Today is not only the final day of Army Ant Week, but Charles Darwin‘s 202nd birthday.
So I close Army Ant Week with a bit of speculation about evolution, and what army ants suggest about the nature of the evolutionary process.
The 300 or so army ant species vary in behavior, but most are specialized ant predators that hunt in soil or leaf litter. Their limbs are short and stubby, ideal for subterranean maneuvering, and the workers are largely to completely blind.
The “classic” army ants, the surface-foraging swarm raiders familiar to most people, are in fact a minority of species (Eciton burchellii, Labidus praedator, and several Dorylus). Where did these spectacular swarm raiders come from?
You might think the swarm raiders are ancient, and that the specialist ant predators evolved recently. After all, a generalized surface predator should be able to birth all manner of more specialized descendants. But the available evidence suggests the opposite.
Here’s a figure from Kronauer et al (2007) showing key transitions in the African genus Dorylus:
The early lineages of Dorylus were underground ant specialists, and it is these that gave rise to the later swarm raiders. The evolutionary relationships of the new world army ants (not shown) indicate the same pattern. Swarm raiders emerged from below ground stock.
All told, swarm raiding has evolved at least three times, each advent independent of the others, and each from similarly ant-eating ancestors.
In my opinion, this threefold replicate of leggy swarm-raiders emerging from stubby underground ant predators cannot be coincidence. Nor do I think it chance that swarm raiders don’t evolve from non-ant predators (except perhaps here). The swarm raiding lifestyle is obviously a successful one- so why has it only been achieved via detours through underground ant predation?
Ultimately, the answer may not be knowable. But, the convoluted path to swarm raiding is a reminder that the evolutionary process does not draw direct lines progressing from ancestral to descendant lifestyles. Rather, natural selection works only in the moment, unable to see what might be advantageous generations ahead. Swarm raiding may simply be easier to evolve by taking the long road underground.