EEB Tuesday Seminar Series - Multi-environment fitness landscapes suggest effectively endless adaptation

Seminar Summary - The number of beneficial substitutions (N) required for an adapting population to reach a fitness peak is a fundamental but unknown parameter of evolution. We find that N increases linearly with the number of variable sites in empirical fitness landscapes, regardless of prior adaptation in another environment. Extrapolation suggests a minimal N of 10⁵ for typical prokaryotes and 10⁷ for mammals. In contrast, in highly rugged shuffled landscapes, N is markedly reduced, while fitness gains also shrink. The relatively smooth empirical landscapes therefore enable greater fitness gains while lengthening adaptive walks, rendering even local fitness peaks effectively unreachable because natural environments inevitably change. These results explain the persistence of fitness gains in long-term evolution experiments under constant environments and the widespread occurrence of beneficial mutations across species. They suggest that populations continuously adapt while remaining far from fitness optima, challenging the long-held view that adaptation culminates at fitness peaks.