Have you ever wondered why the theory of evolution is just that- a theory? Is it just a 'best guess'? Why isn't evolution a law, just like gravity?
Evolution comes from the root word evolve, which generally means to change over time. Many things evolve...the stock market, global temperature, the Earth's magnetic field...many many things.
But when we say theory of evolution we're referring to a specific kind of change over time. We're talking about the change over time in the proportion of individual organisms differing in one or more inherited traits.
Some of the misunderstanding surrounding this topic largely stems from the general public's faulty definition of a theory. Colloquially, a theory means something that someone has literally made up- sort of a best guess. For example, I theorize that the blue-colored rain today is due to the snow yesterday. This isn't a theory at all...it is a hypothesis- it is an attempt to explain an observation by using a measurable variable. Obviously, this isn't a very good hypothesis, as it is more likely that there is something wrong with my measuring device (ie/ there is something wrong with me if I am seeing blue-colored rain).
A hypothesis is something that we make up, usually grounded in some sort of logic, that must be tested to see if it can describe the outcome of an event. A theory, on the other hand, began as a hypothesis, and has been tested and tested (and tested), and has been proven correct so many times that it is now used as a mechanism to explain why something happens. For example, why are birds different then dinosaurs- they evolved.
So, evolution clearly isn't just a hypothesis- it's not something that needs to be tested to be sure that it has some merit. But why isn't it a law?
In order to become a law, a theory must be universal (meaning that it has to apply to every single situation in every part of the universe). Evolution has never been witnessed on other planets, only here on Earth, and so it cannot become a law until we can prove that it occurs elsewhere in the universe in the same way. Gravity is the same force here as it is on Mars...but is evolution the same in some distant planets?
Looks like we're stuck with a theory (but a pretty good one at that!)