Natural selection should eliminate harmful mutations. An individual carrying a deleterious mutation reproduces less efficiently than its unmutated competitors. Over time, the mutation's frequency drops. In a large, well-mixed population, this works — the fitness cost acts as a drag, and the mutation is purged.
Madeira, Ortgiese, and Penington ask what happens when the population is not well-mixed but spatially expanding — a wave of organisms colonizing new territory. They construct a rigorous mathematical framework (a functional law of large numbers for a spatial Muller's ratchet) and prove that harmful mutations can surf the expansion wave.
The mechanism is spatial. At the leading edge of an expanding population, density is low and stochastic effects dominate. A mutant that happens to be near the frontier gets carried forward by the wave regardless of its fitness. The wave doesn't select — it transports. The mutant's location, not its quality, determines its fate. Behind the frontier, in the dense interior, selection operates normally and would eliminate the mutation. But at the front, the geometry of expansion overwhelms the arithmetic of fitness.
The result is that deleterious mutations achieve frequencies far higher than fitness alone would permit. They don't succeed because they're fit. They succeed because they're in the right place — the thin edge of an advancing wave where the population is too sparse for selection to act effectively. The mutation hitchhikes on geography.
This is a spatial analog of genetic drift, but with a directional driver. Drift in a well-mixed population is symmetric — mutations wander randomly. Surfing on a wave is directional — the expansion pushes the mutation forward, amplifying it at each generation. Growth itself becomes the mechanism by which harmful traits propagate.
The general principle: an expanding system carries its contents forward indiscriminately. The wave does not curate what it transports. Whatever is at the frontier when expansion occurs gets amplified — including the things that selection would otherwise remove. Growth is not inherently purifying. It can be the vehicle by which defects escape their own fitness consequences.