How does evolution work? Again, in a nutshell, not every member of a generation is identical, and not all of them reproduce. This is called selective pressure. One of the ways it works is that adverse outside circumstances can exert so much pressure on a population that what was once a rare trait can become common.
This is why hospitals, where powerful antibiotics and high-powered cleaning solvents are commonly used, are the most likely sources for new antibiotic-resistant germs. It isn't that the bacteria with antibiotic resistant genes are necessarily more fit in other ways. They may even be slower growing or slower moving than their compatriots, or even be roughly equal in other ways. But when a new pressure is applied, suddenly a trait that may never have been especially useful now determines whether or not that organism's genes will go on to the next round.
Works just the same in every other type of organism, though it can take longer to show up.
As an example, consider the population of Europe. One in 10 people have inborn HIV resistance, a gift of over 300 years of getting hit with wave after wave of hemorrhagic fever.
...But researchers at the University of Liverpool in England said computer modeling, based on the changing demographics of Europe from 1000 to 1800 AD, showed how hemorrhagic fever forced up the frequency of this mutation from 1 in 20,000 at the time of the Black Death to values today of 1 in 10. ...
So, to the Native Americans among us, sorry about that.
It's estimated that diseases carried by the first European explorers wiped out about 80-90% of the native population of North America. From the first contacts, an advance wave of plague is thought to have spread in every direction. When Pizarro first visited the Inca, he was able to take over the country with a tiny expeditionary force ... because an advancing plague had just finished tearing through the population, setting off a civil war when the Inca himself died in the epidemic. When the first Europeans later came to the Pacific Northwest, they described villages where bodies were laying everywhere, with no one left to bury the dead.
After 300 years of plague, which happened mainly because hygiene hadn't been invented there yet, it's no wonder the visitors were carrying some nasty invaders. The European explorers were ruthless, it's true. But they never would have been able to overwhelm the inhabitants of a land that in 1491 was more populous than Europe.
The remarkable thing is that this transformation of the European genome occurred in such a relatively short time frame. Human generations are long compared to those of other organisms, and the population numbers involved are in the millions, so you generally expect change to be quite slow.
Developing a resistance to plague might not be anything showy like a sixth finger or blue skin, but biochemically, it's damn impressive.
Update: Minor fact-check edit, thanks James.Posted by natasha at March 10, 2005 07:56 PM | Health/Medicine/Health Care | Technorati links |