Final answer:
Disruptive natural selection occurs when phenotypes in the middle of the range are selected against, resulting in two overlapping phenotypes. The black-bellied seedcrackers may have distinct phenotypes with different beak shapes specialized for consuming different sizes of seeds. Mexican spadefoot toads may exhibit omnivorous and carnivorous morphs due to intraspecific competition for different types of prey.
Step-by-step explanation:
Disruptive natural selection occurs when phenotypes in the middle of the range are selected against. Resulting into 2 overlapping phenotypes, one at each end of the distribution. In the case of the black-bellied seedcrackers, disruptive natural selection might occur if there are two phenotypes of the bird species with different beak shapes. If the environment provides seeds of different sizes, birds with smaller beaks will be more successful at consuming small seeds while birds with larger beaks will be more successful at consuming large seeds. This would create two distinct phenotypes of the species, each specializing in consuming a different size of seed.
Mexican spadefoot toads also exhibit intraspecific competition between omnivorous and carnivorous morphs. In this case, disruptive natural selection might occur if the environment provides a variety of prey types. If the toads with the omnivorous morph are better at consuming plant matter and the toads with the carnivorous morph are better at consuming animal matter, their competition for resources would increase. This could lead to two distinct morphs within the species, each specializing in a different type of diet.