Again, neither condition is sufficient to maintain polymorphism - the heterozygote must still have higher fitness, averaged over time or space. Multiple niche selection Depends on the graininess of the environment and whether selection is hard or soft. Population size is reduced. Population size is the same, but allele frequencies change. Coarse-grained environments and soft selection are more likely to maintain polymorpisms.
Wide bills can process hard sedge seeds, and narrow bills can process soft seeds.
Intermediate bill size has lower survival rate. Frequency-dependent selection Fitness of a genotype depends on its frequency in the population. New kid in town syndrome. If the fitness of a genotype is greater the more frequent it is in a population, then whichever allele is initially more frequent will be fixed. Heterozygote disadvantage, or underdominance Fitness of heterozyote is less than either homozygote.