Bio 1B, Spring, 2007, Evolution section 1 of 4
Lecture 2
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The only change is on line 6 of page 3: “separation” is replaced by “separate.”
 2 Difficulties and Explanatory Power
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Reading: 7th edition 448-451; 6th edition 438-442.
 The pattern of evolution in the Origin
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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.”1 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.
Darwin acknowledged that, if intermediate forms are not found either as living
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All quotations are from the first edition of The Origin, available at
http://www.gutenberg.org/etext/1228.
Bio 1B, Spring, 2007, Evolution section 2 of 4
Lecture 2
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Updated 2/26/07 10:56 AM
species or as fossils, we cannot be certain of what they looked like and what
advantage they had.
In discussing the vertebrate eye, which Darwin described as “an organ of extreme
perfection,” he argued on the basis of plausibility. He described a sequence of
steps leading from very primitive light-gathering organs to complex eyes, each of
which is was improvement on the earlier form.
Darwin emphasized that organs arising for one function can later serve another.
He used the swim bladder of fishes as an example. The swim bladder is used for
flotation in all fishes but it is also used as an organ of hearing in some species.
Darwin said that some structures appear to be of little or no direct use. He said
that some of those structures have a use that we do not understand and others may
have had a use in the past. But he rejected the idea that any structures were
“created for beauty in the eyes of man or for mere variety.” And he continued
“This doctrine, if true, would be absolutely fatal to my theory.” In other words,
he introduced the idea that his theory is falsifiable.
 Explanatory power
The rest of the Origin was devoted to showing that Darwin’s theory could explain
what was known about nature.
 Fossil record
• Darwin said that the fossil record was incomplete, but that what was known was
consistent with his theory.
• For example, he said that new species and new groups do not appear all at once.
New major groups, such as mammals, appear first in the fossil record as a few
species.
• Also, extinct groups are related to each other and to existing groups. Many
extinct mammals were known but they were all regarded as similar to existing
mammals.
 Geographical distribution
• Darwin said that geographic barriers to dispersal determine where a group of
related species is found. The geographic range of a group is determined by its
ability to disperse from where it originated.
• Darwin used the agouti and the capybara, which are found in South America, as
examples. They arose in South America and could not disperse to North
America. They are very similar to the hare and muskrat, which are found in
North America.
 Linnaean classification
• Classification of plants and animals was and is done using a hierarchical system
introduced by Linnaeus in the 1700s. Species are grouped into genera, families
and larger groups. Darwin said that a hierarchical system is to be expected under
his theory. The different hierarchical levels indicate different ages of common
ancestors. The only figure in The Origin illustrates his point.
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Bio 1B, Spring, 2007, Evolution section 3 of 4
Lecture 2
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 Darwin vs. separate creation
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In the Origin, Darwin frequently compared the ability of his theory with the
theory of separate creation of species to explain what was known about nature.
Imperfection of adaptation
Darwin emphasized that natural selection would not result in perfection of
adaptation: “Natural selection tends only to make each organic being as perfect
as, or slightly more perfect than, the other inhabitants of the same country with
which it has to struggle for existence.”
One of Darwin’s strongest arguments against the separate creation of species is
the apparent lack of perfection resulting from common descent: “He who believes
that each being has been created as we now see it must occasionally have felt
surprise when he has met with an animal having habits and structures not at all in
agreement.” He cited the webbed feet of geese that never or rarely go near water
as an example.
Homology
Darwin said that that similarity of structures in different species that are used for
different purposes, what is now called homology, is explained by descent from a
common ancestor having that structure. Darwin used the hand of a human, the
front paw of a mole, the leg of a horse, the paddle of a dolphin and the wing of a
bat as an example. These limbs are not necessarily perfectly adapted to their
functions.
Similarities during development
Species that are quite different as adults are sometimes very similar early in
development. Darwin cited the example of the larvae of flies, moths and beetles.
An example given in the text is of pharyngeal pouches that appear during the
development of all vertebrates. As in the case of homologous organs, similarities
during development indicate common descent, not optimal design.
Rudimentary organs
Darwin described the rudimentary pelvic and hind limbs found in some snakes as
showing that snakes descended from species with fully developed hind limbs.
Darwin said that “rudimentary or atrophied organs are imperfect and useless.” He
noted that their presence is simple to explain with his theory but they present a
“strange difficulty” for the “doctrine of creation.” In some snakes, including
boas, rudimentary legs are now known to play a role during courtship.
 Questions (correct answer is underlined)
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d.
In the Origin of Species, for what purpose did Darwin use rudimentary organs?
They illustrated the struggle for existence.
They illustrated the perfection of adaptation.
They illustrated the importance of geographic variation.
They illustrated that species that possessed them descended from other species in
which those organs were functional.
e. They illustrated that evolution could happen quickly.
Bio 1B, Spring, 2007, Evolution section 4 of 4
Lecture 2
Updated 2/26/07 10:56 AM
 In the Origin of Species, what did Darwin say about the possibility that some
characteristics of plants and animals were present because they appeared
beautiful to humans?
a. If that were true, his theory would be false.
b. It would illustrate the power of natural selection.
c. It would demonstrate descent with modification.
d. The importance would depend on whether the characteristics were found in only
one geographic region.
e. It could happen in animals but not plants.
 In the Origin of Species, what was the point of Darwin’s discussion of the swim
bladder of fishes?
a. The swim bladder is found only in some fishes.
b. The swim bladder first served for flotation and in some fishes became later an
organ of hearing.
c. The swim bladder is a rudimentary character.
d. The swim bladder appears early in the development of mammals.
e. The swim bladder is found in some reptiles.
 In the Origin of Species, what was the point of Darwin’s discussion of the
rudimentary legs of some snakes?
a. There must be some purpose they serve.
b. They indicate descent from a species with fully developed legs.
c. They must have evolved because of sexual selection.
d. They would not be present in a domesticated species of snake.
e. They do not present a problem for his theory because some species of snakes do
not have rudimentary legs.