What did Darwin contribute to understanding evolution, and how did he develop his ideas? The following is a very brief overview of the key points in history that may have led Darwin to his theory of evolution.
Charles Darwin (1809-1882) went on board the H.M.S. Beagle as the ship's naturalist and the voyage lasted 5 years, from 1831 to 1836. His duty was to observe the biology and geology of the South American rainforests. When comparing the flora and fauna of the Galapogos islands, Darwin found that the species were different from island to island. Also, he compared extant to extinct animals and saw that there was modification over geological time. Before the voyage, Darwin had believed in special creation and the "fixity of species". However, by 1837, after making his observations, Darwin abandoned the "fixity of species" concept.
During the Beagle voyage, Darwin read Lyell's book Principles of Geology. Charles Lyell (1787-1875), a scottish geologist, was a uniformitarian and he did not believe in evolution. "Uniformitarianism" simply means that geological changes occur uniformly and continously over time. In other words, the forces that shape the world today are same as those that acted in the past -- and the world is not affected by major catastrophies (e.g. volcanoes, hurricanes, etc) as a whole. Lyell believed in the "fixity of species" -- species could not evolve or actively adapt to their environment, and so could go extinct. One problem was how did new species arise? Extinction causes a decrease in diversity, yet there appeared to be an increase. Lyell invoked the "perpetual intervention hypothesis" -- i.e. God continously created new species to replace the extinct ones. Darwin saw the argument as weak, as Lyell provided no evidence to support his claims. Darwin may have developed "gradualism" as an important concept in evolution from the uniformitarian view, but more importantly, it was the question of how new species arose that Darwin attempted to answer.
On October 1939, Darwin read Malthus' "Essay on the Principle of Population" from 1798. This essay gave Darwin a mathematical interpretation of "the struggle for existence" (e.g. populations increase geometrically -- or exponentially -- while food resources only increase arithmetrically, thus there is struggle in nature for resources). By abandoning the "fixity of species", Darwin saw it was the individual in a "struggle for existence" that was important in evolution. Over the subsequent twenty years, Darwin amassed evidence for his theory of evolution -- some of this came from animal breeding. Darwin coined the term "natural selection", as opposed to "artifical selection" by humans, to account for the preservation of individuals with certain advantages over other individuals, in the struggle for existence.
In 1859, Darwin published his book (actually, it was an "abstract" of a book he had intended to write later on) On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life (otherwise known as The Origin of Species or simply The Origin) at the suggestion from his colleagues -- in 1858, Alfred Russel Wallace (1923-1913) had sent Darwin a manuscript, "On the Tendency of Varieties to Depart from the Original Type", detailing a view that was remarkably similar to Darwin's own theory of evolution. Darwin was an already well known naturalist -- which was the reason Wallace sent his manuscript to Darwin -- and he continued to write several books on a variety of topics. Wallace, on the other hand, went on to do other things and so, Darwin is given the credit for the theory of evolution by natural selection.
In summary, there are at least three main factors that led Darwin to develop his concepts of "the struggle for existence" and natural selection: 1) his position on board the H.M.S. Beagle, 2) Lyell's book Principles of Geology, and 3) Malthus' "Essay on the Principle of Population". In "the struggle for existence" (or "the struggle to reproduce"), there is competition between individuals -- competition between members of the same species is more intense because of similar requirements -- and there is a struggle with the environment (e.g. climate). Natural selection acts as a filter that preserves favorable variations, while eliminating the injurious variations and leaving alone the neutral variations.
Darwin did not originate the concept of "evolution". Others before Darwin had already proposed "evolution" to explain the diversity of life, such as Lamarck.
Ernst Mayr has indicated that The Origin actually had five independent theories (Futuyma, 1998): 1) evolution as such, 2) common descent, 3) gradualness, 4) populational speciation, and 5) natural selection. Descent with modification and natural selection acting on individual variation were key concepts Darwin's theory: Evolution occurs through selection acting on the variation within a species and favorable traits are passed on through the generations, which causes changes in composition of populations -- and all this happens over long time periods (i.e. it is gradual). Natural selection was a much needed mechanism to explain evolution. However, what Darwin didn't know was how variation passed from the parents to the offspring -- although he had his own theories on the matter.
Evolution After Darwin
Mendel (1822-1884) (the founder of Genetics) discovered how variation can be passed on through the generations with his work on pea plants (published in 1865-1869), yet his work was ignored until the early 1900s. Mendel discovered two laws: 1) the law of segregation, and 2) the law of independent assortment. The law of segregation states that half of the genes from one parent are carried by each gamete (sex cell). Each gamete has one allele (or one variation of a gene) out of two from the parent cell (each parent cell in a diploid species can have only two alleles at one gene locus -- i.e. genes come in pairs in diploid organisms). The law of independent assortment states that different gene pairs assort independently of other gene pairs into gametes. As it turns out, these two "laws" have several exceptions to them. For example, the second law only applies to genes that are far apart on the same chromosome or are on different chromosomes.
At the turn of the century, after Mendel's laws were re-discovered, two groups with differing views on evolution emerged: 1) the mutationalists, who believed that Darwin was wrong and Mendel was correct (i.e. evolution occured rapidly through macromutation), 2) the biometricians, who believed Mendel was wrong and Darwin was right (i.e. evolution occured slowly and gradually via natural selection). As it turns out, both Darwin's and Mendel's theories could be incorporated into one. The Modern Synthesis of evolution was developed around 1920 - 1947, unifying the principles from both Darwin and Mendel. Many people were involved in the development of the synthesis and these include Ronald A. Fisher (1890-1962), J.B.S. Haldane (1892-1964), Sewall Wright (1889-1988), Theodosius Dobzhansky (1900-1975), George Gaylord Simpson (1902-1984), and Ernst Mayr (b. 1904) to name a few. In Futuyma's Evolutionary Biology (1998) textbook there are 20 tenets listed (pp. 26-27) that have been contributed by population geneticists, paleontologists, systematists, and geneticists who studied either laboratory populations or natural populations. Not all of the tenets are accepted by everyone, but the importance of the Modern Synthesis was in the "consistency argument" -- the facts of genetics had to agree with the fossil evidence. Thus, the various biological fields became unified through evolutionary theory.
So, the modern paradigm of evolutionary theory is that natural selection acts on genetically different individuals in populations, thus species change over time. However, selection is not the only driving force in evolution. There are other forces such as genetic drift, gene flow, mutation and recombination (defined in "Basic concepts in Evolution"). Also, the modern paradigm rejects the notion of the "inheritance of acquired characteristics" or Lamarckism. There are however, some cases from embryology (developmental biology) and microbiology (e.g. experiments with ciliates) that provide evidence for inherited structural changes (i.e. not genetic changes) which are ignored by mainstream biology. I have posted a short essay that I wrote for an exam that summarizes some of the details.