Several theoretical studies of niche breadth evolution have shown that niche breadth expansion can only occur under a limited set of conditions. These studies have assumed that mating pairs form at random within spatial subpopulations. I show here that nonrandom mating within spatial subpopulations can greatly alter niche breadth evolution. Niche breadth expansion is restricted by assortative mating but is permitted by sexual selection. When new populations are formed by a single pulse of immigrants, the ability to persist in novel but similar environments is enhanced by sexual selection. When new populations receive a constant stream of immigrants, sexual selection allows the evolution of increased niche breadth even when the novel environment is very different from the ancestral environment. These results suggest that species lineages that exhibit sexual selection will have broader niches than species lineages that mate randomly or assortatively.