Darwin's Evidence for Evolution
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© 1997
David H.A. Fitch
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Lecture notes

Darwin's Origin of Species:  Introduction

I.  Darwin (1859, in his Origin of Species) proposed 2 main theses:
 

A.   Descent with Modification ("Evolution") has occurred by branching (different species share common ancestral species)—
The evidence Darwin cited to support this thesis (and to refute alternatives, such as Lamarckian transformism and special creation) is reviewed in the pages that branch from this page (see the Site Tree for an overview of the structure of this site).
 

B.  Natural Selection is the main (but not exclusive) mechanism for this evolutionary divergence of lineages, and explains adaptations—
For a review of how Darwin presented this mechanism, go to the page titled Darwin's Mechanism.
 

II.  Evolution (what Darwin more descriptively called "Descent with Modification") embodies 2 separable aspects:
 

A.  Change within a lineage (Modification = Anagenesis)
 

B.  Branching (different species share ancestors:  Descent = Cladogenesis)
(Adaptation is not explained by this divergence alone, and requires the explanation provided by Natural Selection)
 

III.  Formal scientific hypotheses make testable (falsifiable) predictions
One reason for the long term success of Darwin's book was his consideration of alternative hypotheses, the predictions of these hypotheses, and how the results of his observations and experiments could be explained (or left unexplained) by these hypotheses.
 

A.  Separate ("Special") Creation (e.g., Cuvier)—Species are fixed entities that neither change nor give rise to different (descendant) species; no mechanism is proposed that is measurable (thus severely limiting the ability to make predictions).
 

B.  Transformism (e.g., Lamarck)—Species lineages change over time along a predetermined pathway (e.g., toward increasing complexity), but branching does not occur; orthogenesis is one proposed "mechanism".

  • Optional reading:  Stephen J. Gould.  1993.  What the immaculate pigeon teaches the burdened mind.  In:  Eight little piggies.  W. W. Norton, New York.  (A lucid example of the use of an orthogenetic mechanism in the study of pigeon variation in the early part of this century.  On reserve in Bobst Library.)
     

C.  Branching "Evolution" (as per  Darwin)—Species lineages usually change over time (though rates of change may vary) and are derived from common ancestral lineages; natural selection is the main, but not exclusive, mechanism.

  • Optional reading:  Stephen J. Gould.  1977.  Ontogeny and Phylogeny.  Belknap Press of Harvard University Press, Cambridge, Massachussetts.  (Take special note of the account of the origin of the modern meaning of "evolution" from preformationist ideas about developmental mechanisms.)
     

IV.  Darwin's evidence for evolution

In his Origin, Darwin presents his evidence supporting "descent with modification" (branching evolution)  in several chapters:

A.  In Chapters 1 and 2, he considers how patterns of variation in domesticated and wild fauna and flora support descent with modification.

B.  In Chapters 10-12, he shows how the patterns of species distributions in time and space support branching evolution from common ancestral lineages.  He considered this evidence to be especially strong.

C.  In Chapter 13, he shows how branching evolution explains a range of different phenomena:  e.g., why a hierarchical system is used for classification, why morphological features used for completely different functions nevertheless show "homology" in form, and why vestigial ("rudimentary") organs exist.

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Exercises

  1. What is a "testable" hypothesis with regard to historical events?  What kind of "evidence" is required to test such hypotheses (short of time travel)?  (Hint:  how are murder cases solved in the absence of eyewitnesses?)
     
  2. What is meant by "prediction" in this case?  Do any of the mechanisms proposed by the alternative hypotheses listed above necessarily "predict" that any particular species should have arisen?  Do any of the hypotheses "predict" what species will appear or disappear in the future?  If not, why is the word "prediction" used?  (Hint:  what are the kinds of "predictions" made by detectives in murder cases?)

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 Variation  Fossils  Geographic
Distribution  Morphology  Classification  Vestigial
Organs  Embryology
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[Variation] [Fossils] [Geographic Distribution] [Morphology] [Classification] [Vestigial Organs] [Embryology]

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