Darwin's Evidence:  Variation
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© 1997
David H.A. Fitch
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Lecture notes

Darwin's evidence for evolution: Variation

I.  Chapter 1:  Variation under domestication
 

A.  What is the biological nature of "variation"? (How does it arise?)
1.  There's lots of variation (even more than between related wild species).
2.  Darwin suggests that the source of variation (i.e., mutation) is in "reproductive elements (i.e., germ cells) prior to conception"
3.  Variation is "random" (i.e., no "inherent trends")
4.  Variation is heritable
5.  Variation in domestic varieties is different than in wild populations.  (In what way?  Why does Darwin stress this observation?)
 

B.  How did different varieties originate?  Darwin uses pigeons as a model to study Variation and its origin
1.  Why pigeons?
2.  How did different breeds originate?  2 alternative hypotheses:
a.  Many origins (from many aboriginal species), or
b.  Single origin (1 species, the rock pigeon, Columba livia).  If true, this would suggest that several divergent varieties could originate from a single ancestral variety.  What is the evidence that supports this alternative?  How does Darwin extrapolate from this conclusion to suggest that different species arise from a common ancestral species?  (Hint:  use his arguments about the kind of variation that exists in domestic varieties along with his next arguments about varieties being incipient species.)
 

II. Chapter 2:  Variation in nature
 

A.  It is very difficult to differentiate species and varieties (especially when you become an expert about a particular group)
B.  Metaphor:  This lack of delineation suggests a series, which suggests a "passage"
C.  Terms "species" and "variety" are "arbitrary"
D.  Darwin tests the hypothesis that varieties are "incipient species" (species arise from varieties, and varieties are thus potential species) (Data is on page 55 of Darwin)
1.  Prediction of cladogenetic Evolution if this hypothesis is correct:  Where many species have arisen (e.g., in larger genera), there should be many "incipient species"
2.  Prediction of Transformism:  The number of varieties should be unrelated to the number of species in a genus (e.g., the number of varieties could be the same, regardless of which genus a species came from)
3.  Prediction of Separate Creation:  No particular pattern is predicted
 

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Exercises

  1. What are 3 patterns observable in the variation of domesticated pigeons that support branching evolution?  How does each pattern support this hypothesis?  Do any of these patterns falsify alternative hypotheses (e.g., special creation or transformism)?  If so, how?
     
  2. How does Darwin's data on the numbers of varieties in large versus small genera support branching evolution?  What are the other patterns in this tabulation of data that he uses to support evolution?  How does this data appear to refute both special creation and Lamarckian transformism?

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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]

[Darwin]

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