What is the difference between multiple allelism and polymorphism? From the Scott Freeman textbook Biological Science 4th Edition: Multiple allelism is defined as "the existence of more than two alleles of the same gene within a population." Polymorphism is defined as "the occurrence of more than two distinct phenotypes of a trait in a population." I don't really understand the difference - unless it means that two alleles may not result in two different phenotypes? Additionally, another definition of polymorphism also in the textbook was "the existence of more than one allele at a certain genetic locus." What does this mean? I was under the impression that all genes had at least two alleles.

The term polymorphism is broad and can have different meanings. Here are your definitions

> Multiple Allelism: The existence of more than two alleles of the same gene within a population.
>
> Polymorphism: the occurrence of more than two distinct phenotypes of a trait in a population.

Considering your definitions only, then multiple allelism has to do with genetics while polymorphism has to do with the phenotype.

Most loci (=position in the genome) that contain more than one allele (=genetic variant) have no effect on the phenotype. So multiple allelism does not necessarily yield to phenotypic polymorphism.

On the other hand, plenty of phenotypic variation in a population is due to environmental variation (including what an individual eats) and not to genetic variation. So phenotypic polymorphism is not necessarily caused by a case of multiple allelism.