Evolution, as understood by biologists, is the change through time that occurs in populations of organisms in response to changing environments. The changes, coded in the molecules of DNA, are transmitted from generation to generation and over the history of the Earth have resulted in progressively more complex life forms. The name of Charles Darwin and his theory of natural selection are inexorably attached to evolution and, together with the mechanisms of genetics, form the basis of the modern theory of evolution.
Simplifying and paraphrasing from Darwin's book, On the Origin of Species by Means of Natural Selection, and adding current interpretations, the main points of his theory are: all life came from one or a few kinds of simple organisms; new species arise gradually from preexisting species; the result of competition among species is extinction of the less fit; gaps in the fossil record account for the lack of transitional forms. These assertions set the stage for the next part of the theory, why life evolves: the number of individuals increases at a geometric rate; populations of organisms tend to remain the same size because the resources are limited, and only the fittest survive; the survivors are variable, and those that survive reproduce, perpetuating the favorable traits.
Natural selection, according to Darwin, is similar to artificial selection. The environment acted as the selecting force in natural selection. Unlike the relatively rapid selection pressures instituted by breeders, however, natural selection took long periods of time to accomplish change. Darwin was familiar with the then new conclusions of geologists that the Earth was far older than previously thought, which gave his theory of natural selection sufficient time in which to work.
A major problem was an explanation for how the favorable selections were perpetuated. In the 1860s the idea that offspring were blends or mixtures of the traits of their parents, the so‐called “blending theory of inheritance,” was unable to accommodate transmittal of favorable adaptations from one generation to the next. With botanist Gregor Mendel's ideas and the development of genetics, the inheritance portion of Darwin's theory no longer posed a problem.