Most organisms are diploid. That is important because an organism only needs one good copy of a gene to function. A deleterious gene can be maintained in a population unless it is dominant allele. Heterozygotes with deleterious but recessive genes are therefore not negatively selected. Further, there are many traits that are not directly related to survival, so there is virtually no selection, and many versions of the same trait can simultaneously exist. Polymorphism is a term that describes traits that can vary within a species. Blood type is a good example. A, B, AB and O blood types are not selected against, so this trait can be polymorphic

In some species, coloration can be highly polymorphic, so much so that scientists have mistaken the different color phases as different species, since in most organisms that is strong selection on color, either because it is used as a species identificationsignal to identify the individual to other individuals of the same species or because the color must be a close match of the background. For example, almost all leopards have spots, although some are melanistic. In some species that are active mostly at night, such as the ground snake, Sonora semiannulata, coloration can be highly variable because it doesn't matter what color a snake is at night, since in low light, color vision is not possible, and everything just appears as different shades of gray.

Since natural selection does not and cannot act on all traits all of the time, may traits can be polymorphic. Some traits, whoever, cannot be polymorphic, because there is harsh selection against individuals that are different. For example, a crocodile born without a tail will almost certainly not live long enough to reproduce, because it cannot swim without its tail. Relatively few traits however are subject to such harsh selection, so they can be polymorphic within a species.

Hello,

I am really confused about the concept of natural selection and polymorphism. Natural selection is the process were by good traits are selected out of different traits produced by mutation. The concept of "polymorphism" states that natural selection maintains variation within a given population. How can many different traits be equally "fit" in a given environment? I don't understand? Also, how can a genotype-phenotype vary with "frequency" in a population?

Thanks!