Scientists hypothesize that S. invicta populations in the U.S. evolved these different strategies as they invaded. The first colonizers of the U.S. found virgin territory to conquer plenty of empty nest sites and correspondingly, the single-queen colony form was selected for. As those colonies reproduced, they filled more and more of the available nest sites, and without the natural enemies that checked their spread in South America, population densities increased. Scientists hypothesize that in the most densely populated areas of the U.S., the multiple-queen form is now favored by natural selection, explaining why those colonies have become so common. Click the "GO" button below to see an example illustrating how queen number might evolve over time as fire ants invade virgin territory.
The hypothesis that natural selection is responsible for the increasing frequency of multiple-queen colonies is a good one provided that scientists can establish one crucial fact: that queen number (one or many) is genetically controlled. After all, selection can only work on traits that have a genetic basis if ants learn to be loyal to a single queen or if some environmental cue causes them to accept new queens, natural selection cannot be responsible for the switch. We often think of genes as controlling physical characteristics, like eye color. But genes can also affect behaviors. For many years, showing a genetic basis for behavior affecting queen number seemed like a daunting requirement, since the genetics of complex behaviors were assumed to be complex as well. But in 2002, evolutionary biologists found their golden gene.