© 1997
David H.A. Fitch
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• Lecture notes
• Exercises

Lecture notes

Darwin's evidence for evolution:  "Geological distributions" of species

I.  Previous chapter (Ch. 9):  Imperfection of the geological record
Despite Darwin's argument that the fossil record is likely to be a poor and biased sampling of historical events leading to the origin and evolution of species, he nevertheless goes on to show how it is consistent with branching evolution.
 

II.  Geological Succession (Ch. 10)

A.  New species have appeared gradually (i.e., gradual increase in the numbers in a particular group in successive strata)
Suggests diversity originates from a single, early source

B.  All species have changed, but rates of change are unequal (e.g., there are some "living fossils" that have changed little over long periods of time)
Each species varies independently (rate is not intrinsically determined, and thus does not conform to an orthogenetic mechanism of transformism)

C.  Rates of change may be unequal in consecutive strata
Could be due to unequal rates of sedimentation (e.g., lakes dry up)

D.  Species do not reappear after they become extinct
Heritable lineages must be continuous

E.  Extinctions are not usually the result of catastrophic events
Extinction is part of the process of Descent with Modification:  parental species are continuously replaced by modified descendants
(Note that Darwin does NOT say that catastrophic events do not occur, but that the extinctions due to catastrophies are rare compared to the normal extinctions that must occur as a result of branching evolution)

F.  Simultaneous changes in form (in different geographical regions) can be explained by waves of migrations (i.e., dispersal)
1.  Changes can only be inherited, and do NOT arise simultaneously, so Darwin must explain these observations by (geologically) rapid dispersal mechanisms
2.  Such "simultaneous changes" are usually demonstrable only for marine forms that have broad and rapid dispersal patterns
3.  Also invokes the great expanse of geological time between strata to allow dispersal

G.  Extinct forms usually show intermediate features and make a "more perfect classification"
1.  Differences between later (or extant)species in a group are generally greater than between extinct species in a group
2.  As species diverge from a common ancestor, they accumulate more differences

H.  Fossil mammals in a particular area are more closely related to living mammals of that area than to fossils from the same stratum in different geographical regions
1.  In recent history, ancestral and descendent species occupy the same region
2.  Thus, descendents are more closely related to their immediate ancestors than to descendents of other ancestors in different regions
 

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Exercises

1. How can Darwin legitimately spend an entire chapter arguing about the imperfection of the fossil record and then go on to show how the fossil record can be used to support evolution?
    
2. Why does Darwin stress the point that extinctions are not usually catastrophic?  Are catastrophic extinctions good evidence in favor of creation over evolution?  (Hint:  Is there anything about the mechanism proposed for creation that necessarily predicts that extinctions must be catastrophic?  Is there anything about descent with modification that necessarily predicts that extinctions cannot be catastrophic?)
    
3. Why does Darwin take such great pains to explain the observation of "simultaneous" changes in forms in a particular stratum at different geographical locations?  How does he explain such observations (beyond the fact that the observations are overly simplistic about such forms being exactly the same)?
    
4. How is descent implied in each one of his observations?

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