Monday, 7 December 2020

More gliders heard from

Extinction is forever!

Last week I was ruminating on the finding that a single species of primate in SE Asia was more accurately described as three separate species based on their genetic differences. In my post-graduate days I went to college across the river from Ernst Mayr who in 1942 had written The Book Systematics and the Origin of Species. That was one of the bibles of the Modern Synthesis: the reconciliation of genetics with evolution into a mutual admiration society rather than two separate rather fighty camps wedded to Mendel or Biometrics. Mayr's long-lasting contribution was defining the BSC - the biological species concept: "a species is a group of organisms fertile amongst themselves but reproductively isolated from other such groups".  

Wait up there; how do you determine if the members of two populations are inter-fertile? You can't really hang around waiting to catch sight of a shag, follow the female for weeks or months: to preclude any other congress and to be sure to be sure that it's the same female who gives birth. Then wait weeks or months to ensure that the offspring themselves are fertile. Horses and donkeys jump around jump around but mules are an evolutionary dead-end. And it really doesn't answer to lock the prospective parents up in a cage because in captivity some animals will shag anything out of boredom or delinquency. One way to achieve reproductive isolation between "incipient species" is behavioural: if you prospective mate is making the wrong moves, that can be a massive turn-off. Generally, if the chromosome count/arrangement is a mismatch then that's also a probable no-no.

But we know of many many cases of well-studied species which look similar and are plainly not interested in each other. And other cases where there's an obvious difference but the two morphs can get it on just fine: what's a white wing between pals? With enough of these cases, we can compare the % divergence of their DNA and get a rough fix. In this group of animals, > 15% difference in DNA indicates a pair of "good" species while less than 10% is allowable variation within a single species.

Which brings me to marsupial gliders in the Eastern part of Australia. A neat morphology and sequence analysis study (Genetic evidence supports three previously described species of greater glider, Petauroides volans, P. minor, and P. armillatus). out of Australian National University, led by Kara Youngentob [L with attentive student] has sided with the splitters rather than lumpers w.r.t. Petauroides volans the greater glider, which, when all fluffed up, is about as big as a breadbox and weighs about the same as a human brain [1000-1700 g]. These gliders, like koalas Phascolarctos cinereus, feed exclusively on eucalyptus leaves; and they nest in hollows on eucalyptus trees. Unlike koalas and their closer relatives ring-tailed possums, [Pseudocheiridae] gliders glide . . . for as much as 100m! They have an extensible fold of skin between elbow and ankle and a long tail for a rudder: like a base-jumping wing-suit with the added ability to swing right-angled turns like Grand Theft Auto neeeeeeowwww! They are a bit ungainly, and prone to predation, on solid ground, so being able to leap from tree to tree without setting foot on the earth is adaptive.

What the biologists from ANU have done is sample a few individuals from within the extensive range of Petauroides spp. get out the calipers and do the standard mammalogy length x width measurements of the sticky out bits: head, tail, body, ears, legs and lash that matrix of lengths into a multivariate statistical analysis:
As you see a) almost all the variability in the dataset is captured by two axes of discrimination; b) the morphological variants match rather well with the location from which each specimen was sampled. I've <spoiler alert> included the conclusions of the paper by editing in [IrfanView!] the proposed specific names suggested in the title of their paper. Next step is to treat the genetic variability with another multivariate statistical tool to reveal similar clusters:
These data are bit more noisy than the limb n ear measurements: with 20% [1 - [59.5 + 20.8)] of the data variability unaccounted for by the first two discriminatory axes. The two lads edging right from the Northern cluster are proposed to be hybrids between P. minor and P. armillatus. If so, these are not species in the Mayr (1942) BSC sense. But taxonomy has moved on in the last 80 years and there are less rigid definitions of what it means to be a species.

The conclusion is that after 200 years of study and classification by a succession of worthy experts into a single or multiple species, the splitters have it . . . for now. That will hopefully inform CSIRO and the Australian government about where to direct their efforts in restoring habitat after the disastrous bush-fires of Summer 2019-2020. A nice side-note is this study is that is was largely accidental. The 1st author, PhD student Denise McGregor, thought she was going to investigate the relationships among physiology, climate and latitude and this morphological data got in the way; and had to be dealt with. Having knocked off this pub she's presumably back on track measuring heart-rate, acidosis and VO2max; hard enough as it might be to get a glider on a treadmill. Cool beans for a post-grad.

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