Specific Examples from anatomy

baleen whale

Hind structures in whales

Whales possess internally reduced hind parts such as the pelvis and hind legs (Fig. 5a). Occasionally, the genes that code for longer extremities cause a modern whale to develop miniature legs. On October 28, 2006, a four-finned bottlenose dolphin was caught and studied due to its extra set of hind limbs. These legged Cetacea display an example of an atavism predicted from their common ancestry.

Insect mouthparts

Many different species of insects have mouthparts derived from the same embryonic structures, indicating that the mouthparts are modifications of a common ancestor's original features. These include a labrum (upper lip), a pair of mandibles, a hypopharynx (floor of mouth), a pair of maxillae, and a labium. Evolution has caused enlargement and modification of these structures in some species, while it has caused the reduction and loss of them in other species. The modifications enable the insects to exploit a variety of food materials.

Other arthropod appendages

Insect mouthparts and antennae are considered homologues of insect legs. Parallel developments are seen in some arachnids: The anterior pair of legs may be modified as analogues of antennae, particularly in whip scorpions, which walk on six legs. These developments provide support for the theory that complex modifications often arise by duplication of components, with the duplicates modified in different directions.

Pelvic structure of dinosaurs

Similar to the pentadactyl limb in mammals, the earliest dinosaurs split into two distinct orders—the saurischia and ornithischia. They are classified as one or the other in accordance with what the fossils demonstrate. Figure 5c, shows that early saurischians resembled early ornithischians. The pattern of the pelvis in all species of dinosaurs is an example of homologous structures. Each order of dinosaur has slightly differing pelvis bones providing evidence of common descent. Additionally, modern birds show a similarity to ancient saurischian pelvic structures indicating the evolution of birds from dinosaurs. This can also be seen in Figure 5c as the Aves branch off the Theropoda suborder.

Pelvic structure of dinosaurs

Pentadactyl limb

·The pattern of limb bones called pentadactyl limb is an example of homologous structures. It is found in all classes of tetrapods (i.e. from amphibians to mammals). It can even be traced back to the fins of certain fossil fishes from which the first amphibians evolved such as tiktaalik. The limb has a single proximal bone (humerus), two distal bones (radius and ulna), a series of carpals (wrist bones), followed by five series of metacarpals (palm bones) and phalanges (digits). Throughout the tetrapods, the fundamental structures of pentadactyl limbs are the same, indicating that they originated from a common ancestor. But in the course of evolution, these fundamental structures have been modified. They have become superficially different and unrelated structures to serve different functions in adaptation to different environments and modes of life. This phenomenon is shown in the forelimbs of mammals. For example:

·In the monkey, the forelimbs are much elongated to form a grasping hand for climbing and swinging among trees.

·In the pig, the first digit is lost, and the second and fifth digits are reduced. The remaining two digits are longer and stouter than the rest and bear a hoof for supporting the body.

·In the horse, the forelimbs are adapted for support and running by great elongation of the third digit bearing a hoof.

·The mole has a pair of short, spade-like forelimbs for burrowing.

·The anteater uses its enlarged third digit for tearing down ant hills and termite nests.

·In the whale, the forelimbs become flippers for steering and maintaining equilibrium during swimming.

·In the bat, the forelimbs have turned into wings for flying by great elongation of four digits, while the hook-like first digit remains free for hanging from trees.

Recurrent laryngeal nerve in giraffes

The recurrent laryngeal nerve is a fourth branch of the vagus nerve, which is a cranial nerve. In mammals, its path is unusually long. As a part of the vagus nerve, it comes from the brain, passes through the neck down to heart, rounds the dorsal aorta and returns up to the larynx, again through the neck.

This path is suboptimal even for humans, but for giraffes it becomes even more suboptimal. Due to the lengths of their necks, the recurrent laryngeal nerve may be up to 4m long (13 ft), despite its optimal route being a distance of just several inches.

Recurrent laryngeal nerve in giraffes

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