The Origin of Species

First published on 24 November 1859, The Origin of Species (full title On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life) by British naturalist Charles Darwin is one of the pivotal works in scientific history, and arguably the pre-eminent work in biology. In it, Darwin makes "one long argument" for his theory that "groups" of organisms, (which we now call populations) rather than individual organisms, gradually evolve through the process of natural selection—a mechanism effectively introduced to the public at large by the book. The work presents detailed scientific evidence he had accumulated both on the Voyage of the Beagle in the 1830s and since his return, painstakingly laying out his theory and refuting the doctrine of "Created kinds" underlying the theories of Creation biology which were then widely accepted.

Darwin's theory, as presented

A firm base

Darwin's theory of evolution is based on five key observations and inferences drawn from them, as summarized by the biologist Ernst Mayr:

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• Species have great fertility. They make more offspring than can grow to adulthood.
• Populations remain roughly the same size, with modest fluctuations.
• Food resources are limited, but are relatively constant most of the time. From these three observations it may be inferred that in such an environment there will be a struggle for survival among individuals.
• In sexually reproducing species, generally no two individuals are identical. Variation is rampant.
• Much of this variation is heritable.

From this Darwin infers: In a world of stable populations where each individual must struggle to survive, those with the "best" characteristics will be more likely to survive, and those desirable traits will be passed to their offspring; and that these advantageous characteristics are inherited by following generations, becoming dominant among the population through time. This is natural selection.

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Darwin further infers that natural selection, if carried far enough, makes changes in a population, eventually leading to new species. He puts forward myriad observations as demonstrations of this, and also claims that the fossil record can be interpreted as supporting these observations. Darwin imagined it might be possible that all life is descended from an original species from ancient times. Modern DNA evidence is consistent with this idea.

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A minor mistake - or precaution

One of the chief difficulties for Darwin in his time was the development of a model of heredity which would allow for the specific requirements of his theory of speciation. Darwin's relatively loose understanding of inheritance had many connections to Lamarckian theories which stressed that use and disuse alone could result in inherited traits. For example, in the first edition, he states, "When the first tendency was once displayed, methodical selection and the inherited effects of compulsory training in each successive generation would soon complete the work". Darwin later worked out a more elaborate model of heredity (which he dubbed "Pangenesis") which incorporated various aspects of Lamarckian inheritance as well, though was also key in influencing early non-Lamarckian theories of heredity (such as the biometric model developed by his cousin Francis Galton). Lamarckian inheritance would not be conclusively disproved until after Darwin's death, and Mendelian genetics would not be (re)discovered until the twentieth century.

Related Topics:
Heredity - Lamarckian - Pangenesis - Biometric - Francis Galton - Mendelian - Genetic

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