Aaron11 said:
The point is, the gene had to already be there to be immune to the bubonic plague. Humans didn't react to the disease by evolving immunity.
And nobody says they did.
Evolution is not something that is turned on and off like a switch. There's no "stimulus-response" effect, as in "Oh, gee, it's getting cold. I better evolve some fur now." The fur was there first; the need for it came later.
This is what Charles Darwin proposed:
The theory of natural selection: Creatures live or die because of changes in their environment. Should there be a major change, such as an ice age or a plague, for example, some members of a population, because of mutations, will be better suited to deal with the new environment. Those that aren't will die off.
I'm oversimplifying. Let me give you an example:
You, Aaron11, were born with a mutation. You've had it your entire life, and you never even knew it was there. One of your genes was just a teensy bit "off."
It seems your immune system is just
slightly different from "normal" humans: It works perfectly, and it also renders you immune to the dreaded
MONGOLIAN ORANGE WARTS OF DEATH (
MOWOD for short).
Hey! A benficial mutation! Lucky you.
MOWOD may be a rare disease, but it is 100% fatal to whoever catches it. No treatment, no cure, no hope. Except for you, and anyone else whose DNA is similarly "off."
Let's assume that this is a pretty rare mutation, affecting only 1 in 2.5 million people. And the affected gene is recessive: You would have to mate with someone else who had this mutation in order to pass it on to your kids, and have
them be immune to
MOWOD.
With me so far?
Normally, you'd probably live and die without ever knowing you had this mutation, because, let's face it, what are the odds of you being exposed to
MOWOD? and with the odds being 2,500,000/1, the chances of you passing this mutation along are rather slim....
Unless a
MOWOD epidemic were to suddenly sweep the planet... Nowhere to run, nowhere to hide. It's a calamity of global proportions, as cities, states, and entire nations are depopulated from this disease....
except those of you born with that slightly "off" gene.
1 in 2.5 million, remember? that doesn't sound like good odds, but considering that the planet has a human population of about 6 billion, that means that 2400 people will survive.
2400 people, breeding like rabbits, could repopulate the planet if they had to. And because the gene is passed on when two carriers mate, what happens? The next generation will all be immune as well.
And the next generation...
And the next...
And the next...
Eventually, the human population will be back up to 6 billion, but this time,
all of them will have that gene, so the human race will never again have to worry about the
MONGOLIAN ORANGE WARTS OF DEATH.
That's evolution by natural selection.