This is not a discussion of the controversies popular among many religious fundamentalists regarding Darwinian Evolution. What follows should be acceptable to both extremes of that debate. It is the principles underlying evolutionary theory that are important here and not whether or not the theory accounts for the development of life.
There are three parts to evolution, the first two of which should not be contentious.
Genetic change means that each generation is genetically slightly different from the one before. Biologically, this is caused by various factors such as mutation, genetic drift, and recombination. For instance, you might look a lot like your parents, but you certainly aren't identical.
Natural selection (survival of the fittest
) means that
gene combinations that enable individuals to produce more
surviving offspring will appear with greater frequency in
future generations, while combinations that cause fewer
offspring to survive will appear with less frequency.
For instance, cystic fibrosis is a genetic trait whose victims
typically die before they have any children of their own,
thus slowly reducing the incidence of the damaged gene.
Similarly, if only the plumpest chickens are allowed to breed,
after a few generations the genes that cause skinny chickens
will be nearly eliminated from the flock's gene pool.
The third part is the claim that those two processes alone are sufficient to account for the creation of new species, and in fact do account for the development of all life on Earth from one common ancestor (or a very small number thereof).
Proof or disproof of this will be left as an exercise for the reader.
But I would suggest that when looking at arguments,
notice how often people attribute purpose or choice
to the way evolution works.
Such arguments (from either side) are bogus and should be ignored
as obviously these people don't understand the first two principles.
(e.g. Birds developed hollow bones in order to enable them to fly.
or If mammoths were killed off by the ice-age,
why didn't they simply evolve into something that could survive?
.)
But these two principles, of change and of selection, can be applied to many other processes.
Consider a new strip mall, which will initially have a diverse selection of stores. Within a year or so, some of those businesses will fail and be replaced by other businesses, typically of different types (if a hardware store fails, it would be unwise for anyone to open another hardware store in the same location).
As the years go by, other businesses will come and go, but eventually the mall will become relatively stable. Those that survived are those that fit well into the surrounding community, those that failed are those that didn't.
From time to time circumstances might change, upsetting the balance again. For instance if WalMart moves into town or a nearby factory shuts down, some businesses will prosper while others will fail. What remains though will be those businesses that fit the community best.