This was originally a series of replies made out to someone on youtube who was arguing against the notion that evolution actually occurs. My intention was to point out how simple the theory of evolution actually is.

Humans have DNA which is vulnerable to mutations, and simply put: Evolution is the accumulation of these genetic mutations over time (and is also subject to survival of the fittest).

When a population of a species is separated into two, each daughter population undergoes mutations, some mutations will disadvantage an organism, other mutations will provide an advantage. Those individuals who are best suited to their environment are more likely to survive, and hence those who have advantageous mutations are more likely to survive and reproduce (than those who haven't). Eventually the two daughter populations will individually become better suited to each of their environments, and they will genetically diverge until it is impossible for them to interbreed. That impossibility of interbreeding signals that "speciation" has occurred.

Speciation is simply a term which describes when an initial species diverges into one or more other species. Therefore with mutations, time, and survival of the fittest, evolution occurs. It's really quite a simple concept, but you need to recognise and understand the underlying genetic process (which is also quite simple).

Lastly, there have actually been many noted cases of speciation occuring in our lifetimes (most of these instances regard bacteria, insects, and plants), and there is a vast consensus amongst scientists (and scientifically educated people) that evolution does occur.

True...
I think that they've discovered that human beings once were much taller.