Principios de Taxonomia

150j 6 Biological Species as a Gene-Flow Community
incompatibility is not a straightforward consequence of evolutionary divergence,
which is the spatial and temporal distance of the populations from each other. No
biological law necessitates that phylogenetically distant populations must
automatically be reproductively incompatible. The organisms in evolutionary
distant strains become reproductively incompatible only by chance, not by any
biological law, as the largest proportion of the genes has nothing to do with species
barriers.
It would be too easy to interpret the situation such that, in the case of reproductive
incompatibility, the two populations just had diverged far enough from each other
that they were no longer genetically compatible. Genetic incompatibility is not a
necessary consequence of the divergence between two populations. Instead, reproductive
incompatibility is the result of the mutual incompatibility of the alleles of a
few particular genes, the interactions of which lead to restrictions on vitality or
fertility. Such genes are called speciation genes.
Speciation genes are responsible for speciation. They are responsible for keeping
two species segregated from each other. These genes cause the splitting of species.
They include, for example, the genes that control the mating choice. Their task is to
prevent mating with different species. Speciation genes also control adaptation to
species-specific food plants, habitats or climates. It is an important task of modern
taxonomy to find genes of this kind (Wu, Johnson, and Palopoli, 1998).
The statement that speciation is primarily founded on very few particular genes,
not on overall genetic divergence, is substantiated by the example of the New World
Ruddy Duck (Oxyura jamaicensis) and the Old World Ruddy Duck (Oxyura leucocephala)
(see below). The species were separated from each other for a long time.
They are phylogenetically distant and became phenotypically very different; however,
after the New World Ruddy Duck was introduced to Europe, it began to hybridize with
the endemic European Old World Ruddy Duck. Presently, the latter is seriously
endangered as a separate species because there appears to be no hybridization
barrier. Despite a long period of separation, no genetic incompatibility has evolved
between these two species. If the further expansion of the New World Ruddy Duck
cannot be stopped, the Old World Ruddy Duck may become extinct by hybridization
in the next few decades.
Speciation genes must be separated into prezygotic and postzygotic speciation
genes. Prezygotic speciation genes are those that control the processes that are
responsible for the final goal of sperm-egg fusion and, therefore, zygote formation
between two organisms. For this reason, they are called prezygotic genes.
In the case of external fertilization, prezygotic speciation genes code for signaling
molecules that allow sperm cells to fuse with the eggs of the correct species or that
prevent sperm cells from fusing with the eggs of the wrong species. Furthermore,
they code for receptor molecules on the egg s surface that control the fusion of sperm
cells with egg cells of the correct species. These receptor molecules could also inhibit
zygote formation between different species.
In the case of internal fertilization, prezygotic speciation genes control copulation.
If two organisms belong to different species, the prezygotic speciation genes prevent
a successful copulation. Therefore, in the case of internal fertilization, the term