During the booming of science in 19th century, rationalists tried to categorize and model everything with unambiguous language. The unambiguous "mathematics is the foundation of all exact knowledge of natural phenomena" . "God created everything by number, weight and measure" . It will be almost a century later that the "two clouds"  triggers the destruction of classic deterministic physics. But it is Charles Darwin, who stood in the middle of 19th century, with limited suggestive evidence and unbounded thinking, showed us the beauty of dynamics and complexity in nature and made sense out of the ambiguity and uncertainty that embedded into nature itself. If the nature is formed from simple matters, would it be possible that a self-organized power created the complex world rather than a work of rational God? The evolution theory before Darwin made it sounded like another conscious power that was more appealing to the evolution method created the world in this way. A superior power (or the First Cause) is an inevitable conclusion. To Darwin, it was unnecessary of an omnipotent deity power to make the complex world possible. The power of individual, the incentive to survive was sufficient enough to self-organize the complexity of nature.
It is often misattributed the initial evolution concept to Charles Darwin. But it is worth to note that evolution is not a new idea. Darwin's grandfather, Erasmus Darwin was an evolutionist at that time. And over the English Channel, Lamarck had exhaustively how he perceived that species mutated over time towards a common goal. In many aspects, Lamarck provided early insights into why and how species mutated. The environment factor he stressed also played a critical role in Darwin's theory. However, the major assumptions Lamarck made were about the driving forces  in nature which were merely a hypothesis from scientific view. It was the incompleteness and lack of evidence made early evolution theory, although it existed for a quite long period, still being rejected as the main theory in scientific community. On the other hand, the fixity of nature won the large audience because of its simplicity and the coherence with our day-to-day experience. Darwin's theory didn't provide a resolution either. Rather, it was a more compelling and coherent argument with many suggestive evidences that people willing to believe. Only later discovery consistently proved the correctness of his theory and confirmed his insights.
The main insight in Darwin's theory was the Natural Selection. It was the idea that the nature put an upper bound to the population it could hold in one country which would encourage individual competitions and thus the individual would ultimately developed a niche to survive. The population theory was not new. It borrowed the light directly from Thomas Malthus and somehow appeared to be self-evident. The interesting part was about the description about individual competition. In his On the Origin of Species, Darwin drew his Selection idea first from breeders and argued that if breeder's selection could eventually result mutation of species, it would be possible that Natural Selection could work. He cited Malthus' theory as a basis for how Natural Selection could occurred but provided not quite so citation for individual competition. In Lamarck's theory, the group of species worked together and acted with a collective effort to achieve the evolution goal. The giraffe gained the long neck by the collective effort of generations and finally after several generations, it would have long enough neck to reach the leaf. The idea in Darwin's "individual competition" phrase which later was classified as individualism was first introduced by another English tale Adam Smith. However, it was such a coincidence that Charles Darwin never mentioned about him. One reason was that even after half a century The Wealth of Nations was still criticized and debated. The individualistic factor was only later discovered by going over the assumptions Adam Smith used.
The assumption modern economics made was about individual works for self-interests. Darwin's assumption in Natural Selection was about individual works to survive, which served as the ultimate self-interests. The mutation was not a collective conscious choice; rather, it was the edge that empowered individual to survive. It was the differentiator to distinguish Darwin's theory from earlier work. However, it should be pointed out that Darwin's individualism argument was largely based upon the population cap in Malthus' theory. It was in some sense may imply that if the population didn't meet the cap, some mutations could not gain much advantage to pass on. Profoundly, it contradicted with the modern economics assumption since that if the cap were met, the mutation provided no incentive (self-interest). However, Darwin did elaborately argue that even in that situation, a niche in mutation would be preserved because it made the survival of one individual much easier. It was effectively an alternate saying of that individualism would benefit the society as a whole by improving the efficiency of society.
Charles Darwin raised his interests in evolution from his voyage of Beagle. It's the adaptivity of species that triggered Darwin's thought on the possibility of evolution. The different beak shapes of same species of finches showed the fascinating result of the adaptivity force. However, in the Darwin's theory, the adaptivity phenomenon was not a result of the adaptivity force. It is the individual who leverage the environment to serve self-interests that ultimately adapted to the environment. The implication of individualism itself showed a compelling reason that governs the mysterious adaptivity force. Darwin made it clear that adaptivity was not the driving force for mutation; it was the result of mutation. The one who survived with niches would be the one that leveraged the nature most. In the On the Origin of Species, Charles Darwin put lengthy argument of the potential of mutation based on the observation of breeder. Charles Darwin made an intentional effort to blur the conscious choice made by breeder. He argued that the breeder picked stronger ones, but never had a clear image about what it would be like after generations, making the choice less relevant to a conscious design. To Charles Darwin, the Natural Selection was sufficient enough and the need for an omnipotent deity was unnecessary. Rather than a central governing mechanism, Charles Darwin put his faith on a self-organized power.
It is notable that even after the publication of On the Origin of Species Charles Darwin put together only a few suggestive evidences. His field experiment proved mutation was possible and can be passed on to future generations. But it was one and half century later that we could experimentally show the effect of Natural Selection on individual competition and mutation. Darwin collected evidences and organized them around his assumption. That gave him much freedom on what made more sense. The evolution theory before him made several attempts to be coherent and compelling. It was a hard bet to argue the falsity of such theory with limited evidence gathered. It was the back cast that gave us the illusion that Darwin's theory was correct in the first place. But the pattern of scientific advancement was about hypothesis-evidence-prove . When On the Origin of Species published, though trust himself enough, Darwin humbly admitted that it was a possibility. The astonishing accuracy of his observation and hypothesis didn't demolish all the free choices he made in that theory. Granted that, the individual competition choice to Darwin was a conscious one.
In On the Origin of Species, Charles Darwin showed a more dynamic, chaos and ruthless nature that contradict to the daily experience of an ordered and decipherable world. However, Charles Darwin set out the book as the explanation to resolve several imposed questions in natural history study. There were no deterministic criteria to categorize species. With more and more findings world-wide, naturalists had a hard time to identify if a new finding was a new species or not. The newly discovered fossils showed the existence of distant and extinct species in the Earth's history. It was clear from today's perspective those problems cannot be solved under the old framework in which a conscious deity/power designed everything. Darwin's theory supposed to provide a holistic view to address these issues. So far, the scientific community obsessed on providing explanations to the world of now, to describe the world works at present. If the present could be explained, the past was determined. Darwin's theory provided a very different angle; the nature was changing all the time, with random mutations and selections. It results a chaos and booming world. Science, in its vein, was a deductive method. In 19th century, cell was well observed thanks to the invention of microscope. Scientists were much convinced the idea that the world was constructed from "small building blocks". However, in Darwin's theory, he put a step ahead and made the critical observation that those small building blocks were evolved instead of assembled to the current form.
Throughout the Darwin's theory, the individualism was implied. But, it was not an intentional choice. Darwin, as a naturalist, was very much lived in an orthogonal world to the application of individualism in politics and economics. I would like to attribute the implication to his rationality judgment. After all, it was an industrialized England. The more appealing ideas in Darwin's mind could only be those that fit in the contemporary thinking of that time. When today's heterodox economists  tried to apply evolution method into economic analysis, it worth to point out the connection between Darwinism and the central element in modern economics: Individualism.
 David Hilbert (1862 - 1943)
 Sir Isaac Newton (1643 - 1721)
 From 1900 Royal Institution lecture, Lord Kelvin
 Lamarck asserted the force to complexify and the force to adapt as the two fundamental driving forces in nature evolution
 The structure of scientific revolutions, Thomas Kuhn
 Complexity: the emerging science at the edge of order and chaos, M. Mitchell Waldrop