134 Vestigial Structures

Key Idea: The persistence of vestigial structures provides evidence for common ancestry. Vestigial structures are anatomical features that have been retained through a species' evolution but have lost their ancestral function. Having no obvious function, vestigial structures are no longer subject to natural selection and remain unchanged through a lineage. Their presence can be used to detect common ancestry. Vestigial structures are often homologous to organs that still have a function in other species. For example, the human tail bone (coccyx) has lost its original function (balance and mobility) and is homologous to the fully functioning tail in many other primates.

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:

Ancestors of modern whales

3D

1.8 m long

Pakicetus (early Eocene). A carnivorous, four limbed, early Eocene whale ancestor. It was still partly terrestrial and not fully adapted for aquatic life.

Vestigial organs are common in nature. The vestigial hind limbs of modern whales (right) provide anatomical evidence for their evolution from a carnivorous, four footed, terrestrial ancestor. The oldest known whale, Pakicetus, from the early Eocene (~54 mya) still had four limbs. By the late Eocene (~40 mya), whales were fully marine and had lost almost all traces of their former terrestrial life.

2.5 m long

Protocetus (mid Eocene). Much more whale-like than Pakicetus. The hind limbs were greatly reduced and although they still protruded from the body (arrowed), they were useless for swimming.

Femur

Pelvis

Vestigial hindlimb

20-25 m long

Basilosaurus (late Eocene). A very large ancestor of modern whales. The hind limbs contained all the leg bones, but were vestigial and located entirely within the main body, leaving a tissue flap on the surface (arrowed).

Forelimb

Modern right whale up to 20 m

F&B

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 evolved in the absence of predators to a totally ground dwelling existence.