2012lecture02 charles darwin
TRANSCRIPT
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Lecture 2
Charles Darwin, the Origin of Species,
and tests of his evolutionary hypotheses
The voyage of the HMS Beagle
December 27, 1831 - October 2, 1836
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The Galapagos islands and
Darwins Finches
The most striking and important fact for us inregard to the inhabitants of islands, is their affinityto those of the nearest mainland, without beingactually the same species..... The naturalist,looking at the inhabitants of t hese volcanic islandsin the Pacific, distant several hundred miles fromthe continent, yet feels that he is standing onAmerican land. Why should this be so? why
should the species which are supposed tohave been created in the GalapagosArchipelago, and nowhere else, bear so plain astamp of affinity to those created in America?
...on the view here maintained, it is obvious thatthe Galapagos Islands would be likely toreceive colonists, whether by occasionalmeans of transport or by formerly continuousland, from America; and the Cape de VerdeIslands from Africa; and that such colonistswould be liable to modification;the principleof inheritance still betraying their originalbirthplace.
From the Origin
If all of these species were descended from a
common ancestor, what mechanism could accountfor this divergence and the apparent fit between
their morphology and habits of life?
1. Large cactus finch (Geospiza conirostris)
2. Large ground finch (G. magnirostris)
3. Medium ground finch (G. fortis)
4. Cactus finch (G. scandens)
5. Sharp-beaked ground finch(G. difficilis)
6. Small ground finch (G. fuliginosa)
7. Woodpecker finch (Cactospiza pallida)
8. Vegetarian tree finch (Platyspiza crassirostris)
9. Medium tree finch (Camarhynchus pauper)
10. Large tree finch (C. psittacula)
11. Small tree finch (C. parvulus)
12. Warbler finch (C. olivacea)
13. Mangrove finch (C. heliobates)
The Galapagos islands and Darwins Finches
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Thomas Malthus
(1766 - 1834)
The power of the population is indefinitely greater than the power
in the earth to produce subsistence for man. Population, when
unchecked, increases in geometrical ratio. Subsistence increases
only in an arithmetical ratio. A slight acquaintance with numbers
will show the immensity of the first power in comparison with the
second.
The power of the population is indefinitely greater than the power
in the earth to produce subsistence for man. Population, when
unchecked, increases in geometrical ratio. Subsistence increasesonly in an arithmetical ratio. A slight acquaintance with numbers
will show the immensity of the first power in comparison with the
second.
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The power of the population is indefinitely greater than the power
in the earth to produce subsistence for man. Population, when
unchecked, increases in geometrical ratio. Subsistence increases
only in an arithmetical ratio. A slight acquaintance with numbers
will show the immensity of the first power in comparison with the
second.
The power of the population is indefinitely greater than the power
in the earth to produce subsistence for man. Population, when
unchecked, increases in geometrical ratio. Subsistence increasesonly in an arithmetical ratio. A slight acquaintance with numbers
will show the immensity of the first power in comparison with the
second.
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The power of the population is indefinitely greater than the power
in the earth to produce subsistence for man. Population, when
unchecked, increases in geometrical ratio. Subsistence increases
only in an arithmetical ratio. A slight acquaintance with numbers
will show the immensity of the first power in comparison with the
second.
Publish or PerishAlfred Russel Wallace independently conceived of the idea of
Natural Selection and sent Darwin a manuscript in 1858
Alfred Russel Wallace
1823 - 1913
The problem then was not only how and why do specieschange, but how and why do they change into new and welldefined species, distinguished from each other in so manyways; why and how they become so exactly adapted to distinctmodes of life; and why do all the intermediate grades die out(as geology shows they have died out) and leave only clearlydefined and well marked species, genera, and higher groups ofanimals?
from My Life - 1905
Wallace claims that the idea of natural selection came to
him while laying in bed with a fever.
And the answer was clearly, on the whole the best fitted live andconsidering the amount of individual variation that my experienceas a collector had shown me to exist, then it followed that all thechanges necessary for the adaptation of the species to the
changing conditions would be brought about In this way everypart of an animals organization could be modified exactly asrequired, and in the very process of this modification the unmodifiedwould die out, and thus the definite characters and the clearisolation of each new species would be explained.
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On The Origin of Species by Means of
Natural Selection - C. Darwin. 1859
Darwins Hypotheses
! Evolution per se - That in the history of life on earth
dramatic changes in biological diversity have occurred in a
way consistent with the evolutionary hypothesis.
!Common Ancestry- The hypothesis that all living thingsare related by a history of descent with modification from a
common ancestor.
! Gradualism - The hypothesis that the di"erences
distinguishing di"erent species came about through a
gradual evolutionary process rather than saltational leaps.
! Population Speciation- The hypothesis that it is the
accumulated changes in separated populations that
eventually leads to the formation of new species.
! Natural selection- A hypothesis describing deterministic
changes in populations that lead to adaptation and
divergence in populations perhaps promoting speciation.
Georges Cuvier
(1769 - 1832)
Charles Lyell
(1797 - 1875)
Have life forms changed over time?If the hypothesis of evolution is true, the species that lived in the
remote past must be di!erent from the species alive today.
Cuvier showed the inescapable
reality of extinction.
Charles Lyells Principles of
Geology also discussed fossils
found in stratigraphic rock and that
more ancient strata often had very
different organisms.
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Have life forms changed over time?If the hypothesis of evolution is true, the species hat lived in the
remote past must be di!erent from the species alive today.
Have life forms changed over time?If the hypothesis of evolution is true, the older the sedimentary strata,
the less the chance of finding fossils of contemporary species.
Percentages of Tertiary (65 - 1.8 m a) species still livi g (from Lyell 1854)
Fossil Species Alive Today % Fossil St il l A live
Recent Pliocene
(1.8 - 3 mya)226 216 96
Older Pliocene
(3 - 5.2 mya)569 238 42
Miocene
(5.2 - 23.8 mya)1,021 176 17
Eocene
(33.5 - 55.6 mya)1,238 42 3
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Have life forms changed over time?If the hypothesis of evolution is true, then we would expect to find simpler
organisms in older strata and more complex ones only in more recent strata.
! Be very careful with your thinking on this question. Evolution by natural selection does
not meanthat organisms necessarily must become more complex, only that individuals
better able to survive and reproduce will leave more o"spring and evolution will occur if
the cause of their competitive advantage is heritable.
! However, because evolution can only work with what is present, it is necessary that
complex phenotypes evolved from simpler types.
! Many parasites, for example, become less complex as they lose traits that were important
for living outside of the host.
! Looking at the major evolutionary events depicted in the geological time scale, it is
easy to see this is true. For example, multicellular organisms or metazoans capable
of producing a shell are not found in strata dating to the Archaean.
Jean-Baptiste Lamarck
(1744-1829)
FIRST LAW In every animal which has not passed the limit of its development, a morefrequent and continuous use of any organ gradually strengthens, develops and
enlarges that organ, and gives it a power proportional to the length of time it has
been so used; while the permanent disuse of any organ imperceptibly weakens
and deteriorates it, and progressively diminishes its functional capacity, until it
finally disappears.
SECOND LAW All the acquisitions or losses wrought by nature on individuals, through theinfluence of the environment in which their race has long been placed, and hence
through the influence of the predominant use or permanent disuse of any organ;
all these are preserved by reproduction to the new individuals which arise,
provided that the acquired modifications are common to both sexes, or at least to
the individuals which produce the young.
Pre-Darwinian views of Evolution
Lamarks two hypotheses:
! Adaptationoccurred as a consequence of the inheritanceof acquired characteristics.
! Species could change over time, but, as we will see, he did
not propose a theory of common descent.
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Robert Chambers
1802 - 1871
Pre-Darwinian views of Evolution
1844 - Vestiges of the Natural History of
Creation
Published anonymously.
Was wildly popular with the public.
Denied the plausibility of Larmarks mechanism of
evolutionary change, but he did not provide his
own.
Recall that Darwin had already conceived of his
evolutionary hypotheses by 1838. He read
Vestigesand immediately recognized that it was
not a credible scientific theory.
Darwins theory of common descent
! Darwins theory of common descent was a unique
contribution to our understanding of life.
! The explanatory power of the theory of common
descent led to its rapid acceptance by the majority
of biologists.
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Pre-Darwin views of common descent
Pre-Darwin views of common descent
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Geneticsand Evolutionexplain both similarities and di"erences at
di"erent levels of biological organization.
Major Themes of the Course
Order: Primates
Class: Mammalia
Phylum: Chordata
Family: Hominidae
Kingdom: Animalia
Genus:HomoSpecies:sapiens
Geneticsand Evolutionexplain both similarities and di"erences at
di"erent levels of biological organization.
Major Themes of the Course
Order: Primates
Class: Mammalia
Phylum: Chordata
Family: Hominidae
Kingdom: Animalia
Genus:Homo
Species:sapiens
The reason whymembers of the family
Hominidae more closely resemble each other is
that they shared a common ancestor morerecently than with members of species from
other Families.
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Geneticsand Evolutionexplain both similarities and di"erences at
di"erent levels of biological organization.
Major Themes of the Course
Order: Primates
Class: Mammalia
Phylum: Chordata
Family: Hominidae
Kingdom: Animalia
Genus:HomoSpecies:sapiens
Geneticsand Evolutionexplain both similarities and di"erences at
di"erent levels of biological organization.
Major Themes of the Course
Order: Primates
Class: Mammalia
Phylum: Chordata
Family: Hominidae
Kingdom: Animalia
Genus:Homo
Species:sapiens
The reason whymembers of the class
Mammalia more closely resemble each other is
that they shared a common ancestor morerecently than with members of species from
other Classes.
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Geneticsand Evolutionexplain both similarities and di"erences at
di"erent levels of biological organization.
Major Themes of the Course
Order: Primates
Class: Mammalia
Phylum: Chordata
Family: Hominidae
Kingdom: Animalia
Genus:HomoSpecies:sapiens
Evidence of common ancestry?If the hypothesis if evolution is true, then closely related organisms
should possess similar morphology, physiology, behavior and genetics
Humans, moles, horses, dolphins and bats are all mammals and descended
from a common ancestor that possessed particular bones of the arms and
hands (the humerus, radius, ulna, carpals, metacarpals and phalanges).
Homology
Similarity between species due to their common ancestry.
With some subtle variations, allliving things utilize the same
genetic code, consistent with the
hypothesis that all living things
share a common ancestor.
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Evidence of common ancestry?If the hypothesis if evolution is true, then closely related organisms
should possess similar morphology, physiology, behavior and genetics.
There are striking homologies in early
development among amniotes.
Evidence of common ancestry?If the hypothesis of evolution is true, then organisms may often
posses vestigial structures belying their history.
Vestigial structures are rudimentary versions of structures with
functions in closely related species.
Spur on a rubber boa (remnant
hind limb)
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! Almost all* mammals are capable of making their hair stand on end.
! Insulation when cold! Exaggeration of size when threatened or when trying to achieve
dominance.
! Attached to the base of hair follicles are small muscles called
arrector pili, that cause hair to stand up when contracted.
! Our closest relative, chimpanzee does have arrector pili and uses
hair raising in displays of dominance.
! As naked apes raising hair wouldnt work to keep us warm or
exaggerate our size, but do we still have arrector pili as a
consequence of evolving from an ancestor in which they were likely
functional?
! Yes. And as expected our arrector pili are contracted when we are
threatened or cold.
Evidence of common ancestry?If the hypothesis of evolution is true, then organisms may often
posses vestigial structures belying their history.
Evidence of common ancestry?If the hypothesis if evolution is true, then closely related organisms
should possess similar morphology, physiology, behavior and genetics.
Not all similarities are due to shared ancestry.
Homoplasy (analogous traits)Similarities betweenorganismsnotdue to common descent.Most often
arises as a consequence of the convergent
evolutionof adaptive structures.
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Evidence of common ancestry?If the hypothesis of evolution is true, then there should have been connecting
forms between the major groups (phyla, classes, orders)
Evidence of common ancestry?If the hypothesis of evolution is true, then there should have been
connecting forms between the major groups (phyla, classes, orders)
Archaeopteryx was discovered shortly
after Darwin published the Origin. It
had feathers, but also very reptile like,
with 3-clawed hands, teeth and a long
bony tail.
Other examples of feathered
dinosaurs have now been found,
especially in Chinas Liaoning
Province.
For a discussion of the many transitional fossils of vertebrates that have been found see:
http://www.talkorigins.org/faqs/faq-transitional.html
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Darwins theories of gradualism and population
speciation.
! It was Darwins view that the origin of new species was due to processes occurring within separated
populations (especially the action of natural selection) and not due to saltational leaps whereby newspecies would arise quite suddenly (even instantly).
! Darwin understood that gradualismand the tremendous diversity of extinct and extant life meant thatthe earth must be very very old.
! The hypotheses of gradualism and population speciation are unique contributions by Darwin. Theconcept of population speciation has no antecedent.
Gradualism and the debate between
the Mendelians vs. Biometricians
! The rediscovery of Gregor Mendels work on transmission
genetics in 1900 quickly eroded into a conflict over the
hypothesis of gradualism that occupied scientists for thenext 20 years.
! Mendelians (most strongly advocated by Bateson)
! Believed Mendelian genetics was incompatible with
the gradual evolution favored by Darwin.
! Biometricians(including Karl Pearson and Walter Weldon)
! Held that variation in nature was continuous and not
discrete as the Mendelians suggested.
! The resolution of this debate required the development of
quantitative genetics.Carl Correns
William Bateson
Hugo de Vries
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Antecedents of gradualism:
Charles Lyell and uniformitarianism
! Uniformitarianismis the proposal that geological features are the result of the accumulated
action of processes that can be observed today. This is in contrast to catastrophism.
! This view influenced Darwin in 2 ways:
! There are natural and material explanations for historical processes
! The accumulated e"ect of a process over long periods of timecan have dramatic e"ects
Has there been enough time for evolution?If the hypothesis of evolution is true, the age of the earth must be
very great, possibly many hundreds of millions of years old.
! By Darwins time, the 4004 B.C. date
was seen as impossible. Geologistsreckoned the age of the earth had to be
many millions of years.
! These geologists were already putting
together a relative dating to the rock
formations of Europe.
! Using radiometric dating geologists
have now placed dates on the geologic
time scale originally produced by these
geologists.
James Ussher
Archbishop of Armagh
1625-1656
Estimated the beginning of time as,
upon the entrance of the night preceding
the twenty-third day of October in 4004
B.C.
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! Age of the Earth estimated ~4.6
billion years.
! Origin of life ~3.5 - 3.7 billion years
ago.
The ancient age of the earth
means that there has been
more than enough time for
evolution to have led to the
observed biological diversity.
Has there been enough time for an evolutionary
accounting of the earths biodiversity?If the hypothesis of evolution is true, the age of the earth must be
very great, possibly many hundreds of millions of years old.