The interplay of events played out over billions of years, in the historical process called evolution, dictates the form and structure of the living world today. Thus biology, which is the study of the results of these historical events, differs fundamentally from physics and chemistry, which deal with the essential and unchanging properties of matter. The great insight of Charles Darwin was that all organisms are related in a great chain of being extending from the distant past to the present. The Darwinian principle that organisms vary randomly and the fittest are then selected by the forces of their environment guides biological thinking to this day.
We now know that genes, which chemically are composed of deoxyribonucleic acid (DNA), ultimately define biological structure and maintain the integration of cellular function. The genes encode proteins, the primary molecules that make up cell structures and carry out cellular activities. Alterations in the structure and organization of genes thus provide the random variation that nurtures evolutionary change in biological structure and function.
Even scientists brought up in the evolutionary tradition have been surprised to learn in recent years just how closely the genes of different species are related. During evolution, genes have been conserved to such an extent that some human genes will function in a yeast cell and quite a few will function in a fly cell. Clearly, one feature of evolution is the maintenance unchanged of many aspects of cellular life even while great changes in external form and capability are occurring. Recent progress in determining the sequences of all the genes in a variety of organisms is revealing the subtle changes that have fueled evolution.
The creative part of the evolutionary process is adaptation to rapidly changing environments and the conquest of new environmental niches. During this process, small alterations in cellular structures and functions are selected. Entirely new structures rarely are created; more often, old structures are adapted to new circumstances. More rapid change is possible by rearranging or multiplying previously evolved components rather than by waiting for a wholly new approach to emerge. The cellular organization of organisms plays a fundamental role in this process because it allows change to come about by small alterations in previously evolved cells, giving them new capabilities