Principios de Taxonomia

6.24 The Example of Some Duck Species: Extinction through Hybridizationj169
2006). In contrast to expectations, these hybrids often do not have an intermediary
phenotype. Because of the dominance of many genes, the phenotype of only one
parent can predominate, with the result that the hybrid is not readily recognized
because it resembles only one of the two parental species. This scenario is often the
case in ducks (Randler, 2000).
An increased chance for species hybridization occurs when one of the sexual
partners suffers sexual starvation. For example, this case occurs if one species faces
extinction. In this case, members of the rare species are only rarely found by an
individual looking for a sexual partner, while members of the other species exist in
abundance and are, therefore, in a pinch accepted as sexual partners. The
hybridization with the still frequent species is an additional threat for the rare
species, which can lead to the extinction of the rare species (Frankham et al., 2001).
This scenario was, for example, observed in the case of a rare species of Fur Seals on
Marion Island in the southern Indian Ocean (Wirtz, 2000). Species hybridizations
also occur more often towards the end of the mating season, when the members of
the same species are almost all paired up, while those of the other species are
still available.
Especially impressive are the many hybridizations among almost all species of
Anatidae (ducks, geese, swans). The Anatidae are so-to-speak the orchids among the
animals (Randler, 2000). In an investigation by Scherer and Hilsberg (1982) on
the hybridization of the Anatidae, 418 different hybrids originate from a total of 149
species; 52% of the hybrids even comprise genera:
Mallard (Anas platyrhynchos) with Egyptian Goose (Alopochen aegyptiacus).
Egyptian Goose (Alopochen aegyptiacus) with Greylag Goose (Anser anser).
Greylag Goose (Anser anser) with Mute Swan (Cygnus olor).
Swan Goose (Anser cygnoides) with Mute Swan (Cygnus olor).
Observations speak in favor of at least a part of the hybrids being fertile, so that the
gene exchange between the species is further propagated. However, it is often not
possible to pursue the question of whether the hybrids produced in nature are fertile.
This difficulty occurs because the goslings that follow a couple of geese or the
ducklings that follow a leading duck mother do not necessarily have to descend from
those parents. In Anatidae, there are occurrences of exchanges, including nest
robbery, unfaithfulness and adoptions.
6.24
The Example of Some Duck Species: Extinction through Hybridization
Nevertheless, the frequency of the occurrence of species hybrids should not contradict
the fact that there are biological laws that prevent incidences of this type. The
occurrence of reproductive isolation is still the central objective in the evolution of
species because, if the organisms of one species would not be reproductively isolated
from each other, then mixed types between the species would occur in an unlimited
way. An infinite production of mixed types carries the danger of one of the parental