Barrett, Spencer CH. “Major evolutionary transitions in flowering plant reproduction: an overview.” (2008): 1-5.
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Evolutionary transitions are functionally significant changes in organismal traits that largely result from the action of natural selection. They first appear within populations when novel traits replace the ancestral state because of increased fitness. Evolutionary transitions take on broader significance and can be considered major when newly established traits persist, are maintained through multiple speciation events, and ultimately become well-established features of lineages. The shifts in character state that constitute an evolutionary transition are key elements of biological diversification, and the identification and study of major transitions during the history of life now represent an important research program in evolutionary biology.