, 1858.
1859- After a year of frenzied work, Darwin finished & published Origin of Species.
a. Stressed “descent with modification.”
b. All species came from single, common ancestor.
c. New habitats subjected populations to different selection factors = natural
selection, descendents thus posses different characteristics from sibling species or
original stock.
1860- Thomas Henry Huxley (1825-1895), also called “Darwin’s bulldog,” defended
Darwin vigorously, helped establish Darwin’s theory.
HOW NATURAL SELECTION WORKS
1. Natural selection works on individuals but it is populations that evolve.
a. Natural selection removes the ability of an individual to reproduce.
2. Note difference between acclimation and adaptation.
a. Acclimation = individual (non-heritable) change resulting from environmental
pressure.
b. Adaptation = heritable change in a population resulting from environmental
pressure.
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1) E.g. Darwin’s finches.
a) Finches eat seeds, preferably small seeds.
b) During wet years, seeds are large; in dry years, seeds small.
c) Birds with small beaks eat small seeds most easily.
d) During wet years, birds with small beaks selected against; large-beaked
birds reproduce, pass “large beak genes” to their offspring, thus the
average size of beaks increases.
EVOLUTION AS A HYPOTHESIS, THEORY, PRINCIPLE
1. Properties of a good hypothesis:
a. Testable
b. High prediction value
c. Falsifiable
d. Best explanation of a natural phenomenon
2. Evolution meets the criterion of being good hypothesis; further, it has been tested for
150+ years and has yet to be falsified—thus, its elevation to principal. Any
explanation of origins that does not meet the above criteria is not science.
EVIDENCE FOR EVOLUTION
1. Areas of evidence include biogeography, fossil record, comparative anatomy,
comparative embryology and molecular biology.
BIOGEOGRAPHY
1. Related species (living and fossil forms) tend to be clustered together geographically.
a. This suggests that they evolved where they are clustered.
b. Example seen in global coral distributions with two centers of origin, the oldest
being the W Pacific, the other being in the Caribbean.
2. Center of Origin.
a. The oldest location of a species, the area with most species (living or fossil).
b. Why are there more species here? It’s the area that has had the most time for
evolution to occur.
3. Continental Drift.
a. What is continental drift?
1) Movement of land masses on continental plates over heavier oceanic plates.
2) Plates pushed apart by volcanism at mid-oceanic ridges, creating new crust,
old crust subducted into trenches
b. Pangaea.
1) 200 mya, all of the continental crust land masses were together in one land
mass called Pangaea.
2) It began breaking apart ~180 mya
a) Laurasia—N. America, Europe + Asia and
b) Gondwanaland—So. America, Africa, Antarctica + Australia;
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c. History
1) Ben Franklin noted So. America & Africa fit together.
2) Alfred Wegener (1912) scientifically proposed theory.
a) Not accepted until
d. Explains many organism districutions. For instance,
1) Some closely-related freshwater fishes are found only in equatorial Africa and
South America. How did they get there when they can’t survive marine water?
a) Cichlid distribution
b) Lungfish distribution
2) Fossil distributions explained by continental drift
FOSSIL RECORD
1. Absolutely consistent with evolution—simpler forms appear before more complex
forms, transition/missing link fossils have been and are being found.
2. Fossil record is predictably incomplete and depends on
a. Rarity of species fossilized,
b. Availability of fossilization events,
c. Chances of fossil surviving geological processes,
d. Chances of finding fossil.
3. Fossilization processes: Organisms must be covered by air-tight agent to prevent
bacterial & fungal decomposition.
a. Tar, amber, mud—most common, thus aquatic organisms most common in fossil
record—form sedimentary rocks.
b. Minerals replace tissues, crystallize.
c. Trace fossils form from animal tracks, wave swash, burrows, etc.
d. Organisms with hard shells favored.
4. Relative dating.
a. Younger strata forms on older strata due to seasonal deposition events.
b. Specific fossils may be limited to specific strata = index fossils.
c. Fossils that occur on top of index fossils are younger than those that occur lower
down. May be widespread.
d. Led to geological time scale development
5. Eras in geological time scales
a. Rapid radiations followed by extinctions.
b. Extinctions due to ?
1) Volcanism,
a) Volcanism found at end of all major extinction events
2) Climate change,
a) Sudden drop in sea level at end of each period suggests climate change
3) Asteroids—Alvarez hypothesis
a) At the K-T boundary extinctions ~65 million years ago, iridium deposits
found globally, associated with asteroids
b) Chicxulub crater in Yucatan site of impact.
6. Absolute dating.
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a. Radiometric techniques using radioactive isotopes which decay at known rates
into measurable products.
b. The half-life is amount of time it takes ½ of isotope to decompose.
c. Examples:
1) Organisms absorb C12 and C14 when alive at a known ratio, C14 then
decomposes at death, with ½ of the C14 gone in 5,730 years. C14 good for
estimating ages directly up to 70,000 years. C14 good for directly dating
anything with carbon: shells, plant matter, bones, etc.
2) Common radioisotopes used for absolute dating include:
K40/Ar40; ½ life 1.3 billion years; age range 100,000-4.5 billion years
U235/Pb207; ½ life 710 million years; age range 10 million-4.5 billion yrs
3) Problem: Rock is 75% U235/25% Pb207. How old is rock?
4) Problem: Rock is 87.5% U235/12.5% Pb 207. How old is rock?
e. Used to date index fossils.
1) Volcanic rocks above and below fossil can give exact age range of fossil.
COMPARATIVE ANATOMY
1. Related organisms have a core of similar characteristics, relatedness proportional to
number of anatomical similarities or differences.
2. Focus on homologous structures, characteristics resulting from common ancestry.
Examples: Radius bone in tetrapods suggests common ancestory
3. Beware of analogous structures, characteristics with similar function but different
origins! Examples: Wings of insects, wings of birds—both developed from
completely different types of tissues and showing absolutely no homology.
4. Note that comparative anatomy was used to relate organisms by evolutionary
development before evolution was accepted by science! Linné, who believed in the
fixity of species, classified organisms using comparative anatomy, which suggested
the evolutionary development of organisms, long before Darwin.
COMPARATIVE EMBRYOLOGY
1. Related organisms have similar ontogeny (embryonic development). Comparing
ontogeny yields clues to phylogeny (evolutionary history of organism).
2. For example, comparative embryology suggests terrestrial vertebrates evolved from
fish as all vertebrate embryos have gill slits, fins, fish-like tails, etc.
MOLECULAR BIOLOGY
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1. Theory: Closely-related species have more similar DNA/RNA & amino acid
sequences than those more distantly related.
2. Molecules commonly studied include cytochrome c, hemoglobin amino acid
sequence, mtDNA (mitochondrial DNA), nuclear DNA
3. CLADISTICS see Section 26.3
a. Collects characteristics & creates cladograms of most parsimonious relationships
using mathematical algorithms
b. Show relatedness among groups, thus evolutionary relationships
c. Usually uses molecular data, but may use structural/anatomical data as well
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