The yup alleles of the two species of monkey flower were switched by hybridizing and backcrossing. Visits from pollinators followed the particular yup allele, not the species identity of the flower, indicating that the yup allele determines the pollinator of a given species. This shows that yup is necessary, but it does not show that this is the only reproductive barrier between species. There could be other differences such as flower morphology differences.
Work Step by Step
This question is asking you to summarize p. 516 and to see the difference between necessary and sufficient. Wheels are necessary for a car to move, but not sufficient since other parts like a gas tank and an engine are also necessary. A rocket motor is necessary and sufficient to move a rocket since that is all that is needed.