Before Darwin the general consensus was that species were created independently by some extranatural force, and that they died out without any major change. So, which were the steps needed for a theory of evolution of the biological species to be accepted in the XIX century?
- Accepting the idea that species can change during their life span.
- Accepting the idea that species can change so much that they can even generate new species, either by diversification (cladogenesis) or by a global transformation of one whole species (anagenesis).
- Accepting the idea (that looks like an inevitable result of the previous one) of a common descent, i. e., that all species are related, have a kinship, and they all come from a common ancestor.
- Accepting the idea that the Earth is old enough for these huge changes to have enough time to occur... so slowly that can't even be perceived during a human life span.
- And most notably: knowing of any acceptable mechanism by which these types of changes can occur.
Considering that not so long ago, the Anglican bishop James Ussher had stated that the Earth had been created the night preceding 23 October 4004 BC, the idea of an old Earth was, quite possibly, the necessary precondition for all the others to be even borne in mind by anyone. And this revolutionary idea was just there in the right moment, when Darwin started his five years long journey in the Beagle, and was given the first volume of Charles Lyell's Principles of Geology, which set out the idea of masses of land slowly rising or falling over immense periods of time to finally yield the geological features that can be observed in present time. According to Lyell, the Earth was much older than what was thought by that time, probably even millions of years old (some 4,560 years old, to be more precise, and as we know today).
The concept of a gradual change of biological species, now that Darwin knew that there was enough time for it, arose from the observation of different races of tortoises, finches and others, adapted to the particular environments that the Galápagos islands had to offer. And more particularly, to the different nutritional niches available; while some finches had a beak specialised in cracking hard nuts, others were the perfect weapon for chasing insects, etc. And considering that those islands are some 1,200 km away from the continent, it was clear to Darwin that all those finches (or tortoises) had had to come from one (or a few) small group of ancestors that, departing from the mainland, happened to make their way to the islands. The Galápagos finches had evolved on-site, as it was highly unlikely that all those varieties could have possibly reached the islands from the continent, one by one.
That panorama was clearly speaking about gradual transmutation of those animals by adaptation to different environments (species can change), and with it, Darwin's mechanism of evolutionary change had started to develop.
But was that slow and gradual change powerful enough to produce new species, or were all those differently adapted finches simple varieties of the same species? Friends are to come to the rescue when needed, and so it was with Darwin's mate John Gould, an ornithologist, who announced that the specimens of finches that Darwin brought back to England after his trip belonged, in fact, to three different species. In Darwin's mind, that meant that gradual adaptation to the environment can actually produce new species.
Finally, how do species get to adapt to each one of the many specific environments that Nature provides? How can a population gradually reach its ecological niche, one that allows it to survive and thrive for generations? That was the last and greatest obstacle to overcome, and recent History showed that clearly. Jean-Baptiste de Lamarck had already come up with similar thoughts to Darwin's a couple of decades before the Beagle departed: species evolve, and do it by gradual adaptation to their environment. But Lamarck could never demonstrate that acquired-on-life traits can be passed on to the offspring, and we know nowadays that that just can't happen: the genes of your eggs or sperms will not change because of you dying your hair. And this is how Lamarck passed to History as an unsuccessful attempt to explain the how and the why of the evolutionary process.
But not Darwin. It turns out that among the many different types of individuals that are randomly produced every generation in every species, some happen to be better suited to their environment than others: the former feed better, grow faster, survive longer and as a result... reproduce more than the latter. And as the traits that make them the fittest were inherited, they will also pass those traits to their children. As a final result, those traits will be more present in the next generation than those that conferred a lesser success. This was what Darwin (and Alfred R. Wallace) elucidated, and that's why everyone is now celebrating his 200th anniversary. Or almost everyone.