* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Vestigial Organs
Survey
Document related concepts
Transcript
the tvidenceTor tvoiimon 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. < 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: