Principios de Taxonomia

8
Outlook
What is a species? The answer to this question is clear: a species is a group of
individuals that are connected with each other by bonding and are separated from
another group of individuals. This seems simple, but the fundamental question of
taxonomy is less so: what is bonding and what is separation, and how can they be
recognized and measured? Zebras and Gnus in the Tanzanian savanna are not
separated. Male and female Tigers in the Indian jungle are separated for almost the
entire year. Why and how are they bonded?
Cohesion of the members of a species has something to do with the (vertical)
descent relationships and/or (lateral) sexual gene exchange between the organisms.
There is no other consistent way to define species cohesion. What is not connected by
descent or gene exchange cannot be a species because there is no natural bonding that
holds individuals together.
There is an important difference between sexual cohesion and genealogical
cohesion. Only sexual (lateral) gene flow can be ceased and, hence, terminate species
membership. Genealogical (vertical) cohesion cannot be ceased, and therefore, it
cannot not define the origin of a new species. Apart from species extinction, there is
no way to define the termination of species membership via genealogical decent
(vertical) cohesion because a species membership is terminated only via the
interruption of the gene flow connection.
Interruption of gene flow divides a cohesively bonded group into two separate
groups, resulting in speciation. Only the interruption of gene flow is speciation.
In most cases, the interruption of gene flow is not complete. Most related species
have occasional sexual contacts, which sometimes result in fertile offspring. Pre- and
postzygotic barriers are leaky. Nevertheless, despite the existence of smooth boundaries,
sexual barriers between particular groups of organisms do exist. These barriers
prevent or minimize gene flow among groups, and these groups are species.
Therefore, species do exist.
Finding these barriers represents a major task for the field of taxonomy. There are
particular genes that are responsible for these barriers. These genes are the speciation
genes; they cause pre- or postzygotic incompatibilities. Little is known about the
genes that lead to mating barriers between species or to allelic incompatibilities in a
species hybrid. Furthermore, little is known regarding the number of such genes and
Do Species Exist? Principles of Taxonomic Classification, First Edition. Werner Kunz.
Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.
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