Concept 1.4 Evolution accounts for life’s unity and diversity The history of life is a saga of a changing Earth billions of years old, inhabited by a changing cast of living forms. Charles Darwin brought evolution into focus in 1859 when he presented two main concepts in one of the most important and controversial books ever written, On the Origin of Species by Natural Selection. Darwin’s first point was that contemporary species arose from a succession of ancestors through “descent with modification.” This term captured the duality of life’s unity and diversity: unity in the kinship among species that descended from common ancestors and diversity in the modifications that evolved as species branched from their common ancestors. Darwin’s second point was his mechanism for descent with modification: natural selection. Darwin inferred natural selection by connecting two observations:
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