This comparison is used because the basic principles of natural selection are also 100% observable as well. With gravity all we can observe in our daily lives is that most stuff falls down, but we also see some things that don't fall down or that fall at different rates. As down every place we go has down it makes sense that down is a universal constant so obviously the earth is flat because otherwise it would fall and things would fall off of it.JambalayaBob said:Saying "the theory of gravity" isn't the best example because gravity is 100% observable and always has been. Yeah, we know a lot more about gravity now than when we were all nomadic tribes, but in a sense, the theory has always existed. I'd say a better way of explaining it to someone would be to talk about something like the heliocentric theory or the theory of plate tectonics, just so they can more easily see how ideas evolve over time to give us a better understanding of the universe.
Except nomadic tribes already knew that some things didn't fall (clouds, the sun and the moon all seem to stay up there to a casual observer) and other things could temporarily avoid falling and travel in the sky like birds. Once we figured out that the earth was round it became obvious that down was not a universal direction so we needed a theory a new theory. So we came up with the idea that everything is attracted to everything else with a force directly proportionate to the product of the two object's masses and inversely proportionate to the square of their distances.
Now this goes against most people's everyday understanding of how the world works. After all if you're 2ft in their air gravity isn't 1/4th what it is when you're 1ft in the air. So does that mean the theory of gravity is wrong? Nope it means that this distance is calculated from an object's center of mass when dealing with Newtonian physics, and the earth's center of mass is over 6,300km down so adding or subtracting a foot makes almost no difference.
So I would say comparing the theory of evolution to the theory of gravity is fair. Both rely on things we observe every day being examined in far greater detail to create a set of laws which provide a far better predictive model than our mere intuition.