Ecological niches shape the selective pressures populations face. When related populations exploit different resources or occupy different habitats, they experience divergent selection: traits that improve performance in one niche become more common in that group, while different traits rise in the other. This reduces direct competition between groups because they’re using different parts of the environment, so each population can specialize further. Over time, these differences can lead to reproductive isolation—whether through changes in mating timing, cues, or preferences linked to the distinct niches—so the groups no longer interbreed. That combination of divergence and isolation is how speciation arises. Adaptive radiation fits this idea neatly: when a lineage colonizes a new area with many unoccupied niches, natural selection drives rapid diversification as different populations adapt to distinct resources, leading to a burst of new species filling those niches. Darwin’s finches and various lake-dwelling cichlids are classic examples of this pattern. Why the other ideas don’t fit as well: increasing gene flow between populations would counteract divergence and hinder speciation; niches actually help populations colonize new environments by providing opportunities to exploit different resources rather than preventing it; and forcing identical resource use would heighten competition and blur the distinctions that drive divergence.