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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.
Common Descent
Darwin felt that all diversity of life on Earth emerged from the evolution of one or a few common ancestors. 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.
Gradualism
Darwin felt evolution was a slower process, taking place in innumerable small steps. Based on this, and on the ideas of Scottish geologist Sir Charles Lyell, he estimated the world to be much older than contemporary geological theories accounted for.
Population Speciation
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
Natural selection is often called the most unique part of Darwin's theory. Competition, also known as 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. Males also compete for females by fighting with their long necks. The long-neck giraffe is therefore stronger, lives longer, and is 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:
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Individuals are variable.
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Some variations are passed down.
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More offspring are produced than can survive.
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Survival and reproduction are not random.
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The history of Earth is long.
