 What are the causes of isotope variation in nature?
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the simple answer is that it's due to the radioactive decay series. how isotopes decay and how long...
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Change into a different element altogether.\n\nNatural decay of heavy elements occurs sending off alpha- or beta-particles, i.e. helium4-cores or electrons.\n\nIn the case of alpha radiation, the element will move two positions down in the periodic system, in case of beta radiation it will move one position up.\n\nUranium 238 emits alpha-radiation, forming Thorium 234. See the link below for consecutive decay reactions.
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δ18O\n\nOxygen has 9 isotopes and natural oxygen is a mixture of three stable isotopes. Oxygen 18 is 0.205% of all natural oxygen. Because it is heavier than Oxygen 16 (the normal oxygen atom) it behaves differently during some earth processes, notably as part of the water cycle. As water evaporates from the ocean surface, the heavier molecules (those with the 18O) tend to be left behind. As water vapor forms clouds and rain, the first precipitation is more likely to contain the 18O.  During periods when glacial ice is building up on the planet, water on the continents is isotopically lighter, and the ocean water is isotopically heavier. \n\nBenthic forams, (sea creatures that live on the deep ocean floor) record this change in isotopic composition of sea water as they form their shells of calcium carbonate (which contains oxygen also).  Since the minimum temperature for deep ocean sea water is limited to the freezing point of sea water, the effects of changing temperature on 18O concentration, and the change in the isotopic composition of sea water can be separated and the net change in 18O determined.  These changes, measured from both ocean floor cores and ice cores from polar regions, give both a signal that ice volume on the planet is changing over time and that global temperature is changing over time. \n\nhttp://www2.ocean.washington.edu/oc540/lec01-19/\n\nAn interesting example of how this property of oxygen isotopes can be applied is in this paper on the Eocene (57-36 million years before present) which was a warm period in the Earth’s history.  Their work using cellulose from fossil trees in the Arctic suggested a mean annual temperature for that area of −2.7 °C (± 2.5).  This is not radically different from modern day arctic temperatures, and it disagrees with other evidence of the warmer Eocene climate. These researchers suggested that the δ18O change may have been influenced more by climate patterns than temperature alone, to account for the low δ18O values. \n\nAnother study \nhttp://insci14.ucsd.edu/~tkobashi/paper/geology2001.html \nused marine mollusks to indicate that mean annual sea temperature in the Eocene decreased from 26-27 °C to 22-23 °C in the Oligocene, a 5 degree drop in global temperature averages.
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Atoms of chemical elements may have many isotopes (different forms) with the same atomic numbers but different atomic weights / atomic mass numbers. If a nuclide is radioactive, it is sometimes referred to as a radionuclide or (when the atom in question is understood) a radioisotope.\n\nA radionuclide is an atom with an unstable nucleus. The radionuclide undergoes radioactive decay by emitting a gamma ray(s) and/or subatomic particles. Radionuclides may occur naturally, but can also be artificially produced.\n\nRadionuclides are often referred to by chemists and biologists as radioactive isotopes or radioisotopes, and play an important part in the technologies that provide us with food, water and good health. However, they can also constitute real or perceived dangers.
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Ignore the abusive idiot above. He has no idea what he is talking about.\n\n\nThere are lots of different radioactive elements. Some are dangerous but the vast majority are harmless. \n\nYour body and everybody else's is already full of radiocative elements. You are loaded with isotopes like 13Carbon., tritium, 44Calcium and so on. Every living thing that has ever existed on Earth has been losded with those radicoative elements. We've evolved with them from the beginning and as a result they are quite harmles to us in normal doses.\n\nA few unnatural radioactive isotopes are harmful in small doses, and some natural isotopes are dangerous if they are concentrated in the body. But the general answer is that radiocative elements are quite harmless.
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I was taught in school that it is due to radioactive decay of elements in the core.  That is pure bullcrap.  If it were true, every time a volcano exploded, there would be radioactive toxins spread out across the world.  Others say gravity pushing on the substances create heat and liquification of earth.  This may be somewhat true.  If you put a piece of solid ice in a chamber and put extreme pressure on it, it turns to liquid.  There must be some other source of internal heat generation though.  I have never found a satisfactory answer to date.  I really don't think anyone knows, just like gravity is still a big mystery.
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Natural variations are variations that are often the result of random mutations, that arise when cells divide during development of new organisms. \nWhen these organisms reproduce, they pass on these variations to their offspring – inheritance of natural variation. \n\nOrganisms can vary in size, coloration, ability to fight off diseases and countless other traits. These natural variations and their ability to be inherited makes it  possible to survive as a species.
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It is because the varius atoms and molecules have different sizes, shapes and weights. This means it take different amount of energy to make them vaporize.
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Isotopes are forms of an element whose nuclei have the same atomic number–-the number of protons in the nucleus--but different mass numbers because they contain different numbers of neutrons.\n\nThe word isotope, meaning at the same place, comes from the fact that all isotopes of an element are located at the same place on the periodic table.\n\nCollectively, the isotopes of the elements form the set of nuclides. A nuclide is a particular type of atomic nucleus, or more generally an agglomeration of protons and neutrons. Strictly speaking, it is more correct to say that an element such as fluorine consists of one stable nuclide rather than that it has one stable isotope.\n\nIn scientific nomenclature, isotopes (nuclides) are specified by the name of the particular element by a hyphen and the number of nucleons (protons and neutrons) in the atomic nucleus (e.g., helium-3, carbon-12, carbon-14, iron-57, uranium-238). In symbolic form, the number of nucleons is denoted as a superscripted prefix to the chemical symbol (e.g., 3He, 12C, 14C, 57Fe, 238U).\n\nSee link below for more info.
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Both are required. Variation in the population provides something for natural selection to act on. Mutation is one way to generate variation. Recombination  during meiosis is another. This sounds like a trick question. The correct answer is not one or the other--you would have to explain variation. One part of the process generates choices, the other part of the process "chooses" among the available options.
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Natural uranium consists of three isotopes: U-238, U-235 and U-234, with abundancies of approximately 99.275, 0.72 and 0.054 percent respectively. In fact, however, there is a range of natural variations for each isotope, U234 varies from 0.0059 - 0.0050, U235 from 0.7202 - 0.7198, and U238 99.2752 - 99.2739.
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Isotopes of the same element have *almost* identical chemical properties. They can be distinguished because they have different masses (from a different number of neutrons in the nucleus). But this effect is usually very small. One exception is hydrogen. The isotope consisting of one protoon and one neutron in the nucleus (deuterium) has double the mass of a regular hydrogen atom, and this can have observable effects even on the chemistry.
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Rather than radioactive element, let's call it a radioactive isotope.  Why?  Because a radioactive element can decay and still be the same element but have fewer neutrons.  As other posters sort of explained, if an isotope is radioactive then half of it should become something else in the specific half life time.  It could still be the same element, or it might be a lighter element.  Read the link about carbon dating and you might understand how this is useful.
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Genetic mutaton happens for MANY reasons. Cosmic radiation is the least of them.\n\nTranscription errors is the largest way a mutation occurs. When a cell is copied, the DNA is copied and sometimes the mecahnism for doing this gets confused and makes slight changes. This is the most common way for mutation to happen.\nThere are outside factors liek environment. Toxins, pollutants. It has been shown that toxins have altered the germ (eggs and sperm) cells in amphibians.\n\nAll of this changes happens very slowly. It is not like a fish a born with 4 legs and walks to shore. More like a few fish out of millions have a slightly better advantage for their environment, so they eat better, breed more and have more offspring. The trait is then propagated through the population. Many mutations happen that do NOT lead to improvements, these die off. Back to our fish, over hundreds of thousands of generations, more mutations have happened allowing them to bettter compete for resoruces. Eventually many small changes have changed the nature of the fish.\n\nLook at divergent evolution. The North American Whitetail Deer. It used to comingle along the entore easter Seaboard cost. Now, due to habitat loss, highways and development, they are segregated. Over the past 150 years some visible changes have occured. The souther variety is smaller, and has some other characteristics not seen in the Northern deer. Why? Random mutations are congregated in smaller populations and not intermixed. In time, it will become different enough tobe a new species.\nIsolation is another reason for change, or in some cases not changing. A population becomes isolated and has little reason for random mutation to change them and they remain the same ove rtime while the surrounding areas undergo change.\nIt is a fairly neat system how all of these interactions play into one another.
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Probably most of it comes from radioactive decay.
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Radiocarbon dating relies on a simple natural phenomenon. As the Earth's upper atmosphere is bombarded by cosmic radiation, atmospheric nitrogen is broken down into an unstable isotope of carbon - carbon 14 (C-14).
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It would be either an isotope or an ion depending on the number of neutrons or electrons in the atom.\nA good example of this for isotopes is hydrogen:\nWhen there is no neutrons it is H1 or just plain hydrogen\nWhen there is 1 neutron it is Deuterium or H2 also called heavy hydrogen\nWhen there is 2 neutrons it is called tritium or H3\n\nIf the only difference is the addition or removal of one or more electrons it is an Ion.