A Comparison of Adaptive Radiations of Antarctic Fish with those of NonAntarctic Fish
17
McDonald and coworkers [66] used electrophoresis to examine variation
at 30 protein-coding loci in six species of notothenioids from McMurdo
Sound. Estimated divergence times within the Nototheniidae suggested
that Dissostichus separated from Trematomus at about 28-20 Ma BP, and
from Pagothenia at about 13 Ma BP. This would be consistent with the
major phyletic divergence of notothenioids taking place after the isolation
of Antarctica and after the initial period of cooling.
The traditional Bovichtidae is paraphyletic, with Pseudaphritis more
closely related to nonbovichtid notothenioids than are Cottoperca or
Bovichtus. The position of these latter two genera with respect to various
notothenioid outgroups is uncertain [65]. Inference from mtDNA
sequences implies that the radiation of the phyletic ally derived Antarctic
notothenioids took place more recently than previously suspected,
probably within the last 15 Ma [8]. Recent analysis of partial
mitochondrial sequences from 12S and 16S rRNA genes and assuming a
conventional rRNA molecular clock puts the average age of the
trematomid radiation at only 3.4 Ma (mid-Pliocene). This is relatively
recent, and it is intriguing that the age coincides so well with the timing of
the controversial Pliocene warming event. If there was a significant
warming event at around 2.5-5 Ma BP then this may have resulted in
significant changes in available habitat and together with Milankovitchdriven variations in habitat extent may have prompted the radiation of
trematomids.
The mapping of the presence of antifreeze glycoprotein onto the
cladogram suggests that this feature evolved only once, after the
divergence of the Bovichtidae. If the timing of the cooling of the Southern
Ocean to subzero temperatures can be fixed precisely, then this in tum
could fix the timing of the main diversification.
There are three main possibilities for the timing of Southern Ocean
cooling to subzero temperatures. One is the sharp drop in surface
temperatures at the start of the Oligocene, 35 Ma BP, the second is the
marked cooling event at the middle of the Miocene at about 17 Ma BP,
and the third is the final plunge to truly polar temperatures in the late
Pliocene.
The third of these possibilities is not tenable on the grounds of
biogeography, and all molecular evidence, and hence can probably be
dismissed.
More intriguing, however, is the choice between the first two potential
dates. The older date would be consistent with the combined evidence
from biogeography and the cladistic analysis [1]. The molecular evidence,
however, suggests a date for the main notothenioid diversification (after
17
McDonald and coworkers [66] used electrophoresis to examine variation
at 30 protein-coding loci in six species of notothenioids from McMurdo
Sound. Estimated divergence times within the Nototheniidae suggested
that Dissostichus separated from Trematomus at about 28-20 Ma BP, and
from Pagothenia at about 13 Ma BP. This would be consistent with the
major phyletic divergence of notothenioids taking place after the isolation
of Antarctica and after the initial period of cooling.
The traditional Bovichtidae is paraphyletic, with Pseudaphritis more
closely related to nonbovichtid notothenioids than are Cottoperca or
Bovichtus. The position of these latter two genera with respect to various
notothenioid outgroups is uncertain [65]. Inference from mtDNA
sequences implies that the radiation of the phyletic ally derived Antarctic
notothenioids took place more recently than previously suspected,
probably within the last 15 Ma [8]. Recent analysis of partial
mitochondrial sequences from 12S and 16S rRNA genes and assuming a
conventional rRNA molecular clock puts the average age of the
trematomid radiation at only 3.4 Ma (mid-Pliocene). This is relatively
recent, and it is intriguing that the age coincides so well with the timing of
the controversial Pliocene warming event. If there was a significant
warming event at around 2.5-5 Ma BP then this may have resulted in
significant changes in available habitat and together with Milankovitchdriven variations in habitat extent may have prompted the radiation of
trematomids.
The mapping of the presence of antifreeze glycoprotein onto the
cladogram suggests that this feature evolved only once, after the
divergence of the Bovichtidae. If the timing of the cooling of the Southern
Ocean to subzero temperatures can be fixed precisely, then this in tum
could fix the timing of the main diversification.
There are three main possibilities for the timing of Southern Ocean
cooling to subzero temperatures. One is the sharp drop in surface
temperatures at the start of the Oligocene, 35 Ma BP, the second is the
marked cooling event at the middle of the Miocene at about 17 Ma BP,
and the third is the final plunge to truly polar temperatures in the late
Pliocene.
The third of these possibilities is not tenable on the grounds of
biogeography, and all molecular evidence, and hence can probably be
dismissed.
More intriguing, however, is the choice between the first two potential
dates. The older date would be consistent with the combined evidence
from biogeography and the cladistic analysis [1]. The molecular evidence,
however, suggests a date for the main notothenioid diversification (after