• Evolution means descent with modification, or the change in the form, physiology, and behavior of organisms over many generations of time. The evolutionary changes of living things occur in a diverging, tree-like pattern of lineages.

• Living things possess adaptations: i.e., they are well designed in form, physiology, and behavior, for life in their natural environment.

• Many thinkers before Darwin had discussed the possibility that species change through time into other species. Lamarck is the best known. But in the mid-19th century most biologists believed that species are fixed in form.

• Darwin's theory of evolution by natural selection explains evolutionary change and adaptation.

• Darwin's contemporaries mainly accepted his idea of evolution, but not his explanation of it by natural selection.

• Darwin lacked a theory of heredity. When Mendel's ideas were rediscovered at the turn of the century, they were initially thought to count against the theory of natural selection.

• Fisher, Haldane, and Wright demonstrated that Mendelian heredity and natural selection are compatible; the synthesis of the two ideas is called neo-Darwinism or the synthetic theory of evolution.

• During the 1930s and 1940s, neo-Darwinism gradually spread through all areas of biology and became widely accepted. It unified genetics, systematics, paleontology, and classical comparative morphology and embryology.