[Taxacom] Origin of a Seychelles lizard

[John Grehan]

Townsend et al "Eastward from Africa: palaeocurrent-mediated chameleon
dispersal to the Seychelles islands" refer to the origin of the Seychelles
endemic Archaius tigris.
In their Abstract it is stated that "Divergence dating and biogeographic
analyses indicate an origin by transoceanic dispersal from Africa to the
Seychelles in the Eocene–Oligocene,

In their introduction they say

"Based on data from partly dated molecular phylogenies (see electronic
supplementary material), several taxa show clear signatures of Gondwanan
vicariance with their sister species occurring in India (caecilians,
sooglossid frogs). Other taxa have originated possibly by vicariance
(aplocheiloid fishes) or probably by dispersal (hyperoliid frogs, day
geckos) from Madagascar." The Supplementary Material refers to several taxa
that are supposed to have "ancient sister-taxon relationships' ' - not
stated, but presumably have divergence ages corresponding to, or earlier,
than the geographic isolation of the Seychelles.

This is followed by the observation that:

"Although their distribution superficially suggests Gondwanan vicariance,
shallow molecular divergences among the major clades are instead most
compatible with multiple overseas dispersals [7–10]."

This is a bit tricky as it presupposes that 'old' taxa must have 'deep'
divergences. I guess one has to look at it this way if one believes in
steady molecular clocks. But is there really such a necessity? Does not
even conventional Darwinian evolutionary theory accept long periods of
'slow' or quiescent periods of evolution with little or no divergence? I'm
not up to date with current Darwinian theory so I would be interested to
know if this view has now been rejected.

The authors note that in a previous  morphological study, the Seychelles
endemic species 'tigris' was placed within the "otherwise
Madagascar-endemic genus Calumma [8]. This placement
dictates a solidly Tertiary divergence from its Malagasy congeners [10] and
thus suggests trans-oceanic dispersal northward to Seychelles" Why this
placement would suggest any such dispersal is not stated.

This is then contrasted with "a dated phylogenetic analysis of chameleons
based on sequence data from multiple mitochondrial and nuclear loci, we
recovered an unexpected sister-taxon relationship of this species to the
African genus Rieppeleon, thus providing evidence for overseas dispersal
from Africa to Seychelles that was probably favored by currents and river
discharges in the Palaeogene."

Obvously the molecular result is seen to be true, and the morphological
result false. Just how this is necessarily the case is not explained.

But in results in discussion they  note that:

"The genetic results are compatible with morphological evidence" But if the
morphological evidence pointed to Calumma and the molecular pointed to
Rieppeleon then I am a bit confused as to how the two lines of evidence are
compatible. Perhaps I missed something obvious here?

They also note that "Although the Seychelles chameleon shares with other
Calumma two long, flexible bifid papillae on the hemipenis, no other
morphological synapomorphies are known [21]." Not sure of the significance
inferred here. There are two apomorphies between Seychelles and other
genera. Is that a problem?

But back to the biogeography. They then say "Evidence from fossils and
molecular-dating studies clearly indicate repeated Caenozoic dispersals
eastward from Africa to Madagascar in invertebrates, amphibians, reptiles
and most famously, mammals [6]." OK, so a citation is given, but no
explanation of how fossil evidence and molecular dating 'clearly' indicate
Caenozoic dispersals.

Then:

"chameleons provide, to our knowledge, the first documented example of such
a colonization event"

But it is not a documented example. It is just an assertion, and so far in
the paper no explicit explanation of the evidence.

They then also assert "However, recent studies show that dispersal,
especially from Africa, has played a major role in assembling the
terrestrial biota of the western Indian Ocean." But not even a citation,
let alone an explicit explanation of the evidence.

I find this paper does not live up to its title. There is no
explicit evidence based justification for casting the Seychelles lizard as
the descendents of a waif and stray that happened to succeed in rafting to
the islands - just once successfully.  The only hint is the vague reference
to shallow divergence, without demonstrating why shallow molecular
divergence necessitates a 'recent' origin. If shallow morphological
divergence (which is also genetic and molecular at a molecular level' can
be 'shallow' (whatever that really means, but one may think of Latimera
among others I suppose), then why not sequence divergence as well. Or are
there two sets of disparate evolutionary rules at play?

This seems to be another one of those papers that is largely made up of
generalized assertions without lack of detailed presentation of the
'evidence' in support. Regardless of where one stands on these
biogeographic issues, I would think that everyone would expect evidence to
be explicitly stated. Without presentation of evidence there is no science,
I would think.

John Grehan