Charles Darwin has been credited with the theory of evolution, but in fact he was not the first to consider the idea. Darwin's grandfather, Erasmus Darwin, also began to develop a theory of organic evolution, but it was never brought together in a systematic way. Jean-Baptiste Lamarck is remembered by historians of evolution as another early theorist, but his theory differed in one important respect from modern understanding: Lamarck postulated that acquired characteristics could be passed to offspring; an oversimplified example of this would be the idea that a cat whose tail is amputated would them produce tailless kittens. This was before scientists had a real grasp of Gregor Mendel's laws of genetic inheritance.
Darwin fancied himself a geologist. On his famous round-the-world voyage on H.M.S. Beagle, he brought with him a book by the well-known geologist of the time, Charles Lyell. Lyell is remembered today as an advocate of catastrophism, that is, the idea that major geological features, such as mountain ranges and canyons, were formed quickly by tremendous natural disasters. A much-distorted and oversimplified version of this is seen today in the thinking of some creationists, who attempt to telescope the whole of the fossil record into the global flood in the time of Noah. In Darwin's Journal of Researches (modern editions are often re-titled Voyage of the Beagle), he often commented on the geologic processes he observed in his various ports of call, and the extinct large mammals found fossilized in Patagonia caught his attention. He noted that many of them showed similarities to presently-living species in other parts of the world, yet differed enough to be clearly different species.
Prior to his arrival in Patagonia, Darwin had visited the great rain forests of Brazil, and there had noticed a remarkable pattern. Large rivers often had ever-so-slightly different species on opposite banks. Two species - for example, of butterflies - might be so similar as to suggest they were closely related, yet each was seen only on one or the other side of the great river.
About the same time, Alfred Russell Wallace was noticing the same pattern in the myriad islands of the Malay Archipelago - what we call Indonesia today. Wallace would see, for example, a progression of sorts in the butterflies, with those of each island differing slightly from the last, until at opposite ends of the archipelago, they were quite different. This set Wallace thinking on the same lines as Darwin, though as yet neither man was aware of the other's research.
The famous finches of the Galapagos were relative latecomers to the formulation of Darwin's ideas. By the time he reached the Galapagos, Darwin had already been observing birds for some time, a gradual shift away from his focus on geology. Even so, he very nearly missed the finches' true importance. He began collecting specimens, as naturalists of his day habitually did, but labeled them all as simply coming from "Galapagos Islands." Only after the ship departed the Galapagos was it brought to Darwin's attention that each island of the Galapagos group had different species of finches, as well as of nightingales. Darwin had to go back to his specimens and attempt to reconstruct the variation in them.
Nevertheless, despite this bumbling start, it was the finches which most clearly illustrated what Darwin had been pondering all this time. He figured out that a given complex of finches, differing in size or shape of bill, or in body size, must be ecologically separated, that is, each variant specialized in a slightly different food source than the others. In the case of the three species called the large, medium, and small ground finches, respectively, each was specialized by bill size to fed on a different size seed. Others were specialized to capture insects. Yet all showed the same overall body structure and range of plumage pattern, suggesting that all were somehow related. Eventually, this led Darwin to conclude that originally, in the distant past, there had been but one species of finch in the Galapagos, but that differing conditions of life in the different parts of the islands had meant that different individual variations had different rates of success. No two individuals are alike; but under certain conditions, those with larger bills were more likely to survive, while under other conditions, those with smaller bills had the advantage. After generations of living separate lives, these different variations became fixed as species, so that when they came back together again, they remained separate, living side by side but each keeping its own ways.
While Darwin was formulating his ideas, he received a message from Wallace, still in the Malay Archipelago. Wallace had formulated the very same theory, and had written a scientific paper on it. Darwin knew he had no time to delay; so he, too, wrote up his theory, and his and Wallace's results were presented to the scientific community at the same time. Darwin, and not Wallace, is remembered for the theory of evolution largely because he continued developing the ideas, eventually producing his seminal work, On the Origin of Species By Means of Natural Selection. This was the first work which fully systematized an explanation for the process of evolution.
The theory of evolution met resistance. A literal view of the Bible was still prevalent in Victorian times, and the idea that species changed with time seemed incompatible with the clear words of Genesis. Many in those days believed that each breed of domestic dog had descended from a separate species - an original wild beagle, original wild setter, etc. Darwin's breeding experiments with pigeons showed that, by selecting certain variations at the expense of others, the breeds could be changed within a number of generations. Now, it was known even then that in nature, not every offspring produced, survives to adulthood. So if the conditions of nature were more favorable to the survival of one variation rather than another, that would have the same effect over time as selective breeding by the pigeon fancier. Thus, the mechanism of evolution is called Natural Selection.
This concept of natural selection revolutionized scientific understanding of life. Rather than the idea of Intelligent Design - which states that the Creator designed each species specifically for its particular way of life - the mechanism of natural selection means that the forces of nature, the conditions of life, select those characteristics best suited to survival under existing conditions. Under different conditions - such as the other side of a mountain range - different characteristics would be selected. This had implications for other endeavors in biology. For example, in epidemiology, it explains the origin of new strains of viruses and bacteria, as the natural immune system kills off some variations of a germ, but others survive and become the next wave of infection. In genetics, it has been applied to the study of alleles, that is, variants of genes, of which different populations have different proportions. And in conservation ecology, it emphasizes the importance of maintaining each genetically distinct population of a given species, since unforeseen changes might favor or disfavor one over another.
Evolution by natural selection is indeed seen pervasively throughout every biological and ecological process, so that no aspect of the life sciences can be rightly understood apart from it. From biodiversity to horticulture, every enterprise involving organic life depends on a working knowledge of the mechanism of evolution.