slatkin2 - Bio 1B Spring 2007 Evolution section 1 of 4...

Bio 1B, Spring, 2007, Evolution section 1 of 4 Updated 2/26/07 10:56 AM
Lecture 2
The only change is on line 6 of page 3: “separation” is replaced by “separate.”
 2 Difficulties and Explanatory Power
• Reading: 7th edition 448-451; 6th edition 438-442.
 The pattern of evolution in the Origin
• Darwin argued that natural selection had to produce gradual change. Descent with
modification occurred very slowly but, because of the vast amounts of time
available, very large changes would result. Darwin said that intermediate forms
that represented successive improvements on previous forms had to have existed.
• He explicitly rejected the idea of sudden, discontinuous changes in species.
 Difficulties for Darwin’s theory
• After introducing his theory in the first four chapters of the Origin , Darwin
devoted a chapter (Ch. 6 — Difficulties on Theory) to discussing what he
anticipated would be objections to his theory and presenting his responses to those
objections.
 Lack intermediate or transitional forms
• The fossil record at that time did not provide many examples of forms that were
intermediate between major groups. And living species usually did not represent
intermediates between major groups either. There appeared to be discontinuity
rather than continuity.
• To answer this objection, Darwin argued that transitional forms were not usually
preserved as fossils because the fossil record is “incomparably less perfect than is
generally supposed.”
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He said that the lack of intermediate forms did not prove
that they never existed.
• Darwin said that living intermediate forms would not usually be seen because
species would continue to change. For example, there are no living intermediates
between elephants and other mammals because the intermediate forms evolved
into modern elephants.
• Darwin’s noted that some intermediate forms do exist. He used the example of
the origin of aquatic mammals. He said that the American mink, which has
webbed feet, is intermediate between terrestrial and aquatic mammals.
 Large transitions
• Darwin said that an important difficulty for his theory is understanding how
natural selection can cause major transitions that resulted in “organic beings with
peculiar habits and structures.” If natural selection was the cause of all changes,
then each intermediate type had to be an improvement on the previous type.