Eusociality and other improbable evolutionary outcomes can be accelerated by trait hitchhiking in boom-bust feedback loops

Here I analyze the brush-fire cycle behind the brushy frontier of a grassland, seeking evolutionary feedback loops for large grazing animals and their hominin predators. The burn scar’s new grass is an empty niche for grass-specialized herbivores, which evolved from mixed feeders only in the early Pleistocene. The frontier subpopulation of grazers that discovers the auxiliary grassland quickly multiplies, creating a secondary boom among predators. Following this boom, a bust occurs several decades later when the brush returns; it squeezes both offshoot populations back into their core grasslands population. For both prey and predators, such a feedback loop can shift the core’s gene frequencies toward those of the brush explorers. Any brush-relevant allele could benefit from this amplifying feedback loop, so long as its phenotypes concentrate near where empty niches can open up in the brush; with hitchhiking, improved survival is unnecessary. Cooperative nurseries in the brush’s shade should concentrate the alleles favoring eusociality, enabling their amplification.