Evolution Blogspot 8. Expanded Definitions, Part VII

The possession of vestigial organs is regarded as evidence of evolution.

21. Vestigiality and Atavisms: Vestigial organs are special features of animal heredity which only make sense by recognizing that they represent remnants of traits that were once useful in an evolutionary ancestor. Atavisms are bodily structures that occasionally occur in animals which recapitulate an ancestral trait, such as a demonstrable leg extending from the body of a whale. That leg is an atavism, because whale ancestors had legs. If a human baby is born with a fifth leg, that is a birth anomaly, not an atavism, because human ancestors never had five legs.

Examples include:

1. ~Ostrich wings are vestigial in that the flightless birds descended from flying ancestral birds, and the ostrich is left with definite wings that are too small to allow flight. The wings are now useful for maintaining balance while running, an evolutionary co-option of the small wing remnants—new tricks for old genes.
2.  ~Other flightless birds are: the South American rhea, the New Zealand Kiwi with wings only a few inches long, and flightless rails, grebes, ducks, wood rails, teals, moorhens, the extinct dodo, and the penguin whose flightless wings have evolved into flippers which allows swift underwater swimming. All of these birds have wings; they are just remnants of the powerful wings of flying ancestors. They all have the same wing bones as their flying counterparts. Some of the wings are functional, i.e., they have evolved to perform a function, often a significantly reduced one, other than the original one of flying; some are nonfunctional. Vestigial organs can be either; the crucial issue is that the vestigial structure no longer functions as it did in the ancestor.
3. ~Vestigial eyes are present in several species and are relatively common. An example is the Eastern Mediterranean blind mole rat whose eyes are miniscule, less than a millimeter across and hidden beneath a layer of skin. The eyes are incapable of seeing. Evolution towards loss of sightedness began 25 million years ago from sighted rats. True moles independently lost their sight, also retaining only a vestigial remnant. Blind cavefish, some spiders, salamanders, burrowing snakes, salamanders, shrimp, and beetles lost their eyesight, and some even their eyes completely by prolonged–evolutionarily long–lack of access to light.
4. ~Whales have vestigial pelves and leg bones imbedded in their tissues. They were once part of an ancestor’s skeleton, but that connection is now represented only by string-like connective tissues.
5. 5. ~The human vermiform appendix is of only minimal usefulness, and some human babies are born without one at all. The structure was of significance to an herbivorous ancestor as the cecum, and both the cecum and the appendix decreased in size as the size of the degree of reliance on an herbivorous diet decreased.
6. ~The human coccyx remains to tell us of our long ago ancestors who had long, useful tails. The rare baby is born with an actual tail. Many humans have a specific tail muscle, the extensor coccyges, which is identical to the muscle found in monkeys and other mammals that lifts the tail. In them it is of real function. In humans, it is present and has no tail to extend. The presence of the coccyx and certainly of the tail, is undeniable evidence of evolution in humans—the genetic capacity to produce a tail is there in our genome, but, for most of us has become a relic of the dim past. Arrector pili, the tiny muscles at the base of every hair that cause hairs to stand up during cold or attack have no real function in relatively hairless humans who tend to use their brains rather than their fangs and claws for defense. Humans also have three ear pinna muscles which, for most people, are useless, but for some, a vestigial function for vestigial muscles can be seen in those who can wiggle their ears.
7. ~Atavistic whale legs occur about once in 500 births with many of them having the full complement of the Bauplan tetrapod limb. Some go so far as to have feet and toes. Atavisms are considered by experts to be re-expressions of genes that have lain dormant after having been expressed in ancestors. The whale genome contains degraded genes that have evolved to be relics of past DNA because–by natural selection–they were no longer needed and fell into disuse.
8. ~Modern horses have one toe, the middle ancestral one. However, horse embryos start out with three toes. Rarely, the other two toes continue to grow to become atavistic toes complete with hooves which are very nearly identical to the ancient horse predecessor, Merychippus, which lived 15 MYA [million years ago].

The material presented here and the definitions discussed in these first eight blogspots on evolution give the reader a direction and a set of definitions to use in understanding the next several blogs which delve into the supporting evidence for the theory of evolution. Aspects of natural selection can be taken as observable facts which occur so recently as to be seen by people of today on a fairly regular basis. Others include recent historical observations. The following blogspot will present eight examples that can only be explained by the process of evolution.

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