2012lecture02 charles darwin

8/13/2019 2012Lecture02 Charles Darwin

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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.



unchecked, increases in geometrical ratio. Subsistence increasesonly in an arithmetical ratio. A slight acquaintance with numbers


second.


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second.



unchecked, increases in geometrical ratio. Subsistence increasesonly in an arithmetical ratio. A slight acquaintance with numbers


second.


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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




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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Order: Primates

Class: Mammalia

Phylum: Chordata

Family: Hominidae

Kingdom: Animalia





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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Order: Primates

Class: Mammalia

Phylum: Chordata

Family: Hominidae

Kingdom: Animalia


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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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.


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.

2012lecture02 charles darwin

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