Platypus Genes.

[GoodKat]

So I've been debating a creationist for a while now, and he pointed out the fact that the wikipedia article on the platypus says it has two genes that are only found in fish and birds.
http://en.wikipedia.org/wiki/Platypus#Evolution
I checked the referenced Nature article.
http://www.nature.com/nature/journal/v453/n7192/full/nature06936.html
And even found the two specific genes.
http://www.ensembl.org/Ornithorhynchus_anatinus/Gene/Summary?g=ENSOANG00000007840
http://www.ensembl.org/Ornithorhynchus_anatinus/Gene/Summary?g=ENSOANG00000002187

But I have no idea how to interpret the data on the pages.
Basically I need to know how those genes could have gotten there. Are they simple enough that mutations could have formed them, or are they present in other mammals or reptiles but "switched off"?

 

[Aught3]

Well I didn't read the whole paper but it doesn't seem like that much of a problem. Take a look at one of the figures:

Ok, so the issue that has been raised is that a platypus has some genes that it shares with reptiles and birds but not mammals. Firstly, the paper actually says the platypus shares 80% of its genes with mammals, which is expected given their close evolutionary relationship. Only a very small number of genes will fit the criteria of not being present in mammals. What is most likely to have happened is that (if we take the cyan gene trail in the picture) is that the gene was present in the common ancestor (at the base of the phylogeny diagram) and as species branched off they each took a copy of this gene. So, if it's an ancestral gene then we expect it to be present in all future species. That makes the real question: why is it missing from mammals? It is not clearly shown on the diagram but if you look at the reptile branch the cyan line turns dashed. We can interpret this as a gene deletion for our purposes.

Now we have a likely answer - all descendant species will have a copy of this gene, except for those which no longer required its function allowing the gene to be lost through deletion. Actually, it looks like the loss of a couple of splice sites - but that's my very amateurish opinion.

 

[Pulsar]

So I've been debating a creationist for a while now, and he pointed out the fact that the wikipedia article on the platypus says it has two genes that are only found in fish and birds.

And amphibians. Those genes seem to regulate the production of egg envelope proteins. Looking at the evolution diagram http://www.nature.com/nature/journal/v453/n7192/fig_tab/nature06936_F1.html, I imagine something like this:

The amniotes, which had previously branched off from amphibians, split into the synapsids (mammal-like reptiles) and sauropsids (leading to birds and reptiles), 315 Myr ago. At that point, both groups still had the genes. 166 Myr ago, the primitive mammals split into the prototherians (leading to the platypus) and the therians (all other mammals). The platypus group kept the genes, the therian group has lost them. On the sauropsid side, the bird branch also kept the genes, while the other reptiles lost them. That's at least my interpretation.

 

[scikidus]

Very related:
http://www.wimp.com/platypusevolution/

 

[Squawk]

TBH I'd go about this in a different route. If he is bringing up the platypus it is due to the fact that there aren't any extant species that are even remotely like it and he feels that this one exception (if indeed it was an exception) to taxonomy is a problem for evolutionary theory. It's not all that long ago that it was classified as a mammal in the first place, as has already been pointed out the last common ancestor between all other extant mammal species and the platypus was a long time ago.

In the case of species sharing a common ancestor seemingly not sharing genes that we would expect we should first of all figure out what is more likely. That new genes were acquired after a speciation event in one branch, that those genes were lost in the other branch, or that this particular divergence has been misplaced and we don't actually understand platypus evolution sufficiently well.


Ignoring for a moment the particular example of the platypus you can start to look at the interesting example of new gene acquisition. You can debate horizontal gene transfer for a start, in particular the ability of virii to "cut and paste" dna accross large taxonomic divides. You can look at the structure of various genes and determine the probability of its forming twice in separate organisms.

Looking at the other side of things, the idea that genes were lost is probably much simpler. Map out the genome and see if the genes are still there but switched off, a position which would end the argument instantly. Then you can have fun with vestigiality.

Finally you can discuss the placement of the platypus in basic taxonomy and discuss a couple of other things. If, for example, the platypus is shown to have genes that indicate it does not share a more recent common ancestor with the other mammals than it does with, say, certain fish, would that tell us that it was "designed", or that we fubared its placement in taxonomy?

Personally I would point out the basic flaw in his argument, the fact that he is looking for exceptions, bits that science has got wrong. Point out that by doing so he is actually doing science. If he genuinely found something wrong in taxonomy then science, by its very nature as a self correcting process, would address it. It might take days, weeks, months or years.

In the case of the platypus he is barking up the wrong tree, though it is an interesting example given that it is so dissimilar to any other extant species.