18
J.T. Eastman and A. Clarke
separation of the bovichtid clade) of around 11 Ma BP [8]. If this later date
is correct, it implies a much more rapid diversification than has
traditionally been believed. It would also imply that either the first cooling
event at the start of the Oligocene was not sufficient to require the
evolution of antifreeze, or that antifreeze arose independently in several
notothenioid clades.
The small degree of sequence divergence between the notothenioid
antifreeze glycopeptide and trypsinogen genes suggests that antifreeze
evolved somewhere between 5 and 14 Ma BP [67]. This is satisfyingly
close to the entirely independent molecular evidence from mtDNA [8] and
suggests strongly that the main diversification of the notothenioids has
taken place in the past 10-15 Ma. This matches the end of the first phase
of continental glaciation [46].
Comparison of the Radiation of Antarctic Fishes with
Lacustrine Species Flocks
As a basis for comparison with notothenioids we will mention relevant
aspects of the biology of the well studied flocks of fish in the East African
Great Lakes and in Lake Baikal in Siberia.
East African Cichlids
Lakes Tanganyika, Malawi and Victoria differ greatly in morphometry,
depth and age (Table 1). They have experienced climatic changes that
affected both water levels and degree of isolation of their basins.
Tanganyika and Malawi are old deep rift lakes that are permanently
stratified and anoxic below 250 m. The rocky shorelines with isolated bays
and islands provide a complex array of microhabitats. The present Lake
Victoria, on the other hand, is a shallow depression variously dated from
750000 [68] to 12400 years old [69].
The perciform cichlids in Lakes Tanganyika, Malawi and Victoria
(Table 2) are the most speciose and most extensively studied examples of
fish species flocks [14,70,71]. The age of the various lakes is reflected in
the higher level taxonomic complexity of the fauna. For example, the old
flocks of cichlids in Tanganyika show more tribal and generic
diversification than those in the other lakes. Furthermore, Tanganyika is
home to small flocks of several catfish families and mastacembelids, and
to an offshore fauna consisting of endemic clupeids and centropomids.
Divergence times and phylogenetic relationships based on mtDNA
sequences suggest that the cichlid flock in Tanganyika is polyphyletic,
hence there are actually multiple flocks dating from 3.5 to 5 Ma. The
J.T. Eastman and A. Clarke
separation of the bovichtid clade) of around 11 Ma BP [8]. If this later date
is correct, it implies a much more rapid diversification than has
traditionally been believed. It would also imply that either the first cooling
event at the start of the Oligocene was not sufficient to require the
evolution of antifreeze, or that antifreeze arose independently in several
notothenioid clades.
The small degree of sequence divergence between the notothenioid
antifreeze glycopeptide and trypsinogen genes suggests that antifreeze
evolved somewhere between 5 and 14 Ma BP [67]. This is satisfyingly
close to the entirely independent molecular evidence from mtDNA [8] and
suggests strongly that the main diversification of the notothenioids has
taken place in the past 10-15 Ma. This matches the end of the first phase
of continental glaciation [46].
Comparison of the Radiation of Antarctic Fishes with
Lacustrine Species Flocks
As a basis for comparison with notothenioids we will mention relevant
aspects of the biology of the well studied flocks of fish in the East African
Great Lakes and in Lake Baikal in Siberia.
East African Cichlids
Lakes Tanganyika, Malawi and Victoria differ greatly in morphometry,
depth and age (Table 1). They have experienced climatic changes that
affected both water levels and degree of isolation of their basins.
Tanganyika and Malawi are old deep rift lakes that are permanently
stratified and anoxic below 250 m. The rocky shorelines with isolated bays
and islands provide a complex array of microhabitats. The present Lake
Victoria, on the other hand, is a shallow depression variously dated from
750000 [68] to 12400 years old [69].
The perciform cichlids in Lakes Tanganyika, Malawi and Victoria
(Table 2) are the most speciose and most extensively studied examples of
fish species flocks [14,70,71]. The age of the various lakes is reflected in
the higher level taxonomic complexity of the fauna. For example, the old
flocks of cichlids in Tanganyika show more tribal and generic
diversification than those in the other lakes. Furthermore, Tanganyika is
home to small flocks of several catfish families and mastacembelids, and
to an offshore fauna consisting of endemic clupeids and centropomids.
Divergence times and phylogenetic relationships based on mtDNA
sequences suggest that the cichlid flock in Tanganyika is polyphyletic,
hence there are actually multiple flocks dating from 3.5 to 5 Ma. The