268
L. Bargelloni and G. Lecointre
In summary, molecular evidence from mitochondrial and nuclear
ribosomal genes gives indications that roughly parallel the phylogenetic
picture based on the analysis of morphological characters, if we take into
account those morphologists that have recognized the paraphyly of some
families (i.e. Balushkin [17] for Eleginops). Nevertheless, one exception
exists if we add to this data set the three bovichtid genera. The highly
supported relationships of Pseudaphritis found from two different genes
[10,16] strongly contradict both the poorly argumented monophyly of
bovichtids [8] and the paraphyly of this family as presented by Balushkin
[17].
Phylogenies, Speciation Patterns and Molecular Clocks
As already mentioned, the importance of phylogenies in comparative and
evolutionary studies cannot be overlooked. For instance, the distribution
of present-day species can be superimposed on well-established
phylogenetic trees to infer patterns of speciation [28]. Since Darwin, the
formation of species has represented one of the most important, yet also
one of the most elusive subjects in evolutionary biology, and a large
number of studies have been aimed at understanding speciation. As a
result of these efforts, a basic model of speciation arose, which is now
named "allopatric speciation." The "allopatric" scenario is rather simple: a
large, continuous population is divided in smaller subunits by the presence
of extrinsic barriers, which reduce the genetic exchange among separated
subpopulations. This favors the creation of genetic differences, ultimately
leading to reproductive isolation. In marine systems, however, barriers to
gene flow are far less common if compared to terrestrial or freshwater
environments. In addition, marine organisms often show a high dispersal
potential, further reducing the opportunity for genetic divergence among
populations. It is not surprising, therefore, that the Polar Front Zone (PFZ)
has attracted the attention of several authors, as a unique hydrographic
barrier which isolates the Southern Ocean, playing a pivotal role in the
evolution of the Antarctic marine fauna. Vicariance (i.e. "passive"
allopatric speciation), due to the formation of the PFZ, has in fact been
invoked as the most important mode of species formation in several
Antarctic taxa. Evidence from molecular data casts some doubt on this
scenario. The vicariant hypothesis stems from the observation that the area
cladogram of Nototheniidae shows that the more phyletically derived taxa
(Trematominae and Pleuragramminae) have a High Antarctic distribution,
while less derived groups (Eleginopinae and Nototheniinae) present a
relevant number of subAntarctic or lesser-Antarctic species. However, the
c1adogram itself seems to be seriously flawed, if molecular results are
L. Bargelloni and G. Lecointre
In summary, molecular evidence from mitochondrial and nuclear
ribosomal genes gives indications that roughly parallel the phylogenetic
picture based on the analysis of morphological characters, if we take into
account those morphologists that have recognized the paraphyly of some
families (i.e. Balushkin [17] for Eleginops). Nevertheless, one exception
exists if we add to this data set the three bovichtid genera. The highly
supported relationships of Pseudaphritis found from two different genes
[10,16] strongly contradict both the poorly argumented monophyly of
bovichtids [8] and the paraphyly of this family as presented by Balushkin
[17].
Phylogenies, Speciation Patterns and Molecular Clocks
As already mentioned, the importance of phylogenies in comparative and
evolutionary studies cannot be overlooked. For instance, the distribution
of present-day species can be superimposed on well-established
phylogenetic trees to infer patterns of speciation [28]. Since Darwin, the
formation of species has represented one of the most important, yet also
one of the most elusive subjects in evolutionary biology, and a large
number of studies have been aimed at understanding speciation. As a
result of these efforts, a basic model of speciation arose, which is now
named "allopatric speciation." The "allopatric" scenario is rather simple: a
large, continuous population is divided in smaller subunits by the presence
of extrinsic barriers, which reduce the genetic exchange among separated
subpopulations. This favors the creation of genetic differences, ultimately
leading to reproductive isolation. In marine systems, however, barriers to
gene flow are far less common if compared to terrestrial or freshwater
environments. In addition, marine organisms often show a high dispersal
potential, further reducing the opportunity for genetic divergence among
populations. It is not surprising, therefore, that the Polar Front Zone (PFZ)
has attracted the attention of several authors, as a unique hydrographic
barrier which isolates the Southern Ocean, playing a pivotal role in the
evolution of the Antarctic marine fauna. Vicariance (i.e. "passive"
allopatric speciation), due to the formation of the PFZ, has in fact been
invoked as the most important mode of species formation in several
Antarctic taxa. Evidence from molecular data casts some doubt on this
scenario. The vicariant hypothesis stems from the observation that the area
cladogram of Nototheniidae shows that the more phyletically derived taxa
(Trematominae and Pleuragramminae) have a High Antarctic distribution,
while less derived groups (Eleginopinae and Nototheniinae) present a
relevant number of subAntarctic or lesser-Antarctic species. However, the
c1adogram itself seems to be seriously flawed, if molecular results are