Evolution: Biology
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Evolution: Biology |
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Basic Ideas: Vestigial organs |
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ContentThis article introduces the evolution-theoretical argument of vestigialization and discusses criteria for the identification of vestigial organs. Furthermore it responds to the contention, that there are constructional flaws (faulty design) in the structure of living beings. |
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If living beings developed by evolution, not only organs would have to be formed totally new. It would also have to be expected that already existing organs were altered or even reduced in time. An evolutionary process obviously cannot be “closed for renovation” (G. Osche), the living beings on the contrary have to be capable of surviving in every generation. The hypothetical process of evolution preferentially “carries on building” on whatever already exists: reconstruction instead of new construction. In other words, if a structure is not needed anymore, e.g. because of altered environmental factors or after occupying a new habitat it can atrophy or even disappear totally in time. Two evolutionary concepts are connected to these lines of thought: The concept of vestigialization and the concept of faulty design. |
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Vestigialization can therefore be described as degeneration. An organ is interpreted as vestigial, if it is thought to have developed by degeneration of a former, more functional organ. Usually ceasing of selection is cited as a reason for a vestigialization. If an organ isn´t needed anymore because of a change in the circumstances of life, detrimental mutations are not sorted out any longer; they accumulate and lead to the vestigialization of the organ concerned. In this sense vestigial organs count as an evidence for a regressive evolution. (It has to be noted that generally the process of vestigialization itself has not been observed, but is induced by signs; see “How can vestigial organs be recognized?”). Furthermore vestigialization is of course no proof for a constructive evolution (see The assertion, that vestigialization is not suitable for the idea of creation, cannot convince, for a creation model does not generally exclude vestigialization (see “Theological argumentation”). Many examples of vestigial organs can be interpreted as mere degeneration, for instance the vestigial eyes of cavefish (pic. 74) or the vestigialization of insect wings in special habitats. |
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Coupling of vestigialization and new construction. The situation often gets more complicated, however, if the degenerated organ is not viewed in isolation. The example of blind cave characin Astyanax (pic. 74) may illustrate the problem. These fish live in pitch-dark caves, where eyesight would not be of any use. Still they need some sense of direction. Therefore they are among other features equipped with an area of taste buds around the mouth. All characins have this kind of taste buds, but the cave living variety shows them on a significantly larger area (pic. 75): The loss of eye sight is compensated. Genetic studies show, that an enlargement of the area of taste buds occurs, when modifier genes, by which the development of taste buds are suppressed, are invalidated. Now the genetic program for the development of taste buds on the head´s surface is activated to a great extent. The extension of the area of taste buds is not due to a change in construction but to a failure in control. New structures or genes have not been formed. |
The abdominal bones of whales: compensation by means of macroevolution? Not every compensation can be explained by microevolutionary processes as in above-mentioned example. A new construction of organs linked to degeneration often requires macroevolutionary changes – if it happened on an evolutionary way. A well-known example is the abdominal bones of the cetaceans: paired bones that are embedded in the whale´s muscle mass, exactly where the pelvis of land-living mammals can be found (pic. 76) – without any joint to the spine. Some cetaceans have a second bone pair, the bowhead whale even a third one (pic. 77). Evolution theory interprets these bones as a vestigial pelvis and possibly as the remains of hind legs. This way the bones are taken as a hint for a transition from land living mammals to the whales. Such a transition, though, requires far reaching reorganization of the body – this would certainly imply macroevolution. That means that vestigialization (pelvis hind legs) and macroevolution (reorganization of many organs) somehow would have gone hand in hand. In these cases, vestigial organs are taken as indirect proof for macroevolution. However, one may critically inquire, is vestigialization empirically proven at all? How can the abdominal bones of whales be identified as degenerated pelvis (and leg) bones? This question leads us to the next point. |
How can vestigial organs be recognized? |
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Whenever a species with an organ fully developed is crossbreedable with any other species showing the same organ vestigialized, this capability of hybridization can be taken as an indication for vestigialization that actually happened. This is the case with the cave characin and the seeing variety, that can be crossed without any problems (see pic. 2, pic. 78). The human wisdom teeth are a further typical example. It is most likely, that originally all 32 teeth were functioning and needed. That the wisdom teeth are not necessarily needed anymore today, might be due to a change in human eating habits. A degenerative development of these molars was not of any disadvantage and therefore possible. The vestigialization of the wisdom teeth is different from person to person. A third example are flightless insects that live on stormy islands (see In the case of the whales the criteria of hybridization capability is not effective. There is too big a difference between whales and their presumed land-living ancestors. How then can vestigialization be identified in cases like these? There are mainly two criteria:
1. Uselessness. If it could be proven that an organ is totally useless and has no function at all this could be seen as a hint for the interpretation that the organ once had a function, which now got lost. The abdominal bones of the whales however cannot be called vestigial this way, for they do have a very important function: They form the base for the genital muscles, possibly as well for the elevator muscle of the anus and furthermore support abdominal wall and viscera. Anyway, uselessness is difficult to verify. It can often only be said, that a function is unknown – not, that there is none at all. In many cases, the vital or at least useful function of organs has been discovered long after describing them as useless. This is why the argument of uselessness is problematic and not well founded.
In their studies, the American scientist William Parker and his team came to the conclusion that the human appendix represents a kind of safe house and rescue post for symbiotic bacteria, that supports the increase of useful intestinal bacteria and enables or promotes a repopulation after diarrhea. These bacteria prevent the spreading of dangerous pathogens in the human digestive tract, which is especially important after diarrhea. Details are explained in
Inconsistencies between structure and function. A modified argumentation says that vestigial organs indeed do have a function – but that this function is too insignificant compared to the structural expense. This kind of inconsistencies between structure and function are hardly verifiable, as well. |
2. Comparison with homologous organs. In numerous cases, the presumed vestigialization is explained by a comparison with homolog organs (for the aspect of homology see Evolution theory reasons, that the vestigial pelvis and femur of the whales do have a specific function, but that there is a homologous similarity to the equivalent bones of land living mammals. With this, the similarity is taken as a hint for a connection of origin. So here the actual argument for a connection of origin is the observation of similarity, which is not an independent proof for macroevolution as is shown in Partial loss of function? With this, the argument is touched on as well, that vestigial organs can be recognized by a remaining rest of function. Again, the abdominal bones of the cetaceans give a suitable illustrative material. The loss of function supposed in this case, based on pelvis and legs of land living mammals, is no observable fact but interpretation in the light of evolution theory. It is indeed possible that the abdominal bones of the cetaceans were developed as a pelvis in their postulated ancestors but this cannot be read from the form of these skeleton elements. This interpretation is possible in an evolutionary context, but not compelling. It can as well be thought of abdominal bones that were created as they are formed in today living whales (with specific differences) – a specification that is adequate for a whale-like way of living. (See |
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Faulty design |
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Some structures are seen as phylogenetically caused faulty design (see The chiasm of mammalian oesophagus and windpipe is occasionally cited as an example. Because of the danger of choking this construction seems to be suboptimal and a phylogenetically caused faulty design. On the other hand, a missing chiasm would place the oesophagus right in front of the heart, which would mean its strangulation in the case of an enlarged heart. Besides the chiasm has some advantages: Phlegm that is transported upwards through the windpipe can take its way down the oesophagus. Besides, this construction allows mouth breathing, which is a thankworthy construction under stress, when expectorating foreign bodies or having a cold. Furthermore, the construction is space-saving. A discrepancy of structure and function does not exist. |
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An often cited example in this connection is the retina at the background of vertebrates´ eyes. The photosensitive layer of cells is inversed, so that the incoming light has to pass the nerve tract and layers of cells, before it hits the photosensitive area of the retina (pic. 79, right). It seems to be better the other way around (pic. 79, left). Nevertheless, the eye fulfills its task outstandingly in spite of this “mistake”. The inverse position has the advantage that scattered light is filtered out. Detailed studies of the retina and its function were not able to show any constructional flaws. The actual mistake lies in the isolated view of a structure, ignoring the organ as a whole, the complete organism and its ontogenesis (=individual development). The argument of imperfection is often used too rashly, before detailed functional connections are disclosed. Furthermore, it cannot be spoken of a better working alternative, as long as no alternative has been tested. Details are explained in The eye – a bone of contention. A classical example of “Intelligent Design” criticized. (http://www.wort-und-wissen.de/sij/sij131/sij131-1.html) |
Theological argumentation |
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The argumentation of “vestigialization” and “faulty design” often goes hand in hand with criticizing the idea of creation. It is not seldom argued that vestigial organs contradict a well thought-out creation, because a creator would not create any useless organs. The argument of imperfection (see top) is used similarly. Out of this an indirect proof for evolution is made – what contradicts creation supports an evolutionary point of view. This argument is of theological nature, for it refers to ideas of a creator´s methods. A theological argumentation is out of place within the scope of evolution theories, though. Not so of course, when looking at creational theories. Here this question does have its right (see Furthermore, it has to be taken into account that arguments against creation are not automatically arguments for evolution. Finally, the proof of uselessness or imperfection is difficult to give and was often refuted. |
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Overview 
Vestigialization
