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Theory of Evolution
Evolution is the theory that groups of organisms change with passage of time, mainly as a result of natural selection, so that descendants differ morphologically and physiologically from their ancestors (Microsoft Bookshelf 95). The tree of life below shows the two major branches of animals - vertebrates and invertebrates. As a result of evolution, these two groups are believed to have arisen from the single-celled protozoans.
To see a more detailed evolutionary Tree of Life connected to beautiful pictures of different creatures, click anywhere on the chart above.
According to the theory, some genetic changes in organisms occur due to mutation. Others are due to the mixture of genes carried by the various members of a species. Then, through Natural Selection, those organisms that are best adapted to survive will pass their changes on to subsequent generations. These changes can occur over very long times periods (millions of years), or may occur relatively quickly. Recently, scientists have addressed the problem of the fossil record not supporting the slow and gradual change first postulated by Charles Darwin. They have modified the theory to include a concept called Punctuated Equilibrium. This allows for long periods of little or no change, punctuated by periods of rapid change.
For a more detailed discussion of evolution, including its defense against Creationist criticisms, see http://www.talkorigins.org/faqs/faq-intro-to-biology.html.
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rights reserved. Last modified: Friday August 28, 1998.