A "theory" does not become a "theory" until it has been tested for every possible way it could fail. The reason it is not called a fact is that science will still try to come up with a better theory, or continue to improve on the theory. That is exactly what has happened with evolution - the theory is stronger today than it was when it was introduce.
Those who denounce or criticize the theory of Evolution have not shown that their theory can account for any of the data evolution accounts for, and they have not provided any reason for believing that their theory even has the potential to produce anything useful to science. There are all sorts of findings and experiments that could have falsified evolution. In the century-and-a-half since Darwin published his theory, not one has.
If something is science, hypotheses or theory it makes predictions that could be wrong. If so it will be possible to falsify these ideas. What is found the progression over time seen in the millions of fossils unearthed around the world is exactly what evolutionary theory predicts. Unicellular organisms appear before multicellular ones. Jawless fish precede jawed fish. Lunged fish precede amphibians. Amphibians precede reptiles. Reptiles with scales precede mammals and birds with modified scales (fur and feathers). Apes precede humans. All it would take is one or two exceptions to seriously challenge the theory. No such exceptions have ever been found anywhere. There have been a few claims to this effect, of course, but even most creationists admit that these claims are fraudulent.
Proponents of intelligent design (ID) assert that certain complex biological systems could not emerge from a gradual evolutionary process. They argue instead that such structures are best explained via the deliberate action of an unspecified intelligent designer. Few scientists endorse this conclusion, and they have good reasons for being skeptical. They understand that the prolonged action of natural selection can be expected to leave traces behind in the structure of modern organisms. And when scientists go looking for those traces they invariably find them in droves. Recall that natural selection operates by preserving small, favorable variations that occur naturally in any population of organisms. Over time these variations accumulate to the point that large-scale change is the result. This implies that natural selection works by modifying structures already present in the organism. It does not craft new, complex systems from scratch. This observation is crucial in distinguishing between those systems that could have been crafted by selection and those that could not have been. If we find that a particular organism possesses a complex system made from parts wholly distinct from anything to be found in the organism's closest evolutionary cousins it will be difficult to explain that system via selection. But if we find that the system appears to be cobbled together from parts that were readily available, then natural selection remains a strong candidate. Charles Darwin employed this principle in his studies of the complex systems used by orchids to attract pollinating insects. He discovered that these contrivances, as he called them, were indeed fashioned out of modified versions of parts present in closely related flowers. Stephen Jay Gould famously used the panda's "thumb" to illustrate the same principle. The panda possesses a sixth digit on its front paws that it uses to strip the leaves off of bamboo. This digit is not a true opposable thumb like that possessed by apes and humans. If it were, we would have a strong argument against natural selection in this case, since the panda's closest relatives have nothing like such a thumb. In reality, however, the panda's thumb is cobbled together from alterations in the bones found in the paws of other bears. Since examples like these are ubiquitous in nature, natural selection passes its first big test.
None of the arguments against evolution has been proven. All of the arguments to date have been proven wrong, and no new theory has been presented by anyone. One argument (for instance) is that natural selection could not have evolved cognitive skills (intelligence.) Intelligence evolved as a cognitive strategy in humans to meet the complex demands of their survival. The more difficult it is to survive the more intelligent the organism. Intelligence has to evolve, because evolution is how new traits appear and intelligence is definitely a new trait that evolved well after the basic original bacterium. Evolution operates by natural selection: traits that help an organism survive to reproductive age, and that help it to produce offspring that do the same, will be in evidence in those succeeding generations. Traits that did not do this will disappear with the organisms that died before they could pass them on.
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