quantumaniac:

Vestigiality and Evolution 

Google Chrome, enough with the red line - vestigiality is a word, trust me on this one. Hearing people that say “whales have leg bones” or “dude, the appendix has no purpose” is pretty common nowadays - but people rarely know what this phenomenon is, or the implications of it. 

What our good friends are thinking of is a phenomenon called vestigiality, which refers to genetically determined structures or attributes that have apparently lost most or all of their ancestral function in a given species. Therefore, throughout normal evolutionary processes, features and structures that once had a clearly defined purpose start to lose importance in a changing environment. 

Many people use the phrase vestigial organs, when in fact vestigiality extends to include patterns of behavior, structures, or biochemical processes. Despite common belief, a vestigial feature need not be complete useless - it just simply serves a difference function than it would have in ancestral times. A classic example is the human appendix - though vestigial in the sense of retaining no significant digestive function, it still plays immunological roles.

Vestigiality need not even be macroscopic, as similar concepts apply at the molecular level — some nucleic acid sequences in eukaryotic genomes have no known biological function. Although some of them may be ”junk DNA”, it is very tricky to demonstrate that a particular sequence of a given genome is truly nonfunctional. As stated with macroscopic features, the fact that it is noncoding DNA does not mean that it is functionless - just that the sequence has lost its formal former function. 

It is fairly easy to confuse the concept of vestigiality with that of a similar phenomenon - exaptation. In exaptation a structure originally used for one purpose is modified for a new one. For example, the wings of penguins would be exaptational in the sense of serving a substantial new purpose (underwater locomotion), but might still be regarded as vestigial in the sense of having lost the function of flight. 

In Charles Darwin’s The Descent of Man, he listed a number of human “vestiges,” including the muscles of the ear, wisdom teeth, the appendix, the tail bone, body hair, and the semilunar fold in the corner of the eye. Interestingly, the formation of goose bumps in humans under stress is a vestigial reflex; its function in human ancestors was to raise the body’s hair, making the ancestor appear larger and scaring off predators. These vestigial organs, Darwin argued, are evidence of evolution and represent a function that was once necessary for survival, but over time that function became either diminished or nonexistent.