People generally speak of the "Theory of Evolution" as if it's a single, monolithic concept. It isn't; there are two major parts of this theory.
The first part of the theory is the idea it proposes about where all of the diverse forms of life on Earth came from the "what," part of the theory. This notion Darwin called common descent.
Darwin believed (and virtually all modern scientists agree with him) that all of the millions of species alive on Earth today are ultimately descended from a single ancestral species.
(Actually, Darwin wasn't initially sure whether he thought there had been only one ancestral species or a very few e. g. one for animals, and a separate one for plants. Today, we're pretty much sure there was only one.) Contrary to what many believe, other than a short, rather poetic paragraph in Origin of Species, Darwin's theory doesn't say anything about the origin of that ancestral species the Origin of Life isn't part of the Theory of Evolution. Darwin believed in the common descent of life based upon the magnificent examples of adaptive radiation and other biogeographical phenomena which he observed on his journeys. Today, we find additional and very compelling evidence in the DNA and proteins of living species.
The second part of Darwin's theory was actually more controversial among the naturalists of his day than the notion of common descent was. This was his suggestion of a possible mechanism to explain where all of the diversity of life came from the "how" of his theory. This, of course, was natural selection.
Darwin attributed much of the important part of this idea to the ideas of Thomas Malthus, an economist a generation or so older than Darwin was. Malthus scared the pants off everyone by pointing out that human populations tended to increase faster than their food supplies did unless something like a war, or famine, or plague came along to knock the population size back down.
Darwin saw in this notion the core of a possible explanation for changes in species. Here's the way he reasoned:
From Malthus come these ideas:
Observation: Populations tend to increase in size every generation.
Observation: Resources such as food tend not to increase as rapidly as population sizes do.
Conclusion: Not all members of a population can survive; some must die.
Then Darwin himself added these concepts:
Observation: All populations display diversity in many of their characteristics.
*material*storybehind2.htm*endmaterial* (See "The Story Behind the Story Part 2")
Observation: These variations often influence just which population members are most successful at acquiring the resources they need to survive and successfully rear offspring (thus contributing to the next generation).
Conclusion: The more successful individuals contribute more to the next generation of population members than the less successful ones do.
Observation: Many or all of the variations seen in populations are passed from parent to offspring.
Conclusion: The general characteristics of the next generation will be more like the successful parents than like the unsuccessful ones. The population (and the species) has changed.
Since this goes on every generation, the realization is inevitable that, given enough time, a species can be altered beyond recognition. If you add the final understanding (thanks to Lyell and Uniformitarianism) that the Earth is a very, very old place, you begin to see the incredible power of this concept.
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Updated 25 September 2004