Theory of Evolution
From RationalWiki
The Theory of Evolution? Well, yeah, it's funny that you bring that up, because actually, there is no one theory of evolution. There's actually a few, some of which are beyond dispute (natural selection and mutation), some of which are completely discredited (Lamarckianism), some of which are quite controversial (punctuated equilibrium). It's a really, really long story, but it boils down to this:
Evolution is real, and observed -- we can prove that, moths, finches, dinosaurs, domestic pigeons, horses, whales, Tiktaalik, and all. The question is what makes it work, and we actually a pretty decent handle on that. Start with mutation. The genetic code is chemical in nature, and can be disrupted by any number of things, most of which damage or rearrange the nucleotides in DNA and/or RNA. If the code itself isn't damaged, it can still be altered in duplication (during mitosis, for example), perhaps copying the same section multiple times, or incorporating foreign DNA (such as that from retroviruses). Most such changes have no net effect on the organism (for example changes in junk DNA, which simply are not expressed), or have no effect on the organism's survival. But sometimes they do. This is where natural selection comes in.
Whether a mutation is beneficial to the organism or not depends a lot on the environment. Lethal mutations will generally kill the organism during early development (though some genetic diseases strike only after the organism has reached adulthood and passed on the faulty gene), but mutations that don't directly affect the organism's functioning will usually be winnowed out, if they are at all, by the way the mutation aids or impairs the organism's ability to survive in an environment. (Albinism, for example, will not be conducive to survival in an environment with lots of plant life and a need for camouflage, but would be irrelevant in a lightless cave environment.)
Such changes accumulate with each generation, the eventual result being what creationists call macroevolution and what biologists call speciation, i.e. the differences between two individuals are so great that they are no longer capable of interbreeding. And that, inasmuch as a generalization can give a decent snapshot of a complex story, is the Theory of Evolution.
