Editing Convergent evolution (section)

===Atavism===

{{Main|Atavism}}

In some cases, it is difficult to tell whether a trait has been lost and then re-evolved convergently, or whether a gene has simply been switched off and then re-enabled later. Such a re-emerged trait is called an [[atavism]]. From a mathematical standpoint, an unused gene ([[genetic drift|selectively neutral]]) has a steadily decreasing [[probability]] of retaining potential functionality over time. The time scale of this process varies greatly in different phylogenies; in mammals and birds, there is a reasonable probability of remaining in the genome in a potentially functional state for around 6 million years.<ref>{{Cite journal |last1=Collin |first1=R. |last2=Cipriani |first2=R. |year=2003 |title=Dollo's law and the re-evolution of shell coiling |journal=Proceedings of the Royal Society B |volume=270 |issue=1533 |pages=2551–2555 |doi=10.1098/rspb.2003.2517 |pmc=1691546 |pmid=14728776}}</ref>