Parallel evolution
Parallel evolution is the independent evolution of similar traits, starting from a similar ancestral condition due to similar environments or other evolutionary pressures.  Frequently this is the situation in more closely related lineages, where several species respond to similar challenges in a similar way.

# Parallel vs. convergent evolution
For a particular trait, proceeding in each of two lineages from a specified ancestor to a later descendant, parallel and convergent evolutionary trends can be strictly defined and clearly distinguished from one another.  When both descendants are similar in a particular respect, evolution is defined as parallel if the ancestors considered were also similar, and convergent if they were not.
When the ancestral forms are unspecified or unknown, or the range of traits considered is not clearly specified, the distinction between parallel and convergent evolution becomes more subjective.  For instance, the striking example of similar placental and marsupial forms is described by Richard Dawkins in The Blind Watchmaker as a case of convergent evolution, because mammals on each continent had a long evolutionary history prior to the extinction of the dinosaurs under which to accumulate relevant differences.  Stephen Jay Gould describes many of the same examples as parallel evolution starting from the common ancestor of all marsupials and placentals. Many evolved similarities can be described in concept as parallel evolution from a remote ancestor, with the exception of those where quite different structures are co-opted to a similar function. For example, consider Mixotricha paradoxa, a microbe which has assembled a system of rows of apparent cilia and basal bodies closely resembling that of ciliates but which are actually smaller symbiont microorganisms, or the differently oriented tails of fish and whales. Conversely, any case in which lineages do not evolve together at the same time in the same ecospace might be described as convergent evolution at some point in time.
The definition of a trait is crucial in deciding whether a change is seen as divergent, or as parallel or convergent.  In the image above, note that since serine and threonine possess similar structures with an alcohol side chain, the example marked "divergent" would be termed "parallel" if the amino acids were grouped by similarity instead of being considered individually.  As another example, if genes in two species independently become restricted to the same region of the animals through regulation by a certain transcription factor, this may be described as a case of parallel evolution - but examination of the actual DNA sequence will probably show only divergent changes in individual basepair positions, since a new transcription factor binding site can be added in a wide range of places within the gene with similar effect.
A similar situation occurs considering the homology of morphological structures.  For example, many insects possess two pairs of flying wings.  In beetles, the first pair of wings is hardened into wing covers with little role in flight, while in flies the second pair of wings is condensed into small halteres used for balance.  If the two pairs of wings are considered as interchangeable, homologous structures, this may be described as a parallel reduction in the number of wings, but otherwise the two changes are each divergent changes in one pair of 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.

## Parallel evolution in mammals
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 following the break-up of land-masses such as Gondwanaland roughly 100 million years ago.  In South America, marsupials and placentals shared the ecosystem (prior to the Great American Interchange); in Australia, marsupials prevailed; and in the Old World the placentals won out.  However, in all these localities mammals were small and filled only limited places in the ecosystem until the mass extinction of dinosaurs sixty-five million years later.  At this time, mammals on all three landmasses began to take on a much wider variety of forms and roles.  While some forms were unique to each environment, surprisingly similar animals have often emerged in two or three of the separated continents.  Examples of these include the litopterns and horses, whose legs are difficult to distinguish; the European sabre-tooth tiger (Smilodon) and the South American marsupial sabre-tooth (Thylacosmilus); the Tasmanian wolf and the European wolf; likewise marsupial and placental moles, flying squirrels, and (arguably) mice.