This still will not result in long-term polymorphism unless the overall fitness of the heterozygote is higher than either of the homozygotes. The environment is not always constant Fluctuation environments may favor different genotypes at different times, that is, the environment is temporally variable. Different genotypes may be fittest in different microhabitats, that is, the environment is spatially variable. 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.