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Species Concepts and Reproductive Isolating Barriers

Author(s): Tadzia GrandPré, PhD, Nancy Moreno, PhD, and Lisa Marie Meffert, PhD
Showing Results for: biological evolution Return to Presentation

Patterns of Evolution

In On the Origin of Species, Darwin wrote that all species, whether living or extinct, form a great "Tree of Life," now commonly referred to as a phylogenetic tree. A phylogenetic tree graphically represents the evolutionary history (phylogeny) of species, groups of related species, or other taxonomic groups. A branch point of the tree represents divergence of one or more new populations from a parent population. In some phylogenetic representations, the branch length is proportional to the predicted evolutionary time between branching events. These branches and nodes represent two different patterns of evolution: cladogenesis and anagenesis. 

Cladogenesis, also known as branching evolution, is the evolutionary pattern by which a new group splits from a parent group. Because this results in an increase in the total number of existing taxa, cladogenesis is responsible for the diversity of organisms seen in nature. In contrast, anagenesis, also known as phyletic evolution, results in the linear transformation of a population. There is no branching of the phylogenetic tree and no increase in the total number of species. Instead, anagenesis causes two species that split from the same parent species diverge more and more from one another over time.