The Gene as the Basis of Evolution
Every species of organism reproduces itself. The process of heredity converts the food derived from the environment into more or less faithful copies of the assimilating organism and of its parents and more remote ancestors. The reproductive potentials of many organisms are immense. With abundant food and at favorable temperature, the colon bacteria, Escherichia coli, double in number about every twenty minutes.
Heredity is a conservative force. If children and parents were completely identical, evolution could not occur. Heredity, however, is opposed by a process of change, variability. Self-reproduction occasionally results in an imperfect copy of the parental living unit, and the altered copy, called a mutant, then reproduces the altered structure until new mutations intervene. One of the major achievements of biology during the current century has been the demonstration that the units both of heredity and of mutation are bodies of molecular dimensions, called genes.
In the last analysis evolution is a sequence of changes in the genes. Darwin and other pioneers of evolutionary biology realized very clearly the basic importance of heredity for understanding evolution. However, it is only during the current century, and particularly during the last twenty to thirty years, that a theory of evolution based on the findings of the study of heredity, genetics, has become possible. A brief outline of the fundamentals of genetics is, therefore, essential in a book dealing with evolution. Such an outline is given in Chapters 2, 3, 4, and also elsewhere in this book in connection with the various problems of general evolutionary biology discussed.