Picture the destiny of some rats that stumble into a grain silo full of wheat. It’s an unbalanced diet, but there sure is a lot of it, and the rat population explodes, even though they’re not particularly healthy.
Their population increases until there’s just enough grain pouring in to feed them all (barely), at which point the silo is chock-full o’ rats. Now they’re hungry as well as malnourished. Once in a while, typhus or bubonic plague decimates the rats and there’s plenty to eat for a generation or two, until population catches up again. That doesn’t take long, because rats breed like.. rabbits?
The silo is big and contains an obscene number of rats – all potential mutants – and we keep this up for some 400 rat generations. There are lots of new mutations, some of which pay off in the silo. Natural selection in the silo favors traits that work better there, and the rats, they are a changin’. Eventually we see new versions of metabolic genes that fit them better to this restricted diet [like lactose tolerance, or a new and improved ergothioneine transporter] and new versions of immunological genes that make them more resistant to the plagues made common by the crowding. With the crowding, the optimal behavior changes, and so the rats acquire different dispositions. They even develop an interesting dominance hierarchy, with rat Kings.
I mustn’t forget the spigot that spouts ethyl alcohol, CH3CH2OH. The rats adapt to that too – those that survive.
Not content with our simple selection experiment, we also install complicated mazes with flaming hoops that the rats have to jump through in order to get extra food and mates: we want rats with different brains, and eventually we get them. They’re maze-bright and flaming-hoop-bright. We install treadmills and feed the rats according to their work output, and eventually they produce more work per amount of food eaten. They’ve maximized efficiency rather than surge power, which was more useful back when they were wild and free. Not only that, they eventually come to like being on the treadmill, almost as if it’s some sort of race.
There are other silos – one full of rice and another full of maize. They have different mazes and flaming hoops, built at different times: and there are still wild rats, too, although not as many as in the silos.
But no matter how much they change, they’re still just a bunch of rats.