Genetics of a small, isolated population of Thylacoleonids
Reports of both jet-black and dark
brown through to tan coloured animals are, possibly, the most intriguing
part of this saga. I have decided to deal with this aspect of the phenomenon
separately because, in the absence of actual physical evidence, I believe
that such reports lend the story a measure of credibility which it would
not otherwise enjoy.
In order to account for the presence of both black and tan coloured animals living in the same region, I applied an iterative mathematical model of the well known Hardy-Weinberg Law. It is a primitive model since it assumes that the segregation of melanic\albino alleles is entirely Mendelian. However, I believe it is good for a first approximation of the overall trends in an inbred population.
Briefly, the model indicates that an isolated population of T. carnifex, or, indeed, of any population whose natural coat-colour is agouti, would tend towards melanism ifinbreeding takes place and there is some selection against the reproductive success of the inevitable albino animals.
The drift towards melanism holds whether or not selection against black individuals is less than, equal to or greater than the selective pressure acting on tan coloured (agouti) animals. So long as selection against albinos is greater than it is against the other two colours so that the albino allele is progressively removed from the gene pool, melanism must eventually predominate. Ultimately, the process must lead to a true-breeding melanic race. The predicted outcome is perfectly in accord with evolutionary theory and, I believe, can elegantly account for the reported colouration of these animals.
By fiddling with the selection coefficients and breeding frequency, one can get a feel for how long inbreeding might have been going in order to produce the observed black & tan proportions. Most runs come out at 100-150 years, assuming the critters breed every 2-4 years and reach sexual maturity in about 2-3 years.
One is prompted to wonder if the unsophisticated types who are the main informants of this phenomenon would have, or could have, thought to fabricate their stories in order so neatly to concur with the requirements of genetic theory? It was this thought which first opened the door to doubt.
Assuming that there is a grain of truth in the reports and the mystery animal really is T. carnifex, one is tempted to speculate as to how it came about that its numbers were reduced so drastically that now only a small and severely inbred population (possibly) remains.
Could it have been because of changes in populations of native prey species caused by European settlers? And did the T. carnifex population recover somewhat because alternative, introduced species became available?
If so, one imagines that the population crash must have occurred early in the period of European settlement of Victoria, say in the second half of the nineteenth century and that numbers recovered slowly as the population of foxes and rabbits increased later on. Could that be the reason nineteenth century naturalists missed them? All that is, of course, idle speculation, of no scientific worth whatever.
Objections I have heard ...
Q:Why speculate that melanism is an acquired trait through inbreeding when you don't know the original colour of the animals, the size of the population, the length of generation time on any one of a dozen other factors?
A: It's not an acquired trait - it's segregated. It's in the genotype of most mammals anyway and inbreeding merely serves to bring it out. It seems reasonable to extrapolate breeding frequency from similar sized animals. As for the host of other factors, well, in a case like this, one can but take a reasonable guess!
It's all very subjective I'm the first to admit, but ... how come the witnesses have to report black ones? It is exactly what would happen to a real population of fewer than about 100 individuals. To reiterate, the drift to melanism only requires that selective action against the inevitable albinos is greater than against either the pure melanics or the agoutis.
Q:You're making the mathematics create this conclusion. Why couldn't they all be black to start with? Or why not have a species with a variety of coat colours occurring naturally? Getting a computer program to agree to an hypothesis usually only panders to the weaknesses of the assumptions in the hypothesis.
A: It is suggested by palaeontologists that the structure of T. carnifex's hind limbs indicates that it was a poor cursorial hunter with limited pursuit capability. This makes it seem likely that it was an ambush type of hunter. It therefore seems reasonable to suppose that camouflage would have been central to itspredatory technique. Possums have the classic agouti coat and one branch of that tribe is held to be Thylacoleo's ancestors. I can't think of too many predators whose coat pattern is lurid and easy-to-spot in their usual environment.
Anyway, do I detect a certain animus towards the mathematically literate? It's undeniable that mathematical modeling, cautiously utilised, has great potential to illuminate the pathways of biological phenomena. Particularly in evolutionary and population studies.
It's said, or so I've heard, that descriptive scientists suffer some feelings of inferiority in the presence of practitioners of the "hard" physical and mathematical sciences. Didn't the late Luis Alvarez, famous for his discovery of the iridium layer on the KT boundary, once declare that, in science, you have PHYSICS ... and the rest is STAMP COLLECTING? But I, who have no wish to lose friends and infuriate people, would never give voice to such an unkind sentiment ....
Q: Jaguars and panthers can be black as a natural colour, as are tasmanian devils. The closest relative to a tassie devil is a tassie wolf and they have completely different types of coat, so close relationship is not a good guide to possibly coat colour.
A: Sure, they could be black - just like Tasmanian Devils. It's just that inbreeding is one allowable interpretation and you don't have to plead a special case for coat colour (Ockam's rule). While close relation is no guide to coat colour or morphology, similar lifestyles often are. Why else do thylacines superficially resemble wolves and, apart from the stripes, have a similar colouration? Or, possibly a more illuminating case, why did South American thylacosmilids bear such an astonishing resemblance to sabre-tooth cats? A most compelling example of convergent evolution towards the feline form, I should say.
In any case, the Tasmanian Devil is not a predator but a scavenger. That's a totally different lifestyle.
As for the cats, I understand that you get panthers by inbreeding spotted cats like jaguars or leopards. I'm told that "panther" is simply a generic name for an all-black big-cat, of whatever "stripe".
They wouldn't, of course, breed true unless you carried on artificial selection for many generations. In cases where you have true-breeding races of black panthers, as in parts of SE Asia, I'd hazard a guess that these are descendants of a small founder colony of patterned cats. Allelic segregation and selection against albinos may have led to a melanic race in that instance.
With melanic races of, say, leopards or jaguars, if the gene cluster that determines coat colour and the cluster which determines size and shape of the spots lie on different chromosomes, the two groups of clusters will segregate independently in classic Mendelian fashion. This means that, in a melanic leopard, you would likely retain the spots as discernible circular patterns of black fur, the spots revealing themselves by a slightly different orientation of the fibres to those in the rest of the (black) coat.
NOTE: While we can easily envisage active selection against a white leopard, it's less easy to imagine how selection would act to remove black spots from a black background. Such a configuration would, it seems likely, be selectively neutral and hence likely to linger in the population for a very long time.
If such is the case with true-breeding melanic leopards, it would tend strongly to implicate inbreeding as the mechanism responsible. Must look the matter up, sometime ...
Q: I'm not sure you can invoke Ockham's Razor in this case. The lines of argument are too tenuous and extended to determine which is the most parsimonious.
A: OK, here's some more tenuous speculation : what about the two size classes the critters reportedly come in? Might be male\female dimorphism, which would fit nicely with the fossil record. Or the big ones could be heterotic individuals resulting from occasional encounters between boys'n'girls from far separated colonies. Another effect of inbreeding? Could we be dealing with a number of isolated family groups scattered through the mountains? If so, what keeps'em isolated?
Q: Strict Mendelian segregation involving only two alleles is, in natural populations, exceedingly rare. Even Mendel, it's now asserted, may have fudged his results in a way that would, today, have led to his papers being rejected. You can't expect such a simple model to give meaningful predictions, can you?
Yes, I've also heard that about Mendel. In the case of rodents' coat colouration, a multi-factorial model would be more appropriate, I agree. (If you're offering me a good software package suitable to the task, by all means speak!!) A lot of the work of pre-1940's classical genetics focussed on such models: the agouti coat of rodents was a particular favourite.
However, the assertion that bi-allelic inheritance of colouration is an extreme rarity may not be totally accurate. Mendel's peas are an example, even if he did make his results look a little too good to be true. A cursory examination of the literature turned up REF #1: it seems that plumage colour of the Southern Giant Petrel is determined, pretty closely, by the action of just two autosomal alleles. It may be that colour bi-allelism in wild populations is more common than supposed. So there's another example, at any rate.
In any case, I think it's true that multi-factorial inheritance would give similar results to a Mendelian model, albeit with more intermediate gradations. So the simple model of inbreeding should give a general picture of overall trends.
By the way, before we get too sanctimonious
about founders and heroes fiddling their lab books, we should recall how
it's seriously held that Charles Darwin would have been flunked
from a modern-day university. Now, I agree we need our time serving hacks
to do the grunt work at rock-face or lab-bench but it's often been the
creative dilettantes who have made the real breakthroughs. In the context
of today's strictly regulated and controlled science, that's something
surely worth thinking about?
Blue Tigers: melanism in felids
My thanks to Prof. Loren Coleman for passing this one on to me.
The Times of India (http://www.timesofindia.com/today/06indi29.htm)
Wednesday 6 October 1999.
The Times of India News Service
The odd man out in the tiger family
NEW DELHI: Ever heard of black tigers? Well, apparently they exist.
Not just black but brown ones and blue ones, too. L A K Singh,
research officer at Orissa's Simlipal Tiger
Reserve, has just finished
a field study, Born Black, on the melanistic
(dark) tiger as an
evolutionary aberrant.
In any population, he says, the chances of
occurrence of two extremes,
black and white, are equal. So why not among
tigers? It's just that
spotting even the usual yellow-and-black-striped
tiger is rare. Seeing
the extreme forms is even rarer, particularly
because nature doesn't
usually allow them to survive.
But black does exist, even it has been overshadowed
by the white
exception. There are, in fact, 14 colour
possibilities. According to
Singh, the white tiger is larger in size
than the usual but the black
one is ``diminutive'' and more agile.
A brown tiger with black stripes was reported
shot in India's Central
Provinces decades ago. There are records
of a brown tiger without
stripes. Old records speak of blue tigers
in China, locally referred
to as blue beard or black devil.
But there was no real proof of the black
tiger till 1992; they were so
rare their existence was generally denied
though sightings had been
reported from Myanmar, Bangladesh, northeast
and southwest India.
In 1992, the skin of one such animal was
seized from smugglers in
south Delhi. Eight-and-a-half-feet in size,
it was displayed at the
National Museum of Natural History here.
The second documented case is from Simlipal. In July 1993, a young melanistic tigress was killed there by a boy in self-defence.
This wasn't the last conservationists heard
of the black tiger. In
1996, a forester saw a full-grown black
tiger in Simlipal. There have
been more sightings since.
The study Singh has readied, with WWF-India's
support, makes a strong case for further research on colour aberrations
in tigers and their implications for tiger conservation efforts.
``It's not a good sign to
have aberrations in any population; it indicates
the beginning of some
kind of end.''
Could it be that with increased hunting,
wild animals have become shy
of humans and nocturnal - and melanism may,
therefore, be more
advantageous? Is it possible nature has
already given way to
favourable selection of black panthers and
is experimenting with the
aberrant extremes of the tiger? Just a hypothesis
at the moment.
One problem, he says, may be inbreeding, and this could endanger the future survival of the Royal Bengal Tiger in its original colour, with its tawny and black stripes.
The solution
isn't easy. ``Conservation,'' he says, ``needs to reduce
the possibility of
genetic erosion of the normal colouration of the
tiger in the wild
through habitat improvement.'' Mating within a small
tiger population is
a problem; so try to have large populations, he
suggests. In other
words, a ``site-specific enlargement of population
gene pool''. But he admits this may be difficult.
REFERENCES:
1/ The Genetics of Plumage Phase
Dimorphism of the Southern Giant Petrel Macronectes giganteus.P.D.
Shaughness, Heredity, Vol 25 Part 4, pp 501-506, November 1970.
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