Biologists define evolution as a change in the gene pool of a population over time. One example is insects developing a resistance to pesticides over the period of a few years. Even most Creationists recognize that evolution at this level is a fact. What they don't appreciate is that this rate of evolution is all that is required to produce the diversity of all living things from a common ancestor.

The origin of new species by evolution has also been observed, both in the laboratory and in the wild. Even without these direct observations, it would be wrong to say that evolution hasn't been observed. Evidence isn't limited to seeing something happen before your eyes. Evolution makes predictions about what we would expect to see in the fossil record, comparative anatomy, genetic sequences, geographical distribution of species, etc., and these predictions have been verified many times over. The number of observations supporting evolution is overwhelming.

What hasn't been observed is one animal abruptly changing into a radically different one, such as a frog changing into a cow. This is not a problem for evolution because evolution doesn't propose occurrences even remotely like that. In fact, if we ever observed a frog turn into a cow, it would be very strong evidence against evolution.

Taken from the Talk.Origins: Five Major Misconceptions about Evolution FAQ

While any discussion of evolution is fraught with ambiguity, I contend that evolution has indeed been observed. If we define evolution as the change in the gene pool of a population, such that two or more populations emerge that while both fertile amongst themselves, cannot interbreed to form fertile offspring. This is to say that evolution is a process where selection leads to speciation.

In the '60s, Dobzhansky selected two groups from a population of drosophila, and repeatedly inbred them. He found that the two groups had become so different that while they could reproduce amongst themselves, the two groups could no longer interbreed successfully. This is admittedly speciation through artificial selection, but it demonstrates that evolution can occur in the wild.

Another observed case of evolution is the 'ring species'. Ring species occur when a 'similar' animal has a very wide range (the circumference of the earth, for instance). While nearby individuals can and do interbreed, individuals from points far apart on the 'ring' can look very different, and produce sterile offspring. For an extreme example of evolution, if one was to exterminate enough of the intermediate individuals, those that remained could no longer exchange genes in any way, and would be two different species. Examples of ring species include the herring gull, and salamander.

The final criterion, I suppose, is the appearance of entirely new characteristics through mutation. Dogs, for instance, were not originally 8 inches long. Highland cows used to be black. But does this count? These could all be merely selection of latent, recessive genes. All the really interesting mutations (feathers, bones, jaws, lungs, etc.) are only evidenced in fossils, Archeopterix being the most famous. I'm open to suggestions, as far as mutations leading to new species in historic times goes.
For the sake of argument, let's say that evolution has never been observed. Of course, neither have black holes, or quarks. yet we still know that they are there. This is because instead of direct evidence, we use indirect evidence to know.

One way of indirectly inferring something has happened is by predicting consequences and then seeing if they apply. For example, if there is a black hole in one section of space, then stars would be orbitting around a void. And we see stars orbitting around a void, so therefore there is a black hole.

Now you may be thinking, "But there could be another cause!" If you are, good! That shows you're starting to think rationally. In fact, the example I gave is an often-used logical fallacy called affirmation of the consequent, which takes the form of "If A then B, B, therefore A". If you don't see why this is a fallacy, let me give an extreme example: if magical anti-flight demons existed, then objects dropped will fall to the ground. Objects we drop fall to the ground, so magical anti-flight demons exist.

There are two ways to circumvent this. The first is an "if and only if" statement. That means that if one side of the equation is true, then the other side *must* be true. The first example can be rephased like this, but the second example can't.

Evolution does have a line of evidence like this. If and only if all species rose from a common ancestor, then we would see big genetic similarities between us and other creatures of our genus, and also genetic similarities between us organisms in our kingdom. This is true; we share around 98% of our DNA with certain primates, and about 85% of our DNA with mice (

The other way to circumvent the affirmation of the consequent problem is multiple lines of indirect proof. To illustrate this, pretend we find a dead man on the floor. If taken individually, his broken neck could be attributed to tripping, his cuts could be attributed to his line of work, and his rumpled clothes could mean he put his clothes on in a hurry. All taken together, however, we can assume he was killed during a fight.

Evolution also has evidence like this, too. In addition to genetic similarity, other proofs include vestigal organs, transitional fossils, and the age of the earth in correllation with the timespan evolution would take. All of these have a large pool of evidence backing them up. If taken individually, each could be chalked up to a competing theory of origins, but when taken together it points towards an evolutionary theory.

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