<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> Evolutionary thought, the conception that species change over time, has roots in antiquity--in the ideas of the ancient Greeks, Romans, and Chinese as well as in medieval Islamic science . With the beginnings of modern biological taxonomy in the late 17th century, two opposed ideas influenced Western biological thinking: essentialism, the belief that every species has essential characteristics that are unalterable, a concept which had developed from medieval Aristotelian metaphysics, and that fit well with natural theology; and the development of the new anti-Aristotelian approach to modern science: as the Enlightenment progressed, evolutionary cosmology and the mechanical philosophy spread from the physical sciences to natural history . Naturalists began to focus on the variability of species; the emergence of paleontology with the concept of extinction further undermined static views of nature . In the early 19th century Jean - Baptiste Lamarck (1744--1829) proposed his theory of the transmutation of species, the first fully formed theory of evolution . </P> <P> In 1858 Charles Darwin and Alfred Russel Wallace published a new evolutionary theory, explained in detail in Darwin's On the Origin of Species (1859). Unlike Lamarck, Darwin proposed common descent and a branching tree of life, meaning that two very different species could share a common ancestor . Darwin based his theory on the idea of natural selection: it synthesized a broad range of evidence from animal husbandry, biogeography, geology, morphology, and embryology . Debate over Darwin's work led to the rapid acceptance of the general concept of evolution, but the specific mechanism he proposed, natural selection, was not widely accepted until it was revived by developments in biology that occurred during the 1920s through the 1940s . Before that time most biologists regarded other factors as responsible for evolution . Alternatives to natural selection suggested during "the eclipse of Darwinism" (c. 1880 to 1920) included inheritance of acquired characteristics (neo-Lamarckism), an innate drive for change (orthogenesis), and sudden large mutations (saltationism). Mendelian genetics, a series of 19th - century experiments with pea plant variations rediscovered in 1900, was integrated with natural selection by Ronald Fisher, J.B.S. Haldane, and Sewall Wright during the 1910s to 1930s, and resulted in the founding of the new discipline of population genetics . During the 1930s and 1940s population genetics became integrated with other biological fields, resulting in a widely applicable theory of evolution that encompassed much of biology--the modern synthesis . </P> <P> Following the establishment of evolutionary biology, studies of mutation and genetic diversity in natural populations, combined with biogeography and systematics, led to sophisticated mathematical and causal models of evolution . Paleontology and comparative anatomy allowed more detailed reconstructions of the evolutionary history of life . After the rise of molecular genetics in the 1950s, the field of molecular evolution developed, based on protein sequences and immunological tests, and later incorporating RNA and DNA studies . The gene - centered view of evolution rose to prominence in the 1960s, followed by the neutral theory of molecular evolution, sparking debates over adaptationism, the unit of selection, and the relative importance of genetic drift versus natural selection as causes of evolution . In the late 20th - century, DNA sequencing led to molecular phylogenetics and the reorganization of the tree of life into the three - domain system by Carl Woese . In addition, the newly recognized factors of symbiogenesis and horizontal gene transfer introduced yet more complexity into evolutionary theory . Discoveries in evolutionary biology have made a significant impact not just within the traditional branches of biology, but also in other academic disciplines (for example: anthropology and psychology) and on society at large . </P>

Briefly describe two ideas about evolution that were proposed in the 18th century
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