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The factors that prevent the reproduction (formation of zygote) between two species are called prezygotic barriers. The scenarios of prezygotic barriers include habitat isolation, behavioral isolation, gametic isolation, and physiological isolation. These barriers do not allow the formation of hybrids.

Mimulus lewisii and Mimulus cardinalis are two types of monkeyflowers. They belong to the same species, although they comprise different pollinators due to the action of the yup locus (Yellow Upper). The allele present in this locus controls the carotenoid pigments of flowers.

The yuplocus gives pink to flowers of M. lewisii and red to flowers in M. cardinalis. Pink flowers of M. lewisii are pollinated by bumblebees, and red flowers of M. cardinalis are pollinated by hummingbirds.

In the species of monkey flowering plants, the locus yup is the major reproductive barrier. The adaptive mutation of the alleles of the locus in both the plants acts as a barrier, so they comprise different pollinators. It prevents reproduction between the monkey flowering plants.

However, the pattern of color not only attracts the pollinators towards the flower. Nectar, scent, shape, and size of the flower also influence the pollinators.

Hummingbird favors nectar-rich and high in anthocyanin flowers which are red-flowered M. cardinalis. Bumblebees favor large and low in anthocyanin flowers such as pink-flowered M. lewisii.

Thus, other than alleles of yup locus, the amount of anthocyanin, size of flower, and nectar also act as reproductive barriers.