Polyphenism - Evolution of Polyphenisms

Evolution of Polyphenisms

A mechanism has been proposed for the development of polyphenisms:

1. A mutation results in a novel, heritable trait.
2. The trait’s frequency expands in the population, creating a population on which selection can act.
3. Pre-existing (background) genetic variation in other genes results in phenotypic differences in expression of the new trait.
4. These phenotypic differences undergo selection; as genotypic differences narrow, the trait becomes:
   1. Genetically fixed (non-responsive to environmental conditions)
   2. Polyphenic (responsive to environmental conditions)

Evolution of novel polyphenisms through this mechanism has been demonstrated in the laboratory. Suzuki and Nijhout used an existing mutation (black) in a monophenic green hornworm (M. sexta) that causes a black phenotype. They found that if larvae from an existing population of black mutants were raised at 20˚C, then all the final instar larvae were black; but if the larvae were instead raised at 28˚C, the final instar larvae ranged in color from black to green. By selecting for larvae that were black if raised at 20˚C but green if raised at 28˚C, they produced a polyphenic strain after thirteen generations.

This fits the model described above because a new mutation (black) was required to reveal pre-existing genetic variation and to permit selection. Furthermore, the production of a polyphenic strain was only possible because of background variation within the species: two alleles, one temperature-sensitive and one stable, were present for a single gene upstream of black (in the pigment production pathway) before selection occurred. The temperature-sensitive allele was not observable because at high temperatures, it caused an increase in green pigment in hornworms that were already bright green. However, introduction of the black mutant caused the temperature-dependent changes in pigment production to become obvious. The researchers could then select for larvae with the temperature-sensitive allele, resulting in a polyphenism.