Editing Parallel evolution (section)

==Parallel vs. convergent evolution==

[[File:Evolutionary trends.svg |frame |Evolution at an [[amino acid]] position. In each case, the left-hand species changes from incorporating alanine (A) at a specific position within a protein in a hypothetical common ancestor deduced from comparison of sequences of several species, and now incorporates serine (S) in its present-day form. The right-hand species may undergo [[divergent evolution]] (alanine replaced with threonine instead), parallel evolution (alanine also replaced with serine), or [[convergent evolution]] (threonine replaced with serine) at this amino acid position relative to that of the first species.]]

Given a trait that occurs in each of two lineages descended from a specified ancestor, it is possible in theory to define parallel and convergent evolutionary trends strictly, and distinguish them clearly from one another.<ref name="Zhang"/> However, the criteria for defining convergent as opposed to parallel evolution are unclear in practice, so that arbitrary diagnosis is common. When two species share a trait, evolution is defined as parallel if the ancestors are known to have shared that similarity; if not, it is defined as convergent. However, the stated conditions are a matter of degree; all organisms share common ancestors. Scientists differ on whether the distinction is useful.<ref>{{cite journal |last1=Arendt |first1=J. |last2=REZNICK |first2=D. |title=Convergence and parallelism reconsidered: what have we learned about the genetics of adaptation? |journal=Trends in Ecology & Evolution |date=January 2008 |volume=23 |issue=1 |pages=26–32 |doi=10.1016/j.tree.2007.09.011 |pmid=18022278}}</ref><ref>{{cite journal |last=Pearce |first=T. |title=Convergence and Parallelism in Evolution: A Neo-Gouldian Account |journal=The British Journal for the Philosophy of Science |date=10 November 2011 |volume=63 |issue=2 |pages=429–448 |doi=10.1093/bjps/axr046 |doi-access=}}</ref>