evolution

evolution The gradual process by which the present diversity of plant and animal life arose from the earliest and most primitive organisms, which is believed to have been continuing for at least the past 3000 million years. Until the middle of the 18th century it was generally believed that each species was divinely created and fixed in its form throughout its existence (see special creation). Lamarck was the first biologist to publish a theory to explain how one species could have evolved into another (see Lamarckism), but it was not until the publication of Darwin's On the Origin of Species in 1859 that special creation was seriously challenged. Unlike Lamarck, Darwin proposed a feasible mechanism for evolution and backed it up with evidence from the fossil record and studies of comparative anatomy and embryology (see Darwinism; natural selection). The modern version of Darwinism, which incorporates discoveries in genetics made since Darwin's time, remains the most acceptable theory of species evolution (see also punctuated equilibrium). More controversial, however, and still to be firmly clarified, are the relationships and evolution of groups above the species level. See also macroevolution; microevolution; mosaic evolution.