Principios de Taxonomia

138j 6 Biological Species as a Gene-Flow Community
6.7
Why do the Individuals of a Species Resemble Each Other?
Why do the organisms of a species resemble each other? There are three possible
ways to explain this.
1) First, resemblance can be the result of a common descent. Children resemble
their parents because of descent; however, mutation and selection cause increasing
alteration of traits. As a consequence, genetic and phenotypic heterogeneity
grow over time. Finally, over long evolutionary stretches, the more or less
homogeneous appearance of the individuals of a species can no longer be
explained by their common descent.
2) Secondly, the homogeneous appearance of the organisms of a species may result
from a repeated stabilizing of the genomes via sexual genome fusion. Thus,
continuing biparental sexual contacts between organisms prevent genomic
divergence. If a group of organisms at the periphery of the distribution area of
the species were to adapt to local environmental conditions, these divergent
developments would be reversed by backcrossing with the organisms living at the
center of the distribution area. How strong are these forces? How often and across
which distances are the genomes of distant organisms stabilized by sperm-egg
fusion and genetic recombination? Can newly mutated alleles really reach all
distant areas of the species range? Does the well-known phenomenon of allele
fixation really reach all distant populations of a species?
The cofounders of the gene-flow-community species concept, Theodosius
Dobzhansky and Ernst Mayr, put substantial weight on the homogenizing force
of genetic recombination. In sexual gene flow, they saw a main reason why species
actually exist (see the Section The why of speciation in Mayr, 2000). The 1930s
and 1940s were the time of the Great Synthesis, which was a joining of
evolution, genetics and taxonomy. Gene flow among the organisms of a geneflow
community was not only perceived as a connecting force that keeps the
individuals of a species together, but also as the cause of the organisms identity of
traits across the entire range of the species. Gene flow was thought to cause the
organisms in a community to have an overall homogeneous appearance and
thus belong to the same species (Dobzhansky, 1937; Mayr, 1942). The morphological
and physiological integrity of a species was thought to be created and
maintained by gene flow.
The reasoning behind this was as follows: because of local adaptations in
different regions, the subpopulations in the long run would follow separate
evolutionary pathways and thus would become increasingly divergent. Backcrossing,
however, would occur repeatedly. Consequently, genomic interchange
among the distant organisms would result in homogeneity of the genotypes.
Because of genetic recombination, in principle, all allelic variants would reach all
of the genomes in a population of a species. If some organisms were to diverge by
mutation and selection, these deviations would again be reversed. Biparental
sexuality would hinder this divergence among the organisms of a gene-flow