Evolution is the most explanatory theory we have for the origin of the species. I wrote up a little essay a while ago on it. I've listed the original page below - I suggest you check it out if you are interested, as it has the original pictures in tact.
Evolution is actually a combination of theories, much to the surprise of the creationists I often encounter. Typically when I encounter a creationist their idea of the multiple theories of evolution involve a difference between microevolution and macroevolution.
This is, of course, nonsense; macro-evolution is just a description of evolution above the species level. It does not actually act on separate principles from microevolution.
I will explain as best I can, using cites from actual experts in the field.
Now then, I made the allegation that the theory of evolution was actually comprised of several theories...so without further ado:
According to Ernst Mayr (who died recently) the five principle theories that comprise the theory of evolution are:
- The nonconstancy of species (the basic theory of evolution).
- The descent of all organisms from common ancestors (branching evolution).
- The gradualness of evolution (no saltations, no discontinuities).
- The multiplication of species (the origin of diversity).
- Natural selection.
So those are the five theories...what do they mean?
1. The nonconstancy of species: What this basically means is that every individual within a species is genetically different. Every individual's DNA has unique variations (not to the point that every individual constitutes a new species though). These variations usually express themselves in the phenotype.
To put this in a simpler fashion; no two individuals in a species are alike.
2. The descent of all organisms from common ancestors: All this means is that we came from our parents, who came from their parents. To put it in more general terms, it also means that we all came from one ancestor. Although if I recall correctly Darwin was open to the idea that there could have been a few original ancestors. In any event, one way to test hypothesis is to look at our genetics and to see if we share a genetic history with other organisms. After all, DNA is made up of four nucleotide bases-Tyamine, Cytocine, Adenine, and Guanine- and there is absolutely no reason that the DNA of one species should be related to that of another species unless at some point both species descended from a common ancestor.
3. The gradualness of evolution: Evolution is a gradual process, in that it happens slowly and over a considerable period of time. It's often at this point in the theory that creationists have the most difficulty. This is because they often confuse the 'saltation' with the process of evolutionary change involved in macroevolution.
Before we go any further I will define a few terms relevant to this part:
Saltation: This was a belief prior to the modern synthesis in the 1940's that scientists attributed to speciation. It was the belief that fully formed organs was responsible for speciation, and it's often also called 'macromutation'-which probably helps the confusion. In the words of Tim Berra, in his book "Evolution and the Myths of Creationism", clarifies the distinction:
No knowledgeable biologists today, and that includes the punctuated equilibrium enthusiasts, advocate single-generation macromutational jumps (mutations with large effects). Amphibians, for example, did not arise from the lobe-finned fishes in a single-generation leap (saltation), but by seeing the muscular fins and lung of the aquatic ancestor gradually refined for terrestrial life. Macromutations do occur, but they are usually harmful (the recipients seldom produce offspring) and therefore relatively unimportant in evolution.