Page 44: Fruit Flies

Evolution speeds vary a lot.

Elephants come to sexual maturity late, have a very long gestation period, and generally stay within a tight social framework. The opportunity for new genes to be introduced to the pool is limited by social bonds and relatively infrequent reproduction holds down the number of mutations. In other words, Elephants will tend to evolve slowly.

On the other extreme, consider the fruit fly. The female fruit fly reaches sexual maturity in 8-12 hours. About 7 days later, she will produce a new generation. Not only that, she will mate with multiple males, producing a sort-of "proportional representation" set of offspring of mixed genetic material. There's a lot of mixing and there are a lot of babies. That means there are lots of new genetic combinations and lots of mutations. In other words, fruit flies evolve very quickly. This makes them useful.

Scientists like three things about the fruit fly. First, their genes are relatively simple and easy to manipulate; second, they produce new generations very quickly; and, third, there aren't a lot of activists marching around with signs that read "save the fruit fly".

I got the fruit fly example from Matt Ridley's great book The Agile Gene, in which he talks about the fruit fly several times. The story he tells in the chapter Learning Lessons (page 180 of the paperback) is fascinating.

Did you know fruit flies can learn? Scientists at CalTech figured this out by spraying a smelly chemical at a box full of fruit flies right before electrifying a metal plate on which the flies were sitting. This may sound cruel, but, hey, they are only fruit flies (see what I mean about the activists?). The scientists found out that some of the fruit flies were able to learn that when they smelled the bad smell they better get flying or else get shocked.

But not all the fruit flies learned. The scientists observed that a fruit fly had to have specific genes (17 of them) in order to remember the link between the stink and the shock. The genes make it possible for the fruit flies to learn, but they don't contain any information. The learning comes from the shocking. Both the genes and the experience are necessary. The shocking experience causes certain neurons to weld together (using a protein made possible by the genes) in a specific part of the fruit fly's brain. It is the whole system of genes-proteins-neurons-brain that enables the fruit fly to recall that the stink and the shock are linked. The system "hard-wires" the memory into the fly's brain.

The breeding characteristics of the fruit fly made this scientific discovery possible. Because of the fast fruit fly lifecycle, it was possible to breed a few flies, see which ones can learn, separate them, then breed them again. After a few generations (a couple of weeks), you have "pure" learners and pure, well, dunces. It's really cool. It's what nature does, only on fast-forward.

Meanwhile, in a separate experiment, some other scientists had inserted a mutant gene in some flies. Believe it or not, this mutation made it so that a particular fly would be paralyzed in hot weather. Literally, at 20 degrees C the fly is perfectly fine, at 30 degrees C, it drops from the sky. Amazing.

Then they combined the two experiments. The scientists switched the heat-sensitive paralyzing gene from the part of the brain that controls motion to the part of the brain that controls memory. So now their flies could fly in hot weather, but couldn't use the memory parts of their brains that had been hard-wired with the stink-shock memory. Sure enough, in cool weather the flies remembered the implications of a bad smell and in hot weather they got shocked. Astounding.

So, basically, they used fruit flies to prove that memories get permanently or temporarily "hard wired" in our heads by specific proteins generated by specific genes. And it was all made possible by our friend the frisky, fast-evolving fruit fly.