Evolution and the theory of natural selection are the principle unifying concepts in biology. This is as true now in the age of molecular biology as it was a century ago. The unification stems from the fact that details of embryology, morphology, molecular biology, physiology, paleontology, animal distribution, etc. only become understandable when considered in the context of evolution and natural selection. Certainly evolution had been considered prior to the publication of Charles Robert Darwin's (1809-1882) the The Origin of Species in 1859. However, a mere consideration was no longer appropriate for biological scientists. Darwin's book was an extensive catalog containing a plethora of evidences that support the reality of evolution. His theory of natural selection provided the mechanism for evolution. The 1859 book was the beginning of modern biology.
Evolutionary theory explains the bewildering array of similarities in related biological groups. For instance, frogs, mice, and humans, while obviously different, share similar organ systems. A histological (tissue) section of a mouse liver is difficult to distinguish from that of a human and both share similarities with frog liver. The frog liver functions in detoxification of xenobiotic substances in much the same manner as that of the mouse and human, and of course, all other vertebrates. Striated (voluntary) muscle is essentially identical in all vertebrates. The similarities of cell biology, anatomy, organ systems and physiology are of course well known to both lay people and scientists. What accounts for this notable similarity? The frogs, mice, humans and other vertebrates descended from a common ancestral species in the distant past. Species with a more recent divergence from a common ancestor, such as humans and apes, share more similarities than creatures with a common ancestor in the remote past. Darwinian evolution seeks to explain these relationships by the mechanism of natural selection.
Natural selection rests on a number of obvious facts. First, in natural populations, reproduction has the potential to result in more individuals than can survive. Consider the frog again. A female northern leopard frog, Rana pipiens , has the capacity to produce about 3,000 offspring per year. What is true for frogs is true for all species, i.e., all species have the competence to produce offspring in greater numbers than can survive. This results in competition between individuals and not all individuals of the species survive. Second, scrutiny of any population of organisms will reveal that not all individuals are identical. Variation exists in form and function. Some of the variation is heritable and that means the variation will be passed from the parents to the next generation. Some variation will affect survival and reproduction. Some differences are advantageous and will enhance survival and reproduction; others will not. It is not difficult to see that, in time and with competition, the inherited differences that impart enhanced survival and reproduction will tend to become more common. The population changes (evolves) as the frequency of a particular variant changes.
The natural world is constantly changing which results in new selective pressures. Where previously a particular population was well adapted for survival and reproduction, another environment may result in a new variant having enhanced survival and reproduction potential. In other words, the environment changes and natural selection results in the evolution of a changed population.
As the environment changes, continued selection leads to a steady change in the structure and physiology of a species; the change will always be toward enhanced adaptation to the environment (which has sometimes been referred to as "fitness"). The surviving individuals of apopulation may become exquisitely fit. Darwin observed all of the above. All that was missing was an understanding of heredity. Modern genetics has augmented understanding of natural selection, which over a century after the publication of the The Origin of Species, remains central to the understanding of all aspects of biology.
This is the complete article, containing 632 words
(approx. 2 pages at 300 words per page).
