Principios de Taxonomia

38j 2 Why is there a Species Problem?
common ancestry (Atran, 1999). One of the classification principles will always be
weighted more heavily than the others.
3) The species concept of the gene-flow community: The gene-flow community
refers to a group of organisms that are cohesively connected to each other by gene
flow. Thus, the species has a gene-pool connection. The species concept of the
gene-flow community represented in this book is similar to the species concept of
the reproductive community by Ernst Mayr (Mayr, 1942) but different with respect
to some essential points (Chapter 6). The species concept of the gene-flow
community, in contrast to the concept of the reproductive community, does not
require that all organisms of a species be capable of mutually reproducing. The
gene-flow community concept only requires that individuals be connected to each
other via gene flow. Often, geographically distant organisms have long lost the
ability to mutually reproduce, but they are still connected by mutually reproducing
organisms through various intermediate populations, similar to the links of a
chain, so that gene flow is not interrupted. The concept of the gene-flow
community only demands that the organisms of neighboring chain links
mutually reproduce (Figure 2.7). The organisms of geographically distant populations
may long have become reproductively incompatible.
Figure 2.7 Scheme of gene flow cohesion
between races. A stem species bifurcates into
species 1 and 2 and into races A, B and C. The
difference between races and species is shown.
In each case, two parents (black and white)
exchange genes and produce offspring. Some
individuals mate with different partners. The
pathway of five newly mutated alleles (1 through
5) is shown along four filial generations (F1 to
F4). In each case, two alleles bridge the distance
between two geographically adjacent races. One
allele (5) even crosses the species barrier. It is
evident that there is cohesion between adjacent
races because they exchange their alleles
frequently. However, alleles in geographically
distant races (A and C) may very rarely or never
bridge this distance. For allele 5 (arisen in race C
of species 1) there is a better chance of bridging
the species barrier and introgressing into
foreign species (2), which lives in the same
region, than reaching a distant race (A) of its
own species (1). Nevertheless the stepwise
cohesion between the adjacent races of a
species is tighter than between different
species. The display of the graph follows de
Queiroz (1998).