Principios de Taxonomia

6.3 The Species is a Gene-Flow Community, Not a Reproductive Communityj131
sexual connections with the grandchildren or great-grandchildren of still other
lineages (Figure 2.7).
Thus, the gene-flow community is a network of biparental sexual connections and
genealogical connections. To find all the lateral (horizontal) connections, one must
step down many generations. Finally, however, all members of the gene-flow
community are connected within this network. An important component of the
gene-flow community is the repeated lateral combinations of genomes after only a
few or hundreds or even thousands of generations. The genes flow among the
organisms of this network, connecting the members of the gene-flow community.
This criterion differentiates the species as a gene-flow community from a group of
organisms that propagates only by uniparental reproduction where there is no gene
exchange between lineages.
6.3
The Species is a Gene-Flow Community, Not a Reproductive Community
It would be of particular interest to follow individual alleles within a gene-flow
community, but the available scientific information is insufficient. For example, the
Willow Tit (Parus montanus) is distributed continuously from Western Europe to
Eastern Asia. Imagine that a single allele mutates at a certain moment. How many
generations would it take for a newly mutated individual allele to migrate from
Western Europe to Eastern Asia? Would this migration ever be possible (see below)?
Almost no information is available to answer this question. The question becomes
even more difficult if one considers that most alleles are short-lived. Individual alleles
disappear by selection or genetic drift (Chapter 5); however, to understand the
connection among the individuals of a gene-flow community, it is sufficient to
consider only gene flow among immediate neighbors. Race A is connected with its
neighbor, race B, and race B with its immediate neighbor, race C, and so on
(Figure 6.2a).
A hypothetical structure of the gene-flow connection is illustrated in Figure 2.7.
In many cases, the connection includes only the members of the geographically
adjacent race. Thus, a single newly mutated allele (1) in race A reaches only the
organisms of the nearby resident race B, not the members of the more distant race C.
In turn, a single newly mutated allele (2) in race B reaches only the organisms of
the nearby resident race A, and a newly mutated allele in race C (allele 3) reaches the
organisms in adjacent race B, but not the distant organisms in race A. Nevertheless,
all races A, B and C are connected like the links of a chain, without direct contact
among all links.
Many alleles that arise in race A may not arrive in race C, and vice versa, at least not
across large geographic distances and within the evolutionary life span of an average
species. In these cases, the gene-flow connection is non-transitive; from the concurrent
connections of A and B and B and C, no connection between A and C can be
inferred. Although lateral gene exchange occurs between A and B and also between B
and C, it may not necessarily occur between A and C (Figure 6.2a).