Spatial-Scale Evolutionary Bias Sheds Light on the Latitudinal Diversity Gradient

The latitudinal diversity gradient (LDG) is observed across various biological groups, but its causes remain debated. Here, using Escherichia coli as a model system, we experimentally tested whether spatial-scale evolutionary bias (SSEB)-the tendency for species to evolve adaptively toward areas with higher fitness gains-exists and whether it may contribute to the LDG. We conducted competition and experimental evolution assays between high-fitness-gain and low-fitness-gain populations. Our results show three key findings: (i) high-fitness-gain populations exhibit significantly faster spatial expansion than low-fitness-gain populations; (ii) under gene flow, high-fitness-gain populations tend to competitively exclude low-fitness-gain populations, whereas the reverse effect is minimal; and (iii) experimental populations are more likely to evolve and spread toward regions with higher fitness gains than toward regions with lower gains. These empirical findings collectively demonstrate the occurrence of SSEB. Given that climatic conditions become increasingly favorable from the poles to the equator, a latitudinal fitness gradient is expected for many taxa. Under such a gradient, SSEB would drive biased adaptation toward lower latitudes, either by suppressing adaptation of low-latitude species to higher latitudes or by promoting evolutionary migration from high to low latitudes. Thus, SSEB may provide a novel evolutionary mechanism contributing to the LDG.