Once upon a time there were some bugs that lived behind a school. They weren't very big, or very interesting, and the children and the teachers mostly ignored them. They would run and hide in the cracks in the playground when the children came out to play, although a few weren't fast enough and got squished.
They weren't very pretty either, being just a sort of dull light grey, although some of them had a few black spots. It was because of this though, that you couldn't see them very well. The birds that lived in the trees around the playground liked to eat them, but they were hard to see aganst the light grey playground surface, so they had lots of babies and there were lots of bugs.
One day the school board decided that the playground was getting old and worn, and there were accidents from people tripping on the cracks. So a big asphalt machine came and put a new surface down. It was black and shiny, and it looked very nice.
From the bugs point of view this tide of hot tar was seen as a Massive Tar Disaster. About half of them died. But the rest scuttled away and hid in the grass until it was all over. They didn't like the grass though, there were too many other insects there, all competing for food, and worst of all, bigger bugs that chased them and ate them, and they couldn't wait to get back to a nice flat surface, where it was easy to find food that the the children had dropped, and they were the ONLY bugs. Besides, they had always lived there, they were playground bugs. It was home. It just felt right.
The problem was, the birds could see them now. They stood out very clearly against the black surface. Lots and lots of them were getting eaten, and more and more were getting squished, as there were no cracks to hide in.
But some of the bugs were easier for the birds to see than others. The ones with black spots were much harder to see, so the plainer ones got eaten first. In fact, before very long there were only baby plain ones left, being much smaller they were harder to see. But then, because there were no adult plain bugs passing on plain bug genes, very few plain babies were being born. And those that were, were getting eaten as soon as they were big enough to see, before they had a chance to breed.
It wasn't long before almost all the bugs had black spots. But some had more than others. The birds were now searching in vain for plain bugs to eat, and were having to watch carefully for bugs with fewer black spots than others. But the same thing happened to the less-black spots, and over time there were fewer and fewer bugs with only a few spots, and more and more bugs with lots of spots. Some baby bugs were being born almost completely covered in black spots.
The mostly-black bugs were so hard for the birds to see that they now scoured the ground for bugs with any hint of grey on them, and ate them all up. They were hungry for bugs but it was getting really hard to find them, and many more were surving to breed. Soon the bug population was up to where it was before the Massive Tar Disaster. Bugs with even a hint of grey were gobbled up by birds, and before very long, all the bugs were completely black. The birds went to live at the factory next door.
The bug population went nuts. Even though many were squished, with no birds to eat them they increased in number to the point that there simply wasn't enough dropped food to eat. So some of them went looking for a new place to live. Not in the grass, where big munching bugs lived, but farther away, to the parking lot outside the factory, which had a nice flat, hard surface ideal for playground bugs. And as it was light grey, and they were black, the birds went "AHA!"..............