March 10, 2005

Marks of a Plague

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 |

Back in the early '80's, before the highly toxic and resitant strains of E.coli were visible, we did some rudimentary research on antibiotic resitances at our lab on the campus of Northwestern U. It seemed that E.coli given the restistance to tetracycline from plasmid transmsission incorporated the resitance gene into a part (possibly a promoter) of their genome that not only gave them the resistance, but in turn stepped up their overall fitness and ability to compete against wildtype E.coli. These superfit microbes then proceeded to take over any environment they encountered. Plasmids can transmit resistance even when an organsism isn't directly in contact with an antibiotic. Subsequent research found no regime or strategy to counter these superfit E.coli once they were entrenched. Seems the micro world models macro systems, and once the superfit is introduced, the wildtype population has no chance of survival unless there are those one or two induviduals that aquire a natural immunity through random mutations. Sad to say, thems are the breaks. Sometimes it's just the luck of the draw and what environmental imprints may have left a operon or two in your genome.

Posted by: SME in Seattle at March 11, 2005 01:02 AM

Well, technically speaking, medieval Europe knew about hygeine and had plenty of baths- until the Church decided that baths were places where bad things happened, and set about closing them. In this they were successful, and thus began the great age of plague and scrofula. The Church also contributed to the plague by keeping the people malnourished.

Posted by: serial catowner at March 11, 2005 07:53 AM

If the disease is infectious enough, and has a high death rate, resistant populations can spring up in very few generations. For example, look at the myxomitosis infection induced into Australian rabbits in the '50s. Some ungodly dieoff rate (90% - 99%) but complete immunity in the population in three or four generations.

Also remember that there were constant reinfections with the plague for about 300 years, so susceptibility was constantly selected again.

Posted by: chris at March 11, 2005 09:39 AM

The massive wave of death set off by European diseases is one of the least known facts of the European conquest. There is still much we do not know and much we will never know.

You make good points, but one problem.



Posted by: James E. Powell at March 11, 2005 02:42 PM

Thanks for catching that, James. I wrote that post pretty late, didn't bother checking.

Posted by: natasha at March 11, 2005 07:33 PM