In 1858, Charles Darwin and Alfred Russel Wallace published a paper detailing the theory of natural selection which provides a mechanism by which biological evolution occurs. Although the two naturalists developed similar ideas about natural selection, Darwin is considered to be the theory's primary architect, since he spent many years gathering and compiling a vast body of evidence to support the theory. In 1859, Darwin published his detailed account of the theory of natural selection in his book On the Origin of Species.
Natural selection is the means by which beneficial variations in a population tend to be preserved while unfavorable variations tend to be lost. One of the key concepts behind the theory of natural selection is that there is variation within populations. As a result of that variation, some individuals are better suited to their environment while other individuals are not so well-suited. Because members of a population must compete for finite resources, those better suited to their environment will out-compete those that are not as well-suited. In his autobiography, Darwin wrote of how he conceived this notion:
"In October 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed." ~ Charles Darwin, from his autobiography, 1876.
Natural selection is a relatively simple theory that involves five basic assumptions. The theory of natural selection can be better understood by identifying the basic principles on which it relies. Those principles, or assumptions, include:
- Struggle for existence - More individuals in a population are born each generation than will survive and reproduce.
- Variation - Individuals within a population are variable. Some individuals have different characteristics than others.
- Differential survival and reproduction - Individuals that have certain characteristics are better able to survive and reproduce than other individuals having different characteristics.
- Inheritance - Some of the characteristics that influence an individual's survival and reproduction are heritable.
- Time - Ample amounts of time are available to allow for change.
The result of natural selection is a change in gene frequencies within the population over time, that is individuals with more favorable characteristics will become more common in the population and individuals with less favorable characteristics will become less common.