In evolutionary biology, parallel evolution refers to the independent evolution of similar traits in closely related lineages of species, while convergent evolution refers to the appearance of striking similarities among lineages of organisms only very distantly related.

One of the most spectacular examples of parallel evolution is provided by the two main branches of the mammals, the placentals and marsupials, which have followed independent evolutionary pathways, after splitting off from some primitive mammalian common ancestor in the late Cretaceous. (Placentals bear their young fully developed, while marsupials give birth prematurely and nurture their young in a pouch.) The marsupials of Australia have evolved in isolation from placental mammals elsewhere yet have given rise to a whole range of similar forms: pouched versions of anteaters, moles, flying squirrels, cats, wolves, etc. Much the same phenomenon occurred in South America, where marsupials independently gave rise to a range of parallel forms.

Parallel evolution can be distinguished from convergent evolution and evolutionary relay. In convergent evolution, unrelated lineages acquire similar characteristics (bird and bat wings).

Similar to convergent evolution, evolutionary relay describes how independent species acquire similar characteristics through their evolution in similar ecosystems, but not at the same time (dorsal fins of sharks and ichthyosaurs).

Examples

• In the plant kingdom, the most familiar examples of parallel evolution are the forms of leaves, where very similar patterns have appeared again and again in separate genera and families.
• In butterflies, many close similarities are found in the patterns of wing colouration, both within and between families.
• Old and New world porcupines shared a common ancestor, both evolved strikingly similar quill structures; this is also an example of Convergent evolution as similar structures evolved in both Hedgehogs and Echidnas.
• Contemporaneous evolution of the extinct browsing-horses and extinct paleotheres both of which shared the same environmental space.