Left handed snails cannot mate with right handed snails?
Because they really can't.
Because of misalignment of their reproductive apparatus.
And all this links back to evolution because if you can't mate you can't pass down the genes and you would expect the gene to vanish.
Right?
Yes, but left handed snails can mate with left handed snails. All you need is someone else with the same mutation in the "handedness" gene as you. Then you could potentially form a left-handed species! Of course, the odds of finding another left-handed snail is low, and therefore left-handed snails are quite a rarity.
But the puzzle gets even weirder, because in South-East Asia there is a higher concentration of left-handed land snail species than most other places in the world. This has been explained (accepting it is up to you) by the presence of the right-handed-snail-eating snake and right-handed-snail-eating crab. So, because snails cannot shatter the snail's hard shell, they extract the snail from it's shell by alternately retracting their left and right mandibles. Upon closer examination of the snakes' jaws, researchers found that the snakes were also right-handed, or rather right-mandibled. Their right mandibles have much more teeth than their left, meaning that their mouth is not symmetrical. And this asymmetry was observed in all snail-eating specialists!
Same for crabs - their right claw is often more powerful and specialized to crush crabs. They just can't deal with left-handed snails!
Hence, this would explain why left-handed land snails are more present in SEA rather than in other parts of the world.
It's quite amusing how all this relates to our lives. I'm a left-hander. Even though I'm not that sporty, but in many competitive sports we see many left-handed pros, in a higher proportion than we see in the general population. (e.g Nadal <3 )Some people say that being left-handed means you're better at the sport; but it's been proposed that it's because players are less well-equipped to play against left-handed players, whom they rarely encounter. If it's really the first explanation, then we would expect left-handed pros to eventually dominate the sport. But that isn't the case! It appears that the reason some left-handers are successful is because they're rare. Right-handed players rarely learn to defend and attack against left-handers. (Then again you have left-handed specialists like Yuuta. But most players aren't like that.)
Well, it's the same thing for the crabs, snakes, and snails. It's an interesting parallel. Because the left-handed snails weren't as popular, crabs learnt to prey on right-handed snails, giving left-handed snails an advantage. However, they still remain a rarity because of the statistical difficulty in finding another lefty to mate with.
Pretty cool, right? Luckily, things aren't that way for humans. Which is a good thing! Imagine if left-handers couldn't have sex with right handers! That'd be horrible!
Anyway. General Bio Geek-ery is over. Hope you enjoyed that and found it as exciting as I did, btw. It's stuff like this that makes Bio interesting, and fun. Esp. evolution.
On another note,
www.guardian.co.uk/society/2009/nov/27/patrick-stewart-domestic-violenceThis is a good read.