Since its publication in 1859, On the Origin of Species has been closely studied by generations of biologists. Many scientists have categorized and augmented Darwin's theories. Standing tall among these scientists, Ernst Mayr divided Darwin's theory into 5 unique parts.
As we saw in the section on Lamarck, the very idea that evolution could occur was much debated. Along with Buffon and Lamarck, Darwin supported the ability of species to change over time. However, the mechanism by which he believed this happened was radically different from Lamarck's or anyone else's.
Darwin felt that all of the diversity of life on earth emerged out of the evolution from one or a few common ancestors. This theory took a step beyond Lamarck's observation that complex creatures probably evolved from simpler ones. The idea was also socially important: it implied a less important role for humans as merely one of many branches in the animal world rather than a separate, privileged lineage, as formerly assumed.
While Lamarck felt that species-wide change could take place within the span of a few generations, Darwin felt evolution was a much slower process, taking place in innumerable small steps. Based on this, and on the ideas of the Scottish geologist Sir Charles Lyell, he estimated the world to be much older than contemporary geological theories accounted for.
This portion of Darwin's theory states that within a population, change in a species occurs as the balance of hereditary characteristics shifts across that population. This differs from Lamarck's idea that each individual in the population must undergo the same change. According to Lamarck, all giraffes living under tall trees would develop long necks. According to Darwin, some would randomly be born with long necks, this hereditary trait would gradually spread throughout the population.
Natural selection is often called the most unique part of Darwin's theory. Competition, also called the struggle for life, had been thought of as a reason that a given species might succeed or go extinct, but Darwin extended the understanding to change within a species. To continue the example of giraffes: when a giraffe is born with a longer neck than its fellows, it gains an advantage because it is able to reach more food. The long-neck giraffe is therefore stronger, lives longer, and more likely to have offspring. These offspring are born with the same long neck as their parent, though some might have even longer necks. The cycle continues. The theory of natural selection depends on five postulates: