Evolution

The birth of modern evolutionary theory can be traced to the mid-nineteenth century, and the publication of Charles Darwin's book, The Origin of Species by Means of Natural Selection in 1859. The book is considered by some to be the most influential in all of biology and by others an unsubstantiated theory. It was not, however, a new concept. Even in the late eighteenth century, the French scientist Maupertuis, the philosopher Diderot, and others entertained the notion that new life forms might arise by natural means, including spontaneous generation. Their definition of evolution more closely resembles our modern concept of development, since the course of evolution is unpredictable. Interestingly, Darwin did not use the word "evolution" in The Origin of Species, except in the form "evolved" which is the last word of the book.

French naturalist Jean-Baptiste Lamarck (1744–1829) was the first to clearly articulate the theory that species could change over time into new species, a process later called speciation. In his 1809 book Philosophie zoologique, he argued that living things progress inevitably toward greater perfection and complexity, through a process guided by the natural environment. A changing environment would alter the needs of living creatures, which would respond by using certain organs or body parts more or less. The use or disuse of these parts would consequently alter their size or shape, a change that would be inherited by the creatures offspring. Lamarck, although not the first, discussed this notion of "acquired characters." His contemporaries accepted. It is therefore unfortunate that Lamarck is famous today primarily because he was wrong in this belief, which Darwin upheld, in that it implies that bodily changes acquired during an individuals lifetime can affect the inherited material (now known as DNA) and be inherited. This hypothesis is not true as depicted in the example that a blacksmith's large muscles (an acquired character) are not passed on to the blacksmith's children.

Charles Robert Darwin (1809–1882) and his contemporary Alfred Russell Wallace (1823–1913) are credited with independently providing the first plausible theory for a mechanism to explain evolutionary change, which they called natural selection. However, Wallace did not present so great a synthesis, or consider all the ramifications of evolution, as Darwin did in The Origin of Species and his later works. One major difference between the two men was that Wallace did not believe that natural selection could have produced the human brain; Darwin rejected his colleague's contention that human intellect could only have been created by a higher power.

In The Origin of Species, Darwin concluded that some individuals in a species are better equipped to find food, survive disease, and escape predators than others. He reasoned that if the differences between the survivors and the doomed are passed from parents to their offspring, these differences will be inherited and gradually will characterize the whole population. Darwin knew that the greatest weakness in his theory was his lack of knowledge about the mechanics of inheritance; his attempt at a theory of inheritance, known as pangenesis, was later refuted. However, he is credited for his remarkable insight into the hidden variation in the form of traits that are genetically recessive and only manifest themselves when they occur as a genetic "double dose"—along. He also helped clarify the disctinction between genotype and phenotype. The genotype is a set of genetic instructions for creating an organism; the phenotype is the observable traits manifested by those instructions. The issue that explains the mechanism of inheritance, however, went unsolved for several decades later.