Wombat Pouch

Backwards-facing pouches are not unique to wombats, they are also found in such species as quolls, Tasmanian devils, and koalas. So it's not particularly an adaptation to burrowing, though it might have come in handy.Nor is it clear which form is basal. Why shouldn't the kangaroo/wallaby type pouch be the adaptation? That vertical posture and all that bouncing up and down might have made it useful.Obviously the pouch didn't just turn around. A plausible intermediate form would involve both an anterior and a posterior flap. Then whichever one of them was basal got smaller, eventually disappearing altogether, while the other one got bigger and took over its function. (This supposes that the difference between the pouches has been explained right and that I'm visualizing it right.)

Does this actually happen?

Or for the modern marsupial ...

You're not taking into account developmental constraints. Genes affecting the exterior of the body are going to be symmetrical in their effect, because it would actually be quite hard to set up a new system distinguishing between the left and right side of the body.We can see this in mutations such as antennapedia, which gives fruit-flies legs where their antenna should be. Always legs plural --- there is no mutation that makes them lopsided, because you'd need a whole new biochemical system to tell the developing fly's body how to break the symmetry. It doesn't take a lot of care and fine-tuning to make the mutation come out symmetrical in its effect --- it would, on the contrary, take a marvel of bioengineering to make it asymmetric. The same could be said of other well-known mutations, such as those giving fruit-flies white eyes or an extra pair of wings.In short, being asymmetric is difficult. So "given that evolution works on the principle of "just goodnuff"", asymmetry won't be produced without a very good reason, whereas symmetry is the lazy way of doing things and occurs by default.So you wouldn't even need natural selection to explain the laterally symmetric placement of the pouches --- it's in the nature of mutations to produce lateral symmetry in animals which are already laterally symmetric. Whereas your argument seems to depend implicitly on the idea that a feature produced by mutation might as well have a 30 or 45 degree skew as be symmetric.Edited by Dr Adequate, : No reason given.

Credit me with a little common knowledge. I know that, which is why I explicitly said "Genes affecting the exterior of the body are going to be symmetrical in their effect". The same does not apply to the internal organs, and so if a mammal underwent a mutation giving it (for example) an extra liver, one might well expect that to be asymmetric, since livers are as a rule produced asymmetrically. But the pouch is not an internal organ.Edited by Dr Adequate, : No reason given.

Like I said, asymmetric on the inside, symmetric on the outside. Which is why, as we observe, random mutations which affect the outside of a bilaterian produce symmetry without even natural selection needing to get involved, let alone "design". I have written computer code, and have a degree in Math & Computer Science. This is nothing like that. Mammals are not externally symmetrical in order to be user-friendly. Er ... not only have I not presented such an argument, but nor, so far as I can see, have you. Where do you think you did so?Edited by Dr Adequate, : No reason given.

Perhaps your point is, stop me if I'm wrong, that the external symmetry of bilaterians reminds you vaguely of the interfaces of software, and the latter are designed.This is a very vague resemblance, since interfaces are not symmetrical and bilaterians are not user-friendly (with the possible exception of dogs).But what it resembles far more is what random mutations observably do to the external form of bilaterians. The mutation that gives fruit flies an extra pair of wings gives them an extra pair of wings. The mutation that gives an American Curl cat curly ears makes both ears curly. Here we have a mechanism that produces bilaterally symmetrical features in bilaterians without design, without even the operation of natural or artificial selection. So if the wombat pouch was produced by evolution we would expect the wombat pouch to be symmetrical even if there were no selective pressures (i.e. no good functional reason) for it to be so.What would we expect if it was produced by design? Well if, as you claim, there is no functional reason for it to be symmetrical, then we might expect anything. The designer could have given it a stylish rakish tilt for aesthetic reasons. Or not. Who knows what a creator of wombats would find aesthetically pleasing? But we would expect evolution to produce symmetry in such a case, because this is what random mutations do.Edited by Dr Adequate, : No reason given.Edited by Dr Adequate, : No reason given.

Look, I myself can't figure out exactly what point you're trying to make with your quotation from WP about symmetry. And I am, as you know, quite smart. If even I can't figure out what your point is, then you haven't made your point very well, and it may not be Big Al's problem if he doesn't get it.

Yeah, but they're still bilaterally symmetric. Since they fall on the axis of symmetry, there's only one of each.My point is that when a mutation happens that affects the exterior of a bilaterally symmetric animal, it almost always does so in a bilaterally symmetrical way. We expect evolution affecting the outsides of bilaterians (including wombats) to be bilaterally symmetric, because the mutations that form the raw material of evolution have bilaterally symmetric results when they affect the outsides of bilaterians. There's no mystery here --- that's a complete explanation. It's when we see a bilaterian with external asymmetry that we have to wonder how it came about.Fiddler crabs, for example, might puzzle us: What is the biochemical mechanism that breaks the symmetry, and how did that evolve? I have no idea. (Possibly someone does, I've not researched it.) But I absolutely know why marsupial pouches are symmetric --- that happens by default. Wondering how that evolved is kind of like wondering how so many species have evolved to not be able to fly. That one's easy.Edited by Dr Adequate, : No reason given.

Well, he actually said that himself (post #57). Also, when he thought that I didn't know that (same post), he was much more gracious in the way he said so.I myself am sometimes spiteful and rebarbative, but I like to think that I only do that when it's called for. Big Al, to his credit, is a perfectly decent guy, no matter how wrong he is about biology. I think we should treat him well until and unless he himself behaves badly.

This is true, though I don't know what proportion of these defects are genetic, rather than developmental, or what proportion of those that are genetic are symmetric. Nor do you.However, it is still true that most mutations that affect the exterior form of bilaterians are bilaterally symmetric. We know this just by cataloging actual mutations. The fruit fly has been used as a model for genetics for the last 100 years or so. How many mutations affecting the external form of the fruit fly can you come up with which are asymmetric?If the answer is "not very many", then I stand pat. I did say, after all "almost all", as you can see by reading my post. It is not necessary to my argument that it is an unbreakable rule, merely that it should be a very general one. Well, you're welcome. So long as you're discussing biology then we're doing the thing that these forums are here for. Unfortunately, these discussions often degenerate into slanging matches about which of us is more like Hitler. (Hint: neither of us is particularly like Hitler, 'cos of neither of us perpetrating genocide.) So long as we're exchanging views about biology, then even if you're completely wrong (which you are) then talking to people like you is exactly why I joined these forums in the first place.Edited by Dr Adequate, : No reason given.Edited by Dr Adequate, : No reason given.

d.p.Edited by Dr Adequate, : No reason given.

It would be rather hard to have a site dedicated to the evolution versus creation debate without people occasionally pointing out that creationists are wrong. Yeah, that's an average of over 2 posts a day. 2.1, to be more precise. Many of which run to more than one paragraph. Who has the time to do that? Yeah, you got me. I actually work for Paleocorp Inc, the multinational company that manufactures all the fossils. They pay me to maintain the illusion that fossils are dug out of the ground rather than made in Chinese sweatshops, 'cos obviously if I were to stop posting on an obscure niche website in one corner of the internet to an audience of literally dozens of people, then the whole ingenious fraud would collapse overnight.In the interests of full disclosure, I should point out that Paleocorp is only one of my employers. Obviously there's other people who pay me for my other still more time-consuming leisure activities such as reading and doing the crossword.Edited by Dr Adequate, : No reason given.

It appears that my statement that Big Al is "a perfectly decent guy" has awakened in him a sudden desire to be snide, and that when I said he was doing the right thing by basing his arguments on the facts of biology, this made him think he should leave off doing that and concentrate more on personal attacks.

If you are really too socially inept to realize the problem with your recent posts, then by all means let us omit that discussion and revert to the question of wombat symmetry.If, on the other hand, you're just pretending that you don't know why your posts came off as stupid and malevolent, then you are a dishonest lump of shit. But I like to think the best of everyone.Edited by Dr Adequate, : No reason given.Edited by Dr Adequate, : No reason given.Edited by Dr Adequate, : No reason given.