Re: What exactly is the Genealogical Species Concept?

Speciation is generally defined (in Biology and Evolutionary Biology
textbooks, such as those authored by Futuyma and Ridley) as the result of
two populations of an organism becoming isolated from one another for a
sufficient period of time that they become distinguishable entities. The
many approaches used to define species derive from the methods used to
distinguish the separate populations. One of the older definitions is the
Biological Species Concept, which defines species as closely related groups
of organisms which cannot produce viable offspring. Unfortunately,
reproductive isolation doesn’t necessarily hold for groups which have
traditionally been defined as species, especially when one looks outside
the realm of vertebrates. Recognition of limitations of the Biological
Species Concept lead to various attempts to create a more robust definition
of species. With the advent of methods for sampling genetic information
from individuals in populations came attempts to use these data as the
basis of a realistic and robust species definition.

Even before genetic data were available evolutionary biologists spent a
great deal of time and effort placing all of life into groups. Systematics
is the subdiscipline of Biology dedicated to the partitioning of life into
groups, generally on the basis of a hierarchical branching pattern called a
phylogeny. Taxonomy is the subdiscipline of Biology dedicated to naming
these groups and assigning them places in an organized system of
classification. Many biologists feel that any robust and realistic
definition of species must have an underlying phylogenetic component. The
difficulty of most phylogenetic species concepts came from determining the
level in the branching hierarchy where one would say “Below this Node lie
Species”. 

The Genealogical Species Concept is one type of phylogenetic species
concept. As defined in a recent paper investigating the mathematical
implications of the Genealogical Species Concept (Hudson and Coyne, 2002,
Evolution: Vol. 56, No. 8, pp. 1557–1565), a genealogical species is “a
basal group of organisms whose members are all more closely related to each
other than they are to any organisms outside the group ("exclusivity"), and
which contains no exclusive group within it”. This idea comes from the idea
(and the wealth of data that support it) that, when two populations become
isolated from one another, they begin to accumulate differences in their
DNA independently of one another. Differences in the DNA sequences for the
same genes (or locations in the DNA that aren’t necessarily considered
genes) are called alleles. Organisms that breed with one another will
exchange genetic material and share alleles at some of their loci. The
longer two groups are isolated from one another the more unique differences
they will accumulate. 

Currently relationships among groups of organisms are measured by the
similarity of genetic sequence information. The genetic data are used to
construct phylogenies of the individuals sampled. Phylogenies are subtly
different from genealogies, as each individual in a phylogeny will connect
with only one ancestor, which will also have only one other descendant. In
genealogies each individual connects to two ancestors, who in turn are
connected to two ancestors, and so on. Genealogical ancestors can connect
to many more than two descendants. Similarity and exclusivity are
determined by the agreement of phylogenies derived from multiple
independent loci. If the data support two independent groups then the
phylogenies are described as being in a state of reciprocal monophyly,
where one state of the data defines one group and another data state
defines the second. Generally, when multiple independent loci are surveyed
only a fraction of them will be reciprocally monophyletic. As more loci are
investigated and the reciprocally monophyletic fraction increases the two
groups are considered better genealogical species. Also, the use of
multiple independent loci should help to eliminate potential exclusive
groups within the hypothetical species due to the presence of rare alleles
in local populations. 

Any recently identified species will have been identified using
genealogical means. The recently discovered wild mouse on Cyprus
(http://msnbc.msn.com/id/15233416/?GT1=8618), the yellow and black
Yariguies Brush-Finch of Colombia (http://colombia.logtar.com/?p=76) and
the Indonesian coelacanth (http://en.wikipedia.org/wiki/Coelacanth) are
examples of recently discovered species defined using molecular
phylogenetic methods, which would make them genealogical species.