Evolution is not a theory; it is a fact. Natural selection is a theory to explain that fact.

This is partly a semantic quibble, but hey, Jyte loves those!

Evolution is change over time -- change in populations' traits over time -- and that that change is an observed fact which must be explained.

'Evolution' has come to be conflated with speciation by evolution, and 'theory of evolution' has come to be understood as short hand for 'theory of evolution by natural selection', which is what Darwin proposed. However, evolution as such has been pretty evident for as long as we've been looking at the fossil record. We see dire wolves change over time as we excavate the La Brea tar pits. We find, in almost unbroken sedimentary layers in South Dakota, a branching family tree from small browsing omnivore to recognizable horse. Most inescapably, marine invertebrates, dying and drifting down to be buried in continuous sediment over millions of years, show changes in form over time.

In the present day, we can see change over smaller periods of time. Ornithology shows us Darwin's finch populations on the Galapagos adapting to changes in food supply. Medicine attests to the yearly struggle to vaccinate against ever-mutating influenza, and strives to contain Mycobacterium tuberculosis populations which have adapted to resist our drugs. An isolated population of wire lettuce in Oregon speciated over a decade or two -- it's now unable to breed with its originating population.

These changing organisms exist and demonstrate, in current time, evolution. These fossils exist. They demonstrate change. Geologic time is established by the science of stratigraphy, which is a logical science based on the observation of rock-forming processes in the real world. In order to discount stratigraphy and its timescale for the earth and its fossils, one must discount not only logic and observations of the existing world, but geology as a whole. In order to dismiss some parts of geological dating, one must dismiss chemistry, our understanding of the atom, physics.

In short, evolution is change over time. Change can be seen, and measured. Time is experienced, or inexorably demonstrated by logical processes. Evolution exists, and must (to a mind interested in science) be explained. Natural selection is one theory to explain the change in faunae we see documented in the rocks.

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