Bio 1B, Spring, 2008, Evolution section 1 of 4 Updated 2/29/08 1:32 PM Lecture 2 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 because of small changes in the composition of a species but, because of the vast amounts of time available, very large changes would eventually occur. Darwin said that intermediate forms which 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 there were rarely extant species that were clear intermediates between major groups either. • 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 in the fossil record 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 species or as fossils, we cannot be certain of what they looked like and what advantage they had. • 1 All quotations are from the first edition of The Origin, available at http://www.gutenberg.org/etext/1228.Bio 1B, Spring, 2008, Evolution section 2 of 4 Updated 2/29/08 1:32 PM Lecture 2 • 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 was an 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 depends on 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. Darwin vs. separate creation • 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.Bio 1B, Spring, 2008, Evolution section 3 of 4 Updated 2/29/08 1:32 PM Lecture 2 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 separation creation of species is the apparent lack of perfection, which Darwin said was caused by 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,
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