I wrote this as a follow up to the original "Macro Evolution" explanation. The purpose of this "Micro Evolution" related piece of writing, was to highlight and explain just how simple the underlying genetic process of evolution really is. I understand that it might be overly technical for some, and you are all welcome to ask questions, or for simplified explanations.

Basic evolutionary genetics: Your DNA made up of genetic subunits called "nucleotides". Those are the four different nucleotides: Adenine, Thymine, Guanine, and Cytosine.

Collectively, The nucleotides form a double-stranded helix, which would appear to be similar to a ladder, if flattened.  (images of DNA [in both helical and flat forms follow]).
http://2.bp.blogspot.com/_cWcuJM9QIG4/T ... a%2B45.jpg

Three nucleotides make up one “codon”(or triplet), and a series of codons make up one gene.  Of course, different combinations of codons will result in different genes.

At a greater level:  Genes code for proteins which ultimately make up the body's structure and control its functions and processes. There are a multitude of different proteins, and each has its own specific and unique purpose/s.

If you were to remove a nucleotide (e.g. Adenine ) from a codon, or if you were to exchange the nucleotide (Adenine) with another nucleotide (e.g. Thymine), this would probably result in a different protein. The alteration of these nucleotides, will cause an alteration of its associated codon, and thus its associated gene, which will ultimately result in a different protein being created.

These changes to the genes are known as "genetic mutations" and the effect of the accumulation of genetic mutations and time is evolution. This is because:  As proteins make up the body and its functions, these genetic mutations (and ultimately protein changes) change the body and its functions.

Simple version; the mini-dna things (nucleotides) which (when combined) make up DNA need to be in specific orders for them to be able to create certain proteins.

(some examples of proteins are red blood cells, or hormone molecules like adrenaline, or even the structures at the back of the eye which are responsible for humans seeing colour). There are heaps and heaps of different proteins which are important parts of the human body, and many of them are necessary for us to be alive.

Sometimes by random chance, the order of the mini-dna things (nucleotides) changes (that's basically what a genetic mutation is), and this means that the resulting protein will also change, and might not work the same way it used to. The change might be for the worse, or for the better.