I've always been a little curious about traits that are useful, but only as long as they are rare. It makes total sense that such things could exist, and how and why they are useful is interesting, but it sort of flies in the face of the simplified version of evolution people normally picture. The example that seems really well-known to me is sickle-cell anemia. If someone has a certain gene on one of the two chromosomes where it can occur, they are resistant to malaria. If they have that gene on both of those chromosomes, though, they have sickle-cell anemia. So if only a few people in a certain population have that gene, it might have no effect or a positive effect, but if a lot of people have that gene, you're going to get children and grandchildren with a very nasty congenital disorder.

Another example I'm familiar with is left-handedness. In any kind of close combat, a left-handed person will be attacking and defending from an angle that the opponent isn't familiar with. I noticed this in Tae Kwon Do; I was usually punching or blocking just a little more easily with my left side than my opponent was. (Although, oddly enough, I think I actually preferred kicking with my right foot.) In real life this is usually outweighed by luck and speed and physical size and other factors, but it's still an advantage. I was amused and pleasantly surprised one time when I read something related to it in a much more serious context than sparring for sport. I medieval castles, spiral staircases always spiraled clockwise going up. The reason was that someone defending the place would be higher up the stairs than an attacker. A right-handed person would have plenty of room to use a sword if they're coming down the stairs, but going up the stairs their reach would be hampered on the right side by the pillar down the middle of the stairs. A left-handed person, though, would have much less of a problem.

I was just reminded of it when I followed some links earlier today and learned that the same might be true with certain other disabilities as well. My father is red-green colorblind (partially, at least), and the family has always thought of it as a clear, if minor, disadvantage. Especially when he was driving on French roads, where the order of traffic lights isn't always the same as it is here. We never got in any accidents because there are plenty other cues to rely on, but I seem to remember we did have one or two close calls. But it's interesting to read that a different visual range, even if it's only different by being more limited, can be an advantage for pattern recognition and noticing camouflage and stuff.

And speaking of left-handedness, it has been on my mind for another reason as well.

20th - James A. Garfield (1831-1881)
31st - Herbert Hoover (1874-1964)
33rd - Harry S. Truman (1884-1972)
38th - Gerald Ford (1913-2006)
40th - Ronald Reagan (1911 - 2004)
41st - George H.W. Bush (1924- )
42nd - Bill Clinton (1946- )
As you can see four of our last six presidents were left-handed.
2008 Presidential Campaign
Barack Obama (D) - Left-handed
Hillary Clinton (D) - Right-handed
John McCain (R) - Left-handed

Seven presidents out of 43 have been left-handed, and four of them were in the past 35 years. Sure, in the past lefties were trained to switch at an early age, but it's still interesting to see so many in recent decades, and the odds are good that the next president will be left-handed as well.