Why do vestigial structures persist in modern organisms?
954249
there is no such thing as a vestigial organ.  they are just parts of the body that we don't know what they are their for.  The appendix was at one time thought to be vestigial but is know known to help fight infection.
666851
Structural aspects of plants (=plant anatomy) are very often fundamental to the higher level classifications (family and above) we have today ... it is surprising how many "old" classifications (dating to the late 19th century) that were based on anatomy have been shown to be good with modern molecular data. Although the more outward parts of plants have taken on many and varied forms, the anatomy is more conservative and can reflect relationships that may not be obvious from leaf or flower characters.
745175
A homologous structure is a structure shared between two species (or between the male and female of the same species) , but with a different function in each.\n\nExamples are: the same bones in the same relative positions in primate hands, bat wings, whale and dolphin flippers, pterosaur wings, horse legs, the forelimbs of moles, and webbed amphibian legs.  The same five bones in the mammalian inner ear correspond to the five bones in the hinged jaws of reptiles.  Testicles in males correspond to ovaries in females.\n\nA homologous structure is evidence that the two organisms are related by evolution because it is evidence of a common ancestor that had this structure, but it became "repurposed" for a different function.  In other words, it provides an explanation of why two radically different functions as those served by the human hand and a bat's wing, would have the same underlying bone structure ... because they are related by ancestry, they were working with the same raw materials inherited from that ancestor.\n\nThe more homologous structures shared between two organisms, the stronger the evidence of common ancestry.\n\nIt is also evidence against intelligent design, because these are examples where the structure preceded the function (the same structure modified for two different functions) ... whereas in design, function precedes structure (a designer sees a function, and then designs the most optimal structure to serve that function).
72921
Many small organisms rely on ciliary or flagellar motion to move. These mechanisms only work efficiently at low dimensions, i.e. when the density and viscosity of the medium are more important than inertia (which is given by greater body mass).\nThere are also ameboidal movements, caused by relative displacement of citoskeletal elements. These movements are more common in single-celled organisms, but also in some of our own cells (some of the white blood cells, for example)
462810
yes it is a vestible(correct spelling?) structure meaning it is not necessary to survival/ used enough to be kept as a part of us.  I.E. our tonsils and appendix.
1130439
Because you are a human organism.   The entire system evolved together, with much feedback from one system to another because that has high survival value.  Why do you question it; it works!  It's Mother Nature's way.
3451
First of all, when dealing with biology, you should never BELIEVE that there is a PURPOSE for anything. This sort of thinking evokes the notion that this structure was disigned to do something. Biological structures evolve into their present form because that form is more fit than any of the alternative forms that came before.\n\nAs for the outer ear (the visable portion that protrudes from the side of your head) the function they provide is as a condensor to collect and concentrate energy waves that are able to be decifered by our inner ear and brain as sound.
342488
I think what you are referring to is "natural selection" aka "survival of the fittest"\n\nBasically the animal most suited to its environment due to mutation (the ones with genetic proclivity to have longer necks, stronger arms etc) will get more food/mates and will tend to, over a long period, tend to make the species move in that direction.\n\nBasic answer - if you need more drop me a line.
1092544
Hey does your question mean - how does fossilation happen\n\nMost fossils are found in sedimentary rocks, those rocks produced by the accumulation of sediment such as sand or mud. Wind and other weathering conditions wash away sediment on land, depositing it in bodies of water. For this reason, fossils of sea creatures are more common than those of land creatures. Land animals and plants that have been preserved are found mostly in sediments in calm lakes, rivers, and estuaries.\n\nThe likelihood that any living organism will become a fossil is quite low. The path from the organic, living world to the world of rock and mineral is long and indirect. In the best conditions, fossilization will occur if an animal or plant dies and is quickly covered over with moist sediment. This prevents the animal or plant from being eaten by other organisms or from undergoing natural decay through exposure to oxygen and bacteria. The soft parts of an animal or plant decay more quickly than its hard parts. Teeth and bones are therefore more likely to be preserved than skin, tissues, and organs. Because of this fact, most fossils come from the time period dating to almost 600 million years ago, when organisms began to develop skeletons and hard parts.\n\n=====================================\nAm not too sure of my German\n\nDie meisten Fossilien werden die von der Ansammlung der Ablagerung in sedimentären Felsen, die Felsen gefunden werden hergestellt wie zum Beispiel Sand oder Schlamm. Wind und andere Verwitternbedingungen waschen weg Ablagerung auf Land, deponierend es in Gewässern. Aus diesem Grund sind Fossilien von Seenkreaturen gemeinsamer als jene von Ländernkreaturen. Landen Sie Tiere und Pflanzen, die bewahrt worden sind, sind gefunden größtenteils in Ablagerungen in ruhigen Seen, Flüssen, und Gezeitenmündungen. \n\nDie Wahrscheinlichkeit, die irgendein lebender Organismus ein Fossil werden wird, ist ziemlich niedrig. Der Pfad vom organischen, Welt zur Welt von Felsen und Mineral lebend, ist lang und indirekt. In den besten Bedingungen wird Versteinerung stattfinden, wenn ein Tier oder Pflanze stirbt, und ist schnell über mit feuchter Ablagerung bedeckt. Dies verhindert das Tier oder die Pflanze, von durch andere Organismen gegessen zu werden, oder erlebend von natürlichen Verfall durch Aussetzung zu Sauerstoff und Bakterien. Die weichen Teile von einem Tier oder Pflanze verfallen mehr schnell als seine harten Teile. Zähne und Knochen sind daher wahrscheinlicher, als bewahrt zu werden, enthäutet, Gewebe, und Organe. Wegen dieser Tatsache kommen die meisten Fossilien vom Zeitraum datierend zu fast 600 Millionen vor Jahren, als Organismen begonnen haben, Skelette und harte Teile zu entwickeln.
207219
They breathe air because as mammals, they have lungs. That's a constraint imposed by their evolutionary history; no mammal has "re-evolved" gills or other respiratory surfaces.\n \nOn the other hand, living in salt water has allowed them to develop huge bodies with little skeletal support. On land, where gravity is not counteracted by buoyancy, the sheer pressure exerted by the weight of their own bodies makes their bodies collapse. In addition, their skin dehydrates rapidly when exposed to the air. Thus, adaptation to a given environment always constrains survival in different environments. Evolutionary paths are like that.
648453
Here's the thing: I'm not an expert on fetal anatomy, but human fetuses do not have gills.  They do have tails for a few weeks, appearing around the 4th week and dispearing around the 8th.  They also have regions that are homolgous to those that develop into gills in aquatic organisms, but will form the throat of the human fetus.\n\nThe gill and tail myth was created by a man named Haeckel.  Now, Haeckel actually (whatever creationists may say) did have some good ideas, but he was a very bad scientist and had an unfortunate tendancy, especially in the later part of his career, to exagerate and even completely fabricate morphological features of embryos.\n\nHOWEVER, there _is_ an association between fetal development and evolutionary lineage.  It's just a bit more subtle than Haeckel claimed.  While embryos don't go through stages that ressemble the adult forms of their ancestors, the is a distinct correlation between the age at which the morphology of embryos of different species diverges and the evolutionary relationship between those species.  For example, human embryos and chimp embryos look really similar for a lot longer than human embryos and snake embryos.  But human and snake embryos look similar to each other long after they have completely diverged from the development of, say, a sea urchin.\n\nYour point about gills and tails isn't very accurate, and the actual connection between evolution and development is not very accesable to the non-scientist, but there is plenty of evidence for evolution that doesn't take a PhD to understand.  Here are a few points that I hope will help you next time you want to defend the theory of evolution:\n\n\n1. Vestigial structures\n\nOne example of a vestigial structure in humans is the appendix. It has no function. Removing it is harmful only in so far as any surgery is associated with certain risks (so I wouldn't have it removed just for fun, but it's because of the surgery not the absence of the appendix). An appendix is found in several mammals. In humans it is small and has no known function. In rabbits and many other herbivores it is large and is used in the digestion of cellulose.\n\nAnother example of a vestigial structure is the hipbones of snakes. Snakes evolved from quadrupeds, and some species still retain not on pelvises, but tiny protrusions of bone in a location that corresponds to the location of the legs in other quadrupeds.\n\n2. Microevolution can be observed in both a controlled laboratory setting and in nature.\n\nCreationists will tell you that microevolution does not prove macroevolution.  This is true.  However, it does provide evidence that macroevolution may be possible. Taken in the context of the other evidence for evolution, microevolution provides a mechanism by which macroevolution could occur. On its own, it doesn't prove that macroevolution occurs, but it does suggest that it is possible.\n\n3. The fossil record.\n\nAlthough the fossil record does not provide a “complete” record as fossils form only under certain conditions, it does show a gradual change in the morphology of species as well as numerous extinct species.  There are a number of methods used to date fossils, and the time period from which a fossil comes can be determined with reasonable accuracy.\n\n4. Imperfect structures (the blind spot of the mammalian eye, for example).\n\nI want to mention the bind-spot of the mammalian eye specifically because creationists often hold up the human eye up as an example of something that is too perfect to occur by "chance" (i.e. as the result of the natural selection of beneficial changes among random mutations).\n\nThere are also numerous examples where the morphology of a species is constrained by similar patterns in its ancestors (quadrupeds, vertebrates, etc.).\n\n5. Developmental biology reflects evolutionary lineage.\n\nCreationists like to bring up a man named Haeckel in response to this argument.
801430
There Sach's organ has a stack of electroplaques, which acts somewhat as a stack of battery plates. There evolutionary history is not know to me.
622375
'Survival of the fittest' works to a lesser degree in the opposite direction too; just as animals with features that allow them to adapt better to their environment are more likely to successfully reproduce, animals with features that are not necessary might find them impeded by them and, thus, might find themselves less able to survive long enough to reproduce.\n\nIf you will, imagine a village of our distant ancestors. As we're not hanging from trees anymore (if we ever did), our tail has no practical purpose, but it's still hanging around (pun partly intended). Now, through random mutation protohuman A's tail turns out to be a little shorter than protohuman B's tail. So when a big predator comes chasing after them, A has an advantage because 1) the tail is not getting in the way of his running as much and 2) the predator has more difficulty seizing on to protohuman A. So protohuman B becomes a predator's lunch and protohuman A gets to pass his short-tailed genes onto his children.\n\nNow, you might say 'well, how can that possibly be enough? A 1- or 2mm difference from one generation to the next?' but there also is a concept of a 'feedback loop', which is quite logical to perceive. It's that, frankly, tails ain't sexy.\n\nAll of the predators munching on long-tailed protohumans might have developed a sense among the protowomenfolk that a long tail does not a good father make (you know, motion of the ocean etc.) So then, through the development of this biological preference (call it 'taste' or 'fashion'), the longtailed boys become the wallflowers - always the bridesmaid, never the mixed metaphor.\n\nIn short, every generation, the shorttails are more popular and have more children, and for all I know strut in a manly way around the longtails. This seriously alters the 'genetic lottery' of DNA, with the result that, over countless generations, we're down to the tiny stub that we have today (excepting the unfortunate).
619772
Because the basic mechanisms of self-replicating molecules such as RNA and DNA were SO successful that they continued to replicate and mutate, to thrive and survive.  Success breeds success.\n\nIf other mechanisms evolved to compete RNA and DNA, they might have lost to competition for resources and so their lineages died out.
420898
Has to do with how the limbs develop in the first place during embryogenesis.  In the octopus, the limbs likely just grow outward from the body (same as for a lizard's tail).\n\nIn humans your limbs do not grow outward from the body.  They develop in a much more complex process during the initial stages of body development that cannot be repeated because the body is never again in such a primitive stage.
1020218
To evolve and survive. The process is called metamorphosis.
103977
Whales evolved from terrestrial four-legged hoofed animals. They've acquired hydrodynamic fusiform bodies, including tail flukes (horizontal, in contrast to the tailfin of fishes, and without internal support), dorsal fins (more or less developed, or absent); their forelimbs are flippers and their hindlimbs are extremely reduced and within their trunks. They've lost all external structures that could offer resistance to the waterflow, such as external ears, hair, or external genitalia, and thus have a smooth body surface. This is important in order to keep a laminar flow of water around the body (turbulent water does not provide lift).\nIn addition to their hydrodynamic shape, they have a well-developed layer of fat (blubber) that provides both insulation and some buoyancy. They move basically by undulations of the caudal vertebral column that move the tail up and down.