Biology Reference
In-Depth Information
Convergence or parallelism is the phyletically independent occur-
rence of the same states of character within different lineages.
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Reversion
is the return to an ancestral state of character.
These notions apply to any category of objects evolving within some
kind of environment, whether natural or artificial, through duplication
and extinction events. Consider the design process of automobiles: two
cars may look very alike, although they are not of the same model. There
are three possible explanations. The cars may be produced by the same
manufacturer and therefore have certain design features in common (this
is straight homology). Alternatively, two manufacturers may design simi-
lar cars based on a comparable perception of consumer taste and car func-
tionality (this is convergence homoplasy). Finally, a newly conceived car
may resemble an earlier model when a certain look comes back into fash-
ion (this is reversion homoplasy).
In evolutionary biology, two textbook examples illustrate the notion
of homoplasy due to shared environmental constraints (homoplasy due
to chance being trivial enough):
(1)
Active flight in tetrapods — thrice upon a time…
Active flight devel-
oped at least three times among the Tetrapoda — a number of times
in ancient reptiles such as pterosaurs, once in avians (birds) — and
once or twice in chiropterans — pteropods (flying foxes) and bats.
Although the forelimbs of all tetrapods are homologous organs,
laws of aerodynamics and not common ancestry are responsible for
the similarities of their wings, which have roughly converged in their
aerodynamic morphology because the function of the forelimbs of
these taxa as true wings is identical.
(2)
Return to the sea
. Cetaceans (dolphins and whales) and sirenians
(manatees and dugongs) are mammals that have fully adapted to an
aquatic habitat. The necessary transformations for this adaptation
occurred long ago twice: starting from two different land-dwelling
mammalian ancestors, the two lineages evolved their capacity for life
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The term “convergence” is used for distant lineages, while “parallelism” is reserved
for closer ones.
