Very rarely do you spend your time thinking about an invasion of mutant crayfish, but you might, as it provides a potent metaphor:

Every marbled crayfish, known as a marmorkreb in German, is female — and they reproduce by cloning themselves. Frank Lyko, a biologist at the German Cancer Research Center, first heard about the marbled crayfish from a hobbyist aquarium owner, who picked up some “Texas crayfish” at a pet shop in 1995. They were strikingly large, and they laid enormous batches of eggs — hundreds, in a single go. Soon, the New York Times reports, the hobbyist was beset with so many crayfish he was giving them away to his friends. And soon after that, marmorkrebs were showing up in pet stores upon Europe. There was something very strange about these crayfish. They were all female, and they all laid hundreds of eggs without mating. These eggs, in turn, hatched into hundreds more females — with each one growing up fully able to reproduce by herself. In 2003, scientists sequenced their DNA and confirmed what many owners already believed to be the case: Each baby crayfish was a clone of its mother, and they were filling Europe’s fishtanks at alarming speed. Just 25 years ago, the marbled crayfish did not exist at all. Now, they can be found in the wild by the millions in Germany, the Czech Republic, Hungary, Croatia, the Ukraine, Japan, and Madagascar.

Invasive species, like generalists or people after too much civilization, are simpler and therefore adapt more easily to a new place, at first. Over time their lack of specialization causes them to become more specialized, and they branch off into local species, but first, they have a much easier time of reproducing and displacing any native species that compete with them.

The same is true of government-sponsored immigration. The newcomers have fewer concerns about upholding the civilization and maintaining it, and only need to follow a basic model of feeding and reproducing. Over time, they displace the natives, who then pass into genetic history and are forgotten.