My exact answer. The simple definition of evolution is "change in a species over time." However, if we go more in-depth now we have this definition; "change to the genetic traits in a population over generations." You see, all species have one thing in common. Three things which have and will always be elementary to observe. Firstly, variation: Individuals of a species are NOT identical. Secondly, inheritance: Individuals pass on traits to offspring. Moreover, thirdly, competition: More individuals are born than can survive...they compete for resources and mates. Now, the three things that I've listed above are all facts, basic observations made by biologists and scientists. Non-disputable. However, we seem to be missing a step, a conclusion. So, from those three steps Darwin, a famous biologist, drew a logical conclusion. Natural selection: "The process whereby organisms better adapted to their environment tend to survive and produce more offspring." So those good traits will increase in the population as natural selection thins out the bad genes until they are gone.