Principios de Taxonomia

4.6 Traits that are Used by the Species to Distinguish Themselvesj79
Another example for the phenomenon that species-specific traits are only realized
in the region of overlap of two species is provided by members of the genus Zygaena
(Burnets) (see Color Plate 4). The three species of the Zygaena-transalpina group,
Zygaena transalpina, Z. hippocrepidis and Z. angelicae differ only in the geographical
regions of joint occurrence where they encounter each other (Ebert et al., 1994). Only
in these small overlap regions do they show their respective otherness and differ in
their choice of habitat as well as larval food plants and respective seasonality for when
the imagines hatch. For example, in the overlap region, Z. angelicae occupies beech
groves, while Z. hippocrepidis prefers dry grassland. Z. angelicae prefers the Scorpion
Vetch (Coronilla coronata) as a larval food plant in the overlap region, while Z.
hippocrepidis feeds on the Horseshoe Vetch (Hippocrepis comosa). Z. angelicae flies
from June to the middle of July in the overlap region, whereas Z. hippocrepidis is there
from the middle of July to the middle of August (Hille, 1995). Through these evasion
strategies, competition and also sexual mixture of the different species is avoided. In
the regions in which the three species occur alone and in which a selective pressure
for distinction and evasion does not exist, these species-specific differences are not
realized. In these regions, it is nearly impossible to distinguish the three species.
It is often argued that this case is an example of incipient speciation, which only has
its real beginning where the species encounter each other. Only there could one
speak of speciation (Ebert et al., 1994). The populations in the regions where the three
species do not occur jointly had not achieved the status of real species. However, no
argument exists that posits that the current status of the Zygaena-transalpina group
has to be transitory.
Indeed, there are examples in the animal kingdom that favor the contrary view that
species differences can disappear in geographical regions where they do not make
sense anymore. For example, the Mallard (Anas platyrhynchos) and the Pintail (Anas
acuta) are closely related species. They live sympatrically in major parts of the
Holarctic. There, the females of both species have to be able to distinguish their males
from the respective alien species. The plumage coloration of the male, which is
distinctly different in both species during the mating season, plays a role in the
partner recognition of ducks.
However, the necessity for correct partner recognition ceases to exist in places
where only one of each species occurs, and thus the discrimination of species is
unnecessary. This is the case with the Mallard in Hawaii and the Pintail in the
Kerguelen Islands, a group of islands in the southern Indian Ocean. Most of the male
Mallards in Hawaii and most of the male Pintails in the Kerguelen Islands are, like the
female, uniformly brown-colored for the whole year, even during the mating season.
Thus, the different species become similar. They do not wear the species-specific
mating plumage, probably because they do not have to contrast with the respective
related species.
In this case, one cannot argue that the similarity in traits between two species is a
sign for incipient speciation that has not been completed. The opposite is true. This is
a case of evolutionary retrogression of species recognition traits because the selective
pressure to maintain them has subsided. The species recognition traits seem to have
become lost because it was no longer required to maintain them. The Mallard in