Continuing from Bell’s form observations in Victoria (previous post) …
New science
After some intensive but unsystematic searching I found two relatively recent scientific publications which answered my main questions about how Bell’s form relates to the ‘normal’ goanna. It’s easier to discuss the more recent one first, so that’s what I will do.
(1) Photographic evidence
Farquhar et al (2022) had a double aim, firstly to show that iNaturalist records could assist research and secondly to use them to define (and perhaps explain) the range of Bell’s form. His results on the second point confirm common knowledge, i.e., Bell’s form is more common in drier country. He goes on to analyse, in some detail, likely environmental causes for that pattern of distribution.
It is clear that Bell’s form is rare but not unknown in both Lindsay’s territory in north-central Victoria (previous post) and mine in the Townsville region (2016 post).
(2) Genetic variation across the range
Smissen et al (2013) analysed DNA from 90 specimens across the whole range of the species and found there were three clades or genetic groups: one along the coast from about Cooktown down to the Burdekin; one from the Burdekin down to southern Queensland and then on the inland side of the Great Dividing Range (GDR) into northern Victoria; and the third on the coastal side of the GDR from the McPherson Range (Qld/NSW border) to southern Victoria.
Both the ALA page on Varanus varius and Wikipedia incorporate these findings.
(3) Subspecies? No
If separate lineages keep drifting apart for long enough, they may become subspecies (noticeably distinct but still able to interbreed) or full species (unable to interbreed). The Lace Monitor clades have not diverged to this point although one (notoriously unreliable) writer has claimed to identify three subspecies by their markings.
Some northern specimens are more spotted than banded, and the difference is great enough that when I saw the one below I wasn’t really sure that it was a Lace Monitor. That doesn’t mean they should be defined as subspecies, however, even after drifting apart for nearly a million years.
Conclusions
If Smissen is correct, Farquhar’s results make sense on the basis of genetics as well as environmental factors. Equally, if Farquhar is correct, we should expect the genetic split identified by Smissen to be advantageous and to persist. (Overlaying the ranges of Smissen’s clades on Farquhar’s map makes the agreement more obvious, but I will leave that to my readers.)
It looks as though ‘Bell’s form’ arises from a mutation existing in only one of the three clades, and the mutation occurs somewhat randomly – much like eye colour inheritance in Homo sapiens. That is, individuals may be ‘Bell’s form’ or ‘Lace form’ and matings of any combination of forms can produce offspring in either form.
There are still some questions to be resolved, of course, starting with the Bell’s form individuals on the NSW mid-North coast and around Townsville. Overlapping ranges? Interbreeding between the clades? Or perhaps every branch of the species carries the genes for Bell’s coloration but the paler form is far less common in wetter areas because it’s disadvantageous there?
But we have to stop somewhere. If Smissen and Farquhar are both broadly correct, as I believe they are, Bell’s form of the Lace Monitor, Varanus varius, is just that: a colour variant. All that we have added is that the mutation seems to occur in only one of three populations of the species.
References
Smissen, P. J., Melville, J., Sumner, J., & Jessop, T. S. (2013). Mountain barriers and river conduits: Phylogeographical structure in a large, mobile lizard (Varanidae: Varanus varius) from eastern Australia. Journal of Biogeography, 40(9), 1729-1740. https://doi.org/10.1111/jbi.12128
Farquhar, J. E., Pili, A., & Russell, W. (2023). Using crowdsourced photographic records to explore geographical variation in colour polymorphism of an Australian varanid. Journal of Biogeography, 50, 1409–1421.