"Geneticists believe," writes Charles C. Mann, "that S. invicta (fire ants) originated in southern Brazil, an area with many rivers and frequent floods. The floods wiped out ant nests. Over the eons, these small, furiously active creatures have evolved the ability to respond to rising water by knitting their bodies together into floating swarm-balls - workers on the outside, queen in the center - that can ride on the flood for days. Once the waters recede, colonies swarm back onto previously submerged land so rapidly that S. invicta can use the devastation to increase its range. Like criminal gangs, fire ants thrive on chaos" (pg 31).
Charles C. Mann continues.
In the 1930s Solenopsis invicta was transported to the United States, probably in ship ballast, which often consists of haphazardly loaded soil and gravel. An adolescent bug enthusiast named Edward O. Wilson, later a famous biologist, spotted the first colonies in the port of Mobile, Alabama. From the ant's vantage, it had been dumped onto an empty, recently flooded expanse. S. invicta too off, never looking back.
More likely, the initial incursion seen by Wilson was just a few thousand individuals - a number small enough to hint that random, bottleneck-style genetic change played a role in what happened next. (The evidence is not yet inclusive.) In its homeland, fire ant colonies constantly fight each other, reducing their numbers and creating space for other types of ant. In North America, by contrast, the species forms cooperative super-colonies, linked clusters of nests that can spread for hundreds of miles, wiping out competitors along the way. Remade by chance and opportunity, new-model S. invictus needs just a few decades to conquer much of the southern United States.
A primary obstacle to its expansion is another imported South American ant, Linepithema humile, the Argentine ant. After escaping its natal territory more than a century ago, L. humile formed its own super-colonies in the United States, Australia, New Zealand, Japan, and Europe (the European colony stretches from Portugal to Italy). In recent years researches have come to believe that these huge, geographically separate ant societies in fact may be part of a single intercontinental unit, a globe-spanning entity that exploded across the planet with extraordinary speed and rapacity, and is now the most populous society on Earth.
Homo sapiens did something similar as it became human (pg 32).
Man, that is some cool stuff.
If you haven't heard of this book already, The Wizard and the Prophet: Two Remarkable Scientists and Their Dueling Visions to Shape Tomorrow's World, by Charles C. Mann, check it out. It's worth the read.
Freakonomics recently aired an episode, Two (Totally Opposite) Ways to Save the Planet, where they interviewed Mr. Mann (what a great name, by the way) and a few contemporaries from either side.
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