Vestigial organ: Difference between revisions

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i used to have a brain but now there is wiki i dont need one, its a vestigal organ :)

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+i used to have a brain but now there is wiki i dont need one, its a vestigal organ :)
-'''Vestigial organs''' are [[Organ (anatomy)|organs]] of the body which are smaller and simpler than those in related [[species]]. They have lost, or almost lost their original feature.
-Vestigiality is evidence for [[evolution]], since they only make sense if evolution has occurred.<ref>Futuyma D.J. 1995. ''Science on trial: the case for evolution''. Sunderland, MA: Sinauer p49 {{ISBN|0-878-93184-8}}.</ref> They were one of the puzzles of pre-Darwinian [[natural history]]. The puzzle vanished once biologists realised they once were working [[adaptation]]s, in the [[ancestor]]s of present-day animals.
-They occur in animals (and plants) which have changed their style of life from their ancestors. Thus [[snakes]] lost their legs as their system of movement changed. But one type of snake – the [[boa]]s – have vestigial rear legs and [[pelvis]]. The human [[vermiform appendix]] is another example. That was much larger, and stored [[microbe]]s which produced cellulase to break down plant [[cell wall]]s. [[Leaves]] are the main diet of [[ape]]s, but they are not a main part of man's diet. [[Cellulose]] cannot be digested by our species.
-An unused organ usually degenerates, and becomes smaller or vanishes altogether. Thus [[amphibia]] living in dark caves lose their sight and their body colour.<ref name=Costa>Charles Darwin. 2009. ''The annotated Origin: a facsimile of the first edition of the ''Origin of Species'', annotated by James T. Costa''. Harvard University Press. p137 {{ISBN|978-0-674-03281-1}}</ref> [[Mutation]]s pile up. All structures need energy for their development, maintenance, and weight. This, and the risk of disease in the part (e.g., infection, cancer), provide some selection for the removal of parts which no longer help an organism's fitness.
-The human [[Vermiform appendix|appendix]] had selection against it because of deaths from [[appendicitis]]. Appendicitis was untreatable until modern times. The selection pressure was probably weak, because most cases occur after the age of peak [[fertility]].
-Observations on the cave-dwelling [[amphipod]] ''Gammarus minus'' showed active selection against their vestigial eyes. When mating, the males remain on their females for a week or so after fertilisation, guarding them against other males. The eye sizes of paired males were compared with those of unpaired males. The eye sizes of paired males were smaller than the unpaired ones. Obviously, on average, the paired males would have higher fertility than the unpaired males. The selection gradient was estimated as –0.30, which means the selection for small eyes was quite strong. The researchers supposed that the loss of sight freed more of the nervous system to process other sensory inputs.<ref name=FutEvol>Futuyma D.J. 2005. ''Evolution''. Sinauer Associates, Sunderland, Massachusetts. Sinauer. {{ISBN|0-87893-187-2}}</ref><sup>p310</sup>
-Occasionally, selection turns vestigial organs to a new function. Thus the [[mammal]]ian [[ear ossicles]] were once bones in the jaw of early proto-mammals, the [[Therapsida]]. The [[halteres]] of [[flies]] is a perfect example. The word [[exaptation]] was coined for this phenomenon.
-== Fitness ==
-All functions and structures develop and change because they affect the biological [[fitness]] of the organism. They make it more or less likely that individuals reproduce and contribute genes to the next generation. So, for example, an ability of a frog to survive the bite of its local [[venomous]] snake increases its fitness. But if it lives in an area where the snake does not, then it reduces its fitness. This is because it bears the "cost" of a complicated mechanism without getting any advantage in survival and reproduction. It works the other way round, too. Snakes which eat [[toxic]] prey have to use ways to survive the prey's toxins. The snake's [[digestion]] and [[metabolism]] is slower, and they move around more slowly. See [[rough-skinned newt]] and [[garter snake]].<ref>Really resistant snakes have slower crawl speeds than snakes with little or no resistance. [http://www.sciencedaily.com/releases/2008/03/080311075326.htm Science Daily]</ref><ref>[http://www.gartersnake.info/news/garter_snakes_w.php Garter snake info]</ref>
-Everything has a cost: either it uses up energy or its structure occupies space that might be used differently.
-== References ==
-{{Reflist}}
-[[Category:Evolutionary biology]]