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Vestigial Organs
Some classes of characters are more valuable than others as
reliable indicators of common ancestry. Often, the less any
part of an animal is used for specialised purposes, the more
important it becomes for classification. Vestigial organs are
an example of this. If vestigial features have no clear function
and are no longer subject to natural selection, the common
ancestry between different species is not clouded by later
adaptation to particular purposes. It is sometimes argued that
some vestigial organs are not be truly vestigial i.e. they may
perform some small function. While this may be true in some
cases, the features can still be considered vestigial if their
new role is a minor one, unrelated to their original function.
Ancestors of Modern Whales
1.8m long
Pakicetus (early Eocene) a carnivorous, four
limbed, early Eocene whale ancestor, probably
rather like a large otter. It was still partly terrestrial
and not fulry adapted for aquatic !ife.
Vestigial organs are common in nature.
The vestigial hindlimbs of modem whales
(right) provide anatomical evidence for
their evolution from a carnivorous, four
footed, terrestrial ancestor. The oldest
known whale, Pakicetus, from the early
Eocene (- 54mya) still had four limbs. By
the fate Eocene (~40mya), whales were
fully marine and had fost almost all traces
of their former terrestrial life.
2.5m long
Protocetus (mid Eocene). Much more whale-like
than Pakicetus. The hindlimbs were greatly
reduced and although they still protruded from the
body (arrowed), they were useless for swimming.
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20~25m long
Basilosaurus (late Eocene). A very large ancestor of
modem whales. The hind limbs contained all the leg
bones, but were vestigial and located entirety within the
main body, leaving a tissue flap on the surface (arrow).
Forelimb
Modern Right whale
up to 20m
Vestigial Organs in Birds and Reptiles
In all snakes (far left), one lobe of the lung is
vestigial (there is not sufficient room in the
narrow body cavity for it). In some snakes
there are also vestiges of the pelvic girdle and
hind limbs of their walking ancestors. Like all
ratites, kiwis (left) are flightless. However,
more than in other ratites, the wings of kiwis
are reduced to tiny vestiges. Kiwis have
evolved in the absence of predators to a totally
ground dwelling existence. .
1. In terms of natural selection explain how structures, that were once useful to an organism, could become vestigial:
2. Suggest why a vestigial structure, once it has been reduced to a certain size, may not disappear altogether:
3. Whale evolution shows the presence of transitional forms (fossils that are intermediate between modern forms and
very early ancestors). Suggest how vestigial structures indicate the common ancestry of these forms: