Principios de Taxonomia

5.13 Intraspecies Morphs in the Burnet Moth Zygaena ephialtesj115
referred to as cryptic species. An example of these are the three species of the
Zygaena-transalpina group: Z. transalpina, Z. hippocrepidis and Z. angelicae. Only
specialists are able to distinguish these three species from each other, and an element
of uncertainty remains even then.
Alternatively, only in the genus Zygaena can an impressive example of intraspecific
polymorphisms be observed, for Z. ephialtes. This species consists of a number of
morphs that can be distinguished from each other much more easily than the
majority of the other Zygaena species can be differentiated at the species level (Color
Plate 4). Anyone who intends to identify Zygaena species can only dream to be able to
identify individual species as easily one can distinguish the individual morphs Z.
ephialtes.
Zygaena ephialtes consists of four different morphs. Therefore, Z. ephialtes also
bears the trivial name variable Burnet Moth. The drastic phenotypic differences
between the morphs are based on only two genes, for which there are each two allelic
variants with two different phenotypes. One gene regulates the phenotype of the hind
wings, which are either colored or black. The other gene regulates the color of the
moth, which can be either red or yellow (Sbordoni et al., 1997).
The alleles of the two genes act in a dominant-recessive fashion. Colored hind
wings are dominant over black wings. Red color is dominant over yellow. Both genes
can be freely combined, because they are located on different chromosomes. This
very simple genetic constitution explains why four different morphs arise.
Zygaena ephialtes is one of the best examples of very different phenotypes that
simulate different species. These differences, however, are based on only two genes,
and the entire phenotype set is a textbook example of the third Mendelian rule. The
genetic crossbreeding of homozygous parents corresponds to two-factor crossing,
with the well-known result that four phenotypes occur in the F2 generation in the
9:3:3:1 ratio. These four phenotypes are the four morphs of Z. ephialtes: a red morph
with colored hind wings (9 genotypes; both genes dominantly control the phenotype),
a yellow morph with colored hind wings (3 genotypes; one gene dominant, the other
homozygous recessive), a red morph with black hind wings (3 genotypes; one gene
dominant, the other homozygous recessive) and a yellow morph with black hind
wings (1 genotype; both genes are homozygous recessive) (Ebert et al., 1994).
The authors who first described and named these different morphs of Z. ephialtes
considered them all to be distinct species. They assigned them different scientific
names, with the genus name Sphinx: Sphinx peucedani, S. athamanthae, S. coronillae,
and so on. This example should be a warning for those making taxonomic classifications
according to trait differences. Through the comparison of traits alone, such
mistakes cannot be rectified. The existence of morphs demonstrates again how
important it is to be skeptical of the use of differences in traits as the only criterion for
species membership. Inferring species membership from trait inequality can quickly
lead to false results.
Apparently, the two allelic pairs of both of the genes of Z. ephialtes that are
responsible for the morphs are especially stable. Generally, multiple alleles in the
population are not especially long-lived. Selection and genetic drift in most cases
ensure that mutant alleles cannot establish themselves in the population. If these