true extinction, with fewer cases of Pseudoextinction. Nearly all species undergo true extinction under the model of punctuated equilibrium. Charles Darwin proposed the idea of stasis in his book, On the Origin of Species. He suggested that species spend the majority of their evolutionary lifespan in the same form, having undergone very little morphological or genetic change. Another concept of species on the tree of life is the composite species concept. It sees one species as occupying all internodes of the tree that have the same combination of (morphological, ecological, etc.) characters. Here, a species starts with the acquisition of a character and ends when another change is fixed in its lineage. This process - of one lineage turning into what is afterwards seen as another species because of the fixation of a novel character - is often called anagenesis. In this situation, a species is considered to end by definition but is not really extinct (it survived, after all, in the form of one descendent species with a different character combination), and so it could also be considered to be a pseudoextinction. On the other hand, under the composite species concept a species continues through a lineage split if only one of the two resulting lineages acquires a new character, an event that is sometimes called speciation through "budding". The one that has a new character is now a new species but the other lineage, the one that looks identical to the common ancestor, is considered to be the common ancestor. An example would be a widespread breeding group remaining unchanged while "budding off" a small isolated population that accumulates changes until it cannot interbreed with the others any more. See also Cretaceous–Paleogene extinction event Human extinction Anagenesis Phyletic gradualism Punctuated equilibrium Extinction References External links The Extinction Website Extinction Evolutionary biology