Evolution by natural selection

Evolution is an ongoing process, affecting all biological organisms, including humans. Mutations are happening all the time. These mutations may or may not be passed on to the next generation, and may or may not have an effect on the phenotype of the organism.

Remember that much of an organism’s DNA sequence is not actually translated into genes (and is called “non-coding”). If a mutation occurs in a non-coding region, it may not have any effect on the phenotype of the organism. Even within a gene, some mutations may not lead to a change in the organism. These are called neutral mutations.

If a mutation does cause some change in the organism, it is called a functional mutation. A mutation - even a single nucleotide change - can cause a gene to no longer function, no longer encode for the same protein, or create some new function. Most of these changes are detrimental, or even lethal, to an organism. If they are lethal, the organism most likely would not have a chance to reproduce, therefore that mutation would not be passed on.

Sometimes, however, a mutation can have effects beneficial to the organism, just by chance. Suppose a change in a gene made an organism more resistant to a disease, or better able to tolerate drought. That organism would be more likely to survive and pass on the new genes to more progeny. This is how organisms are constantly changing and adapting to their environment. This is called natural selection; natural selection is a driving mechanism of evolution.

http://learn.genetics.utah.edu/ University of Utah’s Genetic Science Learning Center
http://www.athro.com/ Science Education on the Web