Most people do not understand precisely what it was that Darwin did when he published his famous The Origin of Species. He was working against the established scientific "theory" of the time, which was known as "Natural Theology". Natural Theology assumed that species had been specially created by God and were immutable. Although they could hover around a kind of idealized form, two different species were assumed not to have had a common ancestor.
Darwin did not come up with the idea of common descent, transmutation of the species, or inheritance. All of that had been "discovered" by others and was already in the public record as hypothetical challenges to "Natural Theology". He did not discover genetics, the mechanism by which animals inherit characteristics. He did not rely primarily on the fossil record, which was nowhere near as complete in the mid-19th century as it is now. Natural Theology had its critics and competitors, but it represented the standard theory of most scientists of that time. Darwin was only one of the challengers, and he laid low for most of his life, preferring to delay publication of what he knew would be a scientific bombshell. But what was his bombshell? His conclusions of transmutation of species and common descent were already out there.
The single greatest insight of Darwin's work was the mechanism of natural selection, which was not itself a totally new idea. Humans had known about artificial selection since almost the beginning of recorded history. The Bible even explained how it worked. People took advantage of natural variation within a species and heritability of traits to create desirable enhancements in animals and plants. It could have been possible to mount a religious theory of transmutation and common descent by taking "natural selection" as God's artificial breeding program. And this seems to be what most people today actually believe evolution is about today--God's special breeding program to evolve humans from the "lower animals".
Darwin's bombshell was his argument that evolution was unguided by intelligent design. His mode of discovery was not just to examine the fossil record, which most people nowadays see as definitive proof of evolution. Rather, Darwin looked at biogeological diversity. He looked at living species that were closely related, and he noticed coincidental patterns in the diversity. Like species tended to cluster together geographically and temporally. Their differences tended to take advantage of the differences in climate and other external factors. The idea of common descent explained this skewed pattern of diversity, and natural selection explained why the differences took the shape that they did.
It was Thomas Malthus who really inspired Darwin. Malthus had pointed out that all species tended to produce more members of a species than could possibly survive in an environmental niche, and population pressure acted like a "wedge" to drive out competing species. Darwin put Malthus's insight together with natural biogeological diversity, and he had his "eureka" epiphany. And he sat on the idea for many years before publishing. He made his real reputation studying natural diversity in barnacles, of all things. For 9 years, Darwin did nothing but study barnacles. He made is initial reputation as the Barnacle Bill of biology. And it was the intraspecies variation that fueled his insight and his interest, because he knew that this was behind the Malthusian mechanism.
Evolution theory exploded into controversy, but it was not universally accepted overnight. Scientists, especially devoutly religious scientists, fought it bitterly for decades afterwards. The opposition still rears up in the so-called "scientific theory" of Intelligent Design. But the clinchers for evolution theory came after Darwin introduced natural selection. The fossil record continued to confirm the expectations that Darwin set scientists up with, and the completely independent discovery of genetics clinched the deal beyond all reasonable doubt. When DNA was discovered, evolution theory was no longer strictly in need of corroboration, but the corroboration has never stopped in well over a century of denial and opposition by very passionate, intelligent deniers. It is a tribute to Darwin's intellect that so many people still find it so difficult to accept the truth of his discovery. Natural selection has no goal and no direction. It just acts as a filter to pass through traits that allow the descendants of living organisms to adjust to environmental change.
Source: David Quammen. The Reluctant Mr. Darwin.
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