I got email one day (from some spineless person forging their identity as my partner!) saying "Anyone can validate the theory of gravity by dropping a stone, but there is no similar validation for the theory of evolution."

This is confusing the observed phenomenon of gravity with the theory of what draws the two masses together. It is true that anyone can witness the phenomenon of gravity, but that it takes time and/or specialized equipment to witness the phenomenon (or "fact") of evolution. For example, observing the change in gene frequencies over time is observing evolution. These and other observations have been done often enough that science no longer disputes the phenomenon of evolution, only the explanation of how and why it occurs (that is, its theory.)

The claim that scientists have made more progress on the theory of evolution than on the theory of gravity I took from Elliott Sober. He is frequently called on as an expert witness in court cases about teaching evolution or creationism / "intelligent design" in schools, and this argument is one of his best techniques for explaining the principle to juries.

Thanks for asking!

• National Science Teachers Association page on teaching evolution.
• Outline I wrote for publicly debating creationists.
• The top ten daily consequences of having evolved, from Smithsonian.