Principios de Taxonomia

5.6 What are Morphs?j101
the morphs. Similar to races, morphs are examples of intraspecific polytypes.
However, there is a clear difference between morphs and races. Morphs do not
merge to become intermediate types if they are crossed; instead, their offspring again
split into distinct morphs. Races, in contrast, produce intermediate types if they
are crossed.
The most commonly known example of morphs is the sexual dimorphism, the
simultaneous existence of females and males. In several species, the two sexes are
phenotypically (not genotypically) very different. Already at the morphological level
(hence the name dimorphism), enormous differences between males and females
within a single species become apparent. With many animal species, it is easier to
distinguish the males from the females than to differentiate between different
species (Color Plates 5 and 6). An example of this scenario is the ducks, whose
females can often be assigned to species only by specialists, whereas the distinction
between males and females of the same species is not difficult for most of the year.
If only two morphs exist, then there is a dimorphism; if more than two exist, then
there is a polymorphism. Morphs are different phenotypes of the same species that
often live at the same location and that reproduce in an unlimited fashion with each
other. Morphs should not be confused with races. It is an important difference
between races and morphs that races maintain their phenotypic integrity because
they are, to a great extent, isolated by distance, even though there are smooth
transitions in geographic regions of contact. In great contrast, morphs, in most cases,
occur alongside each other syntopically and are not limited in any way from mating
with each other. This scenario raises the question of how morphs maintain their
phenotypic integrity, because, in contrast to races, there is no mating barrier by
geographic distance, and, in contrast to species, there is no mating barrier by intrinsic
traits that prevent sexual merging.
Why are there no intermediary phenotypes between morphs in spite of a strong
gene flow? Why is the result of a crossing between a male and a female not an
intermediate sex? Morphs owe their polytypy to specific mechanisms that prevent
phenotypic commixture. In most cases, these mechanisms are genetically based.
Specific genetic mechanisms prevent the formation of intermediary phenotypes,
with the best-known being the example of sexual dimorphism. However, in some
cases of di- or polymorphism, intermediaries arise without being prevented by
genetic control. In these cases, selection dominates. Although organisms that are
intermediates between two distinct morphs are born, they have only limited chances
of growing up. Only a few distinct morphs grow up. The intermediates constitute no
optimal adaptations to any type of existing environmental conditions. Therefore, they
are weeded out by selection more or less shortly after their origin in every generation
and, thus, hardly make an appearance. One of the best examples of this case is the
Garter Snake Thamnophis ordinoides (see below).
One must distinguish morphs that are solely alternating developmental stages of
an organism from those morphs that have the same developmental age and coexist
at the same time of the year alongside each other, for example males and females.
Only the second type of morph is a true morph. An example of the first type of
morphs is larvae, in contrast to imagos. Larvae sometimes live in a completely