It's October 2nd 1836, and the HMS Beagle has just reached Falmoth, Cornwall. Exiting the ship that day was a young Charles Darwin, fresh from his 5-year voyage and brimming with new ideas. Already somewhat of a celebrity in scientific circles, largely due to his geological letters and extensive fossil collections, Darwin was in the process of formulating the theory he would publish 23-years later. In what would later be referred to as "the busiest two years of his life," he began an extensive search for the mechanisms underlying evolutionary processes. Two particularly salient sources of influence came from economics, namely: Adam Smith and Thomas Malthus.
The first of these, Adam Smith, is largely celebrated for his notion of the invisible hand: that individual competition in unfettered markets leads to an efficient economy, functioning for greater good through the production of wealth and prosperity. Basically, in a situation where there is competition between businesses, an incentive exists to introduce improved products and cost-saving innovations, with those providing the best product being more likely to lure a greater number customers. Over a period of time, on the basis of Smith's predictions, rivals will eventually bring in similar products and produce similar innovations that results in a ratcheting process: driving down prices and profits.
Now, whether or not you subscribe to Smith's interpretation of economic markets, the important point to take away is how gradual, unconscious processes lead to change -- be it in markets or species. But it would take our second source of influence, Thomas Malthus, for Darwin to arrive at his mechanism of natural selection. In An Essay on the Principle of Population, Malthus puts forward his observation where the finite nature of resources is in conflict with the potentially exponential rate of reproduction, leading to an inevitable struggle between individuals. What Darwin did was take this basic premise and apply it to nature, as he notes in his autobiography:
In October 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The results of this would be the formation of a new species. Here, then I had at last got a theory by which to work.
Having sketched out a mechanism through which gradual change in biological populations could occur, Darwin spent a considerable amount of time gathering evidence in support of his theory. This took shape in many activities: from the study of barnacles to experiments into artificial selection and inheritance. He would also continue to draw influence from economics. Evidence of this is peppered throughout his writings, including an instance where he was trying to work out how it was that plants and animals, despite sharing a common ancestor, ended up being so different:
I can remember the very spot in the road, whilst in my carriage, when to my joy the solution occurred to me; and this was long after I had come to Down. The solution, as I believe, is that the modified offspring of all dominant and increasing forms tend to become adapted to many and highly diversified places in the economy of nature.
Economy of nature would pop up again and again in Darwin's work, and the influence of political economy would lead to the borrowing of certain conceptual tools, such as the division of labor. Here, individual organisms will gain advantages by adapting to particular ecological niches in a specialised manner, analogous to an individual worker gaining an economic advantage over similarly situated workers by specialising in a new trade. Even though Darwin had been quite content to take his time developing these ideas into a comprehensive piece of work, in 1858 a paper he received from Alfred Russell Wallace changed the game somewhat. In it, Wallace had sketched out a theory almost identical to Darwin's, albeit with subtle differences, which he also arrived at upon reading Malthus. As you can see in the letter below (from the brilliant Darwin Correspondence Project), Darwin was clearly frustrated by his own insistence on thoroughly detailing and gathering supporting evidence for natural selection:
My dear Lyell
Some year or so ago, you recommended me to read a paper by Wallace in the Annals, which had interested you & as I was writing to him, I knew this would please him much, so I told him. He has to day sent me the enclosed & asked me to forward it to you. It seems to me well worth reading. Your words have come true with a vengeance that I shd. be forestalled. You said this when I explained to you here very briefly my views of “Natural Selection” depending on the Struggle for existence. -- I never saw a more striking coincidence. if Wallace had my M.S. sketch written out in 1842 he could not have made a better short abstract! Even his terms now stand as Heads of my Chapters.
Please return me the M.S. which he does not say he wishes me to publish; but I shall of course at once write & offer to send to any Journal. So all my originality, whatever it may amount to, will be smashed. Though my Book, if it will ever have any value, will not be deteriorated; as all the labour consists in the application of the theory.
I hope you will approve of Wallace’s sketch, that I may tell him what you say.
My dear Lyell
Yours most truly
As a consequence, Wallace's paper was read alongside extracts from Darwin's work at a presentation in front of the Linnaean Society in 1858. Ironically, the presentation itself made little impact upon the wider scientific community; aptly captured in a statement made by the President of the Linnaean Society, where he summarised 1858 as a year "which has not, indeed, been marked by any of those striking discoveries which at once revolutionise [...] the department of science of which they bear." Instead, it would be a year later, with Darwin's publication of On the Origin of Species by Means of Natural Selection, that would court much of the success. This is clear in that all 1250 copies were sold on the first day of publication.
Main Reference: Barton et al (2007). Evolution. Cold Spring Harbor Laboratory Press: Cold Spring Harbor, New York.
As a side note: I have purposefully neglected to mention the influence of August Schleicher, and other historical linguists during the 18th and 19th centuries, on Darwin's conception of the tree of life, as I'm planning on writing a more thorough post on this particular topic.