An evolutionary misfire occurs when a trait evolved in one context, where it was an advantage to the organism, but then circumstances change -- and it becomes a significant disadvantage.
One common example is the way moths circle point sources of light, like streetlights and candle flames, and often get incinerated if they fly too close. Why on earth would they do something that foolish?
The whole thing has to do with the way moths navigate. They evolved for millions of years in a context where the only point sources of light at night were (very distant) stars, and they used them for navigation. If there's an extremely distant light source, and you keep it in the same position with respect to you as you move, you'll travel in a straight line. (Remember how in the movie Apollo 13, the astronauts kept a particular star dead center in their window when their navigational system failed?)
The problem is, if the point source is much closer, the whole strategy falls apart. If you keep a light source in the same position in your visual field and it's only a few feet away, you don't travel in a straight line, you travel in a circle around it. So what started out as a perfectly reasonable way for moths to navigate at night has now made them commit suicide around streetlights.
There's another evolutionary phenomenon called heterozygote advantage. This is when heterozygous individuals -- those who have two different alleles at a particular gene locus -- have a distinct advantage over homozygotes. Two commonly-cited examples are sickle-cell anemia, where heterozygotes not only seldom have serious symptoms but are resistant to malaria, and cystic fibrosis, a devastating lung disease for homozygotes, that in heterozygotes results in few respiratory symptoms -- and a lower risk for diarrheal disease, still a major killer of infants in parts of the world with poor medical care.
These two phenomena are often the explanation for what might seem like an evolutionary puzzle; if the fittest survive, why do maladaptive traits persist in populations? This becomes less puzzling if the maladaptive trait used to be beneficial -- or if having a single copy of the gene confers a benefit over being homozygous for either of the alleles.
But put those two together, and you've got serious trouble. Which brings us to the odd situation of the "Celtic curse."
It's been known for years that people of Celtic ancestry, particularly those from western Scotland and northern Ireland, have a much higher risk for a genetic disease called hemochromatosis. People with this disorder absorb iron too quickly, so the iron content of their blood builds up to toxic levels, resulting in eventual liver failure. Fortunately, the treatment is simple; regular blood donation reduces the red blood cell count and thus the iron levels, and significantly lowers the risk of liver damage. But why is such a damaging disease so common? A study this week in Nature Communications mapped out the frequency of the allele, and found three hotspots for the gene -- County Donegal, in the northern part of the Republic of Ireland, the region around Glasgow, and the Outer Hebrides -- where the frequency of the high-risk gene is one in sixty.
That seems really high for a condition that can kill you.
It turns out that the "Celtic curse" is the result of a combination of a misfire with heterozygote advantage. Having one copy of the high-risk variant of the gene makes you scavenge iron really efficiently from your food, preventing anemia if you have a poor diet. And for centuries, people in these regions did have iron-poor diets, largely consisting of cereal grains with little in the way of meat. So having one copy of this gene did give you a selective advantage, as long as you were living on short commons.
Now that just about everyone in the region has access to much better-quality food, the allele's ability to turbo-charge iron uptake backfires, causing iron loading to the point of illness.
And of course, there's the fact that even if you do have two copies of the gene, the more serious side effects usually don't strike until you're in your forties or fifties -- at which time you've probably already had whatever children you're going to have, and passed the gene on. So honestly, it's not a double-whammy, it's a triple-whammy.
So there's our genetic curiosity of the day. Interestingly, I have ancestry from Paisley (near Glasgow), but apparently I lucked out and don't have the hemochromatosis gene. Good thing, because despite my relative good health, I have serious doctor phobia. If I had a condition that required people to come at me regularly with stethoscopes and needles -- irrational though it certainly is -- I might just take my chances with being on the receiving end of natural selection.
