Evolution is really a very simple idea. It's based on two easily accepted and understood concepts.
The first is that populations of organisms change over time. Even a creationist would concede this point. The differing races of humans on Earth could not all look like Adam or Eve.
The second concept is that some genetic characteristics make an organism better adapted to its environment and other genetic characteristics make an organism more poorly adapted. Those individuals that are better adapted live longer and have more descendents. Those descendents will inherit the genetic characteristics as their ancestor.
An important point to make is that the genetic changes that are beneficial depend on the environment. A genetic change that causes a somewhat thicker coat of fur will be a negative change if global warming is taking place. But it would be a beneficial change if an Ice Age is approaching.
That's it! That's all that there is to evolution.
Some people (they are called "creationists") attempt to distinguish "micro-evolution" from "macro-evolution". They define micro-evolution as genetic change within a "created kind". That's nonsense.
First of all, there is no such thing as a "created kind"; that phrase has no meaning in science because it is not testable.
The word "species" used to be synonymous with "created kind" and the definition of "species" provides a way of testing whether two populations of organisms are different or the same species. Two populations of organisms are the same species if they can inbreed and have fertile descendents. They are different species if those two populations cannot do that. But science has now demonstrated many examples of speciation (the development of new species through evolution). Rather than accept that there is no way to distinguish between micro- and macro-evolution, creationists who insist that there is a difference choose the intellectual cowardly path of not defining what they mean by "created kind".
The second reason why there is no reason to differentiate micro- from macro-evolution is because no barrier to genetic change has been suggested and certainly none has been demonstrated. Changes in DNA persist from generation to generation. DNA can become larger and more complex or less complex. Even chromosomes have been observed to split and fuse.
There is one and only one thing that differentiates micro- from macro-evolution. That thing is time. Given a long enough period of time, any amount of genetic change can take place.
Given the 3.8 billion years or so that life has existed on Earth, single-celled organisms can evolve into all of the varieties of life that we see on Earth.
