EVOLUTIONARY THEORY

Evolutionary Theory

Evolutionary Theory

Introduction

Darwin was not persuaded that evolution occurred by any of his evolutionist predecessors. The true history of his development of his ideas is controversial, but there were perhaps four main influences on him in this respect. One was the Principles of Geology (1931), written by his mentor and friend, the geologist Charles Lyell (1797-1875), which Darwin read at the start of his famous five-year journey on the Beagle (1831-1836). Darwin was profoundly influenced by Lyell's methodological, as well as his factual claims. With respect to the former, Lyell was a uniformitarian. Broadly speaking, uniformitarianism is the view that the laws of nature have always been the same. For Lyell, this meant that geological features are to be explained by natural ("intermediate" not miraculous) processes that can still be observed to be in operation. Since he thought that these tended to bring about only slow and gradual change (e.g., a valley's formation from erosion), Lyell reasoned that the earth must be far older than the biblical 4,000 to 6,000 years. Although not a believer in evolution, Lyell also argued that investigation of the geological layers showed a continual introduction and extinction of species. This paper provide the detailed information on this theory. It also describe the major principles associated with the theory and contributions of three theorists who worked to develop the theory as it exists today.

Discussion

A second major influence on Darwin was his observation of the natural world, especially during his journey on the Beagle. His extensive collection of living and fossil animals, taken from many diverse parts of the world, and their analysis by experts in the relevant fields, convinced him (and through him, much of the scientific community) that, contra Lyell, neither fossil findings nor the present geographic distribution of species could be adequately explained other than by evolution. The task, as Darwin saw it, was to explain the evolution of species in a manner that was consistent with Lyell's uniformitarian principles. (Charig, 2002)

At least by his own account (Darwin 1876/1958, p. 120) Darwin had help with this from a third major influence, An Essay on the Principle of Population (1798), written by the parson and social economist Thomas Robert Malthus (1766-1834). Malthus was no evolutionist; he believed that his understanding of population dynamics supported the view that populations could not change much. His concern was the possibility of social improvement, but his social theory was driven by an observation that applied to all species: Unchecked increases in population always outrun the means of subsistence. As Malthus says:

Malthus's message for the poor was that if they were to reduce their struggle for existence they must reduce their fecundity. According to Darwin, this struggle for existence between members of the same species suggested to him a mechanism by which populations could evolve.

A fourth influence on Darwin that may have been important was his familiarity with the artificial selection of plants and animals for breeding. Such selection showed that differential reproduction could produce a change in ...