- Whales Occasionally Exhibit Atavistic Limbs. - Nestor Galina
Vestiges Versus Atavisms
In a Usenet newsgroup devoted to the discussion of biological origins, Douglas Theobald, PhD defines a vestige as a reduced and rudimentary structure compared to the same complex structure in other organisms ( Talk Origins, 1999-2004, accessed 24 July 2010). Taber’s Cyclopedic Medical Dictionary, 19th Ed defines an atavism as the appearance of a characteristic presumed to have existed in some remote ancestor.
Lars Bejder and Brian K. Hall explain in a 2002 paper in Evolution & Development, that whereas vestigial structures appear in all members of a species, atavisms appear only occasionally in single individuals (“Limbs in Whales and Limblessness in Other Vertebrates")
Vestiges
Vestiges include anatomical structures and physiological processes. For example, naturalists categorize ostrich wings as vestigial because they are rudimentary wings that lack the capacity to function as wings (Theobald). In primates and other mammals, hair often stands erect when anger or fear grips the animal. This makes the animal look larger, augmenting its ability to scare off attackers. This physiological reaction happens in humans too. Richard Dawkins notes, however, that in humans “goose bumps” are a vestigial reaction, as humans have very little hair and no need to raise it in self-defense (The Greatest Show on Earth, 2009).
Atavisms
Atavisms would include the appearance of hind limbs in animals normally lacking them. For example, hind limbs occasionally appear in certain species of whale. In sperm whales, hind limbs appear about once every five thousand births. These limbs, moreover, can attain substantial lengths. A female humpback whale exhibited hind limbs four feet long (125 cm). (Bejder and Hall)
Tails in humans exemplify atavisms too. Although very rare, such tails have been medically documented in more than five dozen cases, according to Theobald. He notes that, like hind limbs in whales, human tails can constitute remarkably complete structures. Sometimes they comprise a complex arrangement of adipose (fat) and connective tissue, bundles of longitudinally arranged striated muscle, blood vessels, nerve fibers, nerve ganglion cells, and specialized pressure sensing nerve organs.
Evidence for Evolution
Vestiges and atavisms are observable facts. They suggest that evolution really happens. Dawkins writes that vestigial wings suggest that ostriches evolved from ancestors who had fully formed wings capable of flight. In parallel fashion, atavistic hind limbs suggest that whales evolved from ancestors who depended on hind limbs for locomotion.
Additionally, vestiges and atavisms, like all visible traits, reflect the genes that give rise to them. Their persistence signifies that those genes, inherited from past generations, are still circulating among members of their respective species. This is exactly what the theory of evolution implies, as evolution can proceed only on the basis of what has gone before (Dawkins).
Since vestiges occur normally in members of the species exhibiting them, vestiges can, and often do, serve functions that still routinely aid the survival of that species. By contrast, atavisms appear to be pure echoes of the evolutionary past.
Michael Drwiega - Since the mid-1990s, Michael W. Drwiega has done both scholarly and fiction writing, his dissertation published by UMI and other works ...
