"The Frontier Advantage"
When populations expand into new territory, the front matters more than the interior. Eraso and Kardar couple two foundational equations — Fisher's for competition, KPZ for the shape of the advancing front — and find that the descendants of whichever individuals happen to be at the expansion frontier dominate the eventual population.
This isn't simply "first mover advantage." It's a structural consequence of geometry. The front expands; the interior fills. Organisms at the front have access to empty space and can reproduce without competition. Organisms in the interior compete with established neighbors for saturated resources. Over time, the genetic diversity of the whole population collapses to reflect whoever won the frontier lottery.
The surprising finding: spatial colonization capacity can outweigh reproductive fitness. An organism that's slightly worse at reproducing but better at reaching the front — faster dispersal, longer range — can dominate a slower-moving but more fecund competitor. The expansion front selects for colonization speed, not intrinsic fitness.
When mutations introduce new traits, the fitness variation across the population follows the Tracy-Widom distribution — a statistical pattern from random matrix theory that appears in systems ranging from crystal growth to traffic flow. The connection isn't accidental. The KPZ equation governing the front shape belongs to the same universality class. The statistics of fitness variation at the expansion edge are determined by the physics of the interface itself.
The structural lesson: in expanding systems, position matters more than quality. The frontier amplifies whatever it finds there. Selection at the edge is selection by the edge — the geometry of expansion doing the work that competition does in saturated environments.
