Re: [asa] Platypus- a transitional creature?

> Since the platypus is part of the protherian mammals (higher up the tree
> than the bird/reptile splits from the Sauropsids), why then the
> pervasiveness of reptilian features "that penetrates to the level of the
> genome sequence"?

The platypus-therian split and the bird-reptile split are on different
branches, so it's not too meaningful to call one higher up the tree
than the other (except if you are talking about how recently they
occurred). The platypus still has the generic ancestral amniote
(reptile+bird+proto-mammal) pattern for some genes in which the
therian mammals have distinctive features. It also has some
distinctly mammalian features and some things unique to itself-exactly
what one would expect from evolution.

In other words, the genuinely "reptile-like" features (as opposed to
convergences) are cases in which neither the platypus nor the reptiles
and birds have changed as much as the other mammals.

Another issue regarding the convergence between platypus venom and
lizard/snake venom is that no other reptiles have similar venom (yes,
we can know that Jurassic Park was wrong on this-venom needs a
delivery system, and you can't spit venom without either having hollow
fangs with tiny exit holes or having your mouth mostly closed). If
your fifth cousin twice removed and you are the only people in the
entire family with red hair, convergence may be more likely than
assuming that there's a red hair gene lurking in the family.

A bit more on all these names:

The very earliest reptiles had no extra openings between skull bones
for jaw muscle attachment and are referred to as anapsids. The
earliest major lineage to split off from this was the synapsids, aka
mammal-like reptiles, with a single opening. This split would have
been somewhere around the latest Carboniferous to earliest Permian, as
generic-looking basal reptiles are first known from the upper
Carboniferous (with the caveat that land verterbates are almost
unknown from the lower Carboniferous, so we don't know what was around
then), and definite synapsids are around fairly early in the Permian.
Later, a group of reptiles developed two openings in the skull and are
known as diapsids. One lineage of anapsids survived into the
Triassic, but apart from them and the mammal-like reptiles in the
Triassic and perhaps earliest Jurassic, all Mesozoic and Cenozoic
reptiles are diapsids. (Turtles were widely thought to be anapsids,
but recent data indicates otherwise). Within the diapsids, two main
groups developed. Lepidosauromorphs include the tuatara, lizards,
snakes, and assorted extinct things. Archosaurs include birds,
dinosaurs, crocodiles, turtles, and assorted extinct things. The
split between those was mid to late Permian.

Sauropsida seems to include Diapsida and most Anapsida (those closer
to Diapsida than to Synapsida).

> Now, I'm certainly willing to give the experts due reverence in their fields
> of expertise, but isn't this perhaps a little begging the question? We are
> told that the phylogenic tree is descriptive of common ancestry, but when
> features appear on two different branches that don't follow the path of
> common ancestry, we are told that "convergent evolution" can occur? Before
> I jump to conclusions, isn't this what the article is saying?
> On the other hand, if modern evolutionary biology can tell us with a
> straight face that convergent evolution is a possibility, then isn't it a
> little disingenuous for them to turn around and claim that consistent
> phylogenic lineages is sure evidence that the evolutionary model is
> correct? I'll grant the possibility that a gene might mutate the same way
> twice in two different lineages, or a duplicate feature might emerge
> regardless of the genetic similarity, but what are the odds?

Convergent evolution is not only a possibility but is strongly
expected based on evolution; parallel evolution is also a possibility.
 There are certain types of features, whether molecular, large-scale
physical features, or whatever, that work well for a given function.
Due to common ancestry, two organisms will have certain shared genetic
and physical features that provide similar raw materials for
evolution. The laws of physics, chemistry, etc. are also the same for
all organisms. DNA is made of A, G, T, and C, so even two random
sequences will be about 25% similar. Thus, it should be highly
unsurprising if two different types of organisms independently hit on
similar end results. (There's also the phenomenon of lateral transfer
of genes, rampant in bacteria but generally less common in
eukaryotes). However, if the results were reached independently,
there will be differences of detail. For example, tunas,
ichthyosaurs, penguins, squid, and dolphins are all rather similar in
body shape-they're streamlined for moving rapidly through water.
However, there are key differences that reflect the different
ancestry. The squid uses jet propulsion, the tuna and ichthyosaur use
left-right flexing of the body and tail, the dolphin uses vertical
flexing of the body (mammals evolved a more restricted ability for
lateral motion of the body for more efficient running on land), and
the penguin uses paddling with its flippers (derived from birds that
had strong flapping wings to fly with). The squid, not being a
chordate, has no tail, just fins. The tuna, ichthyosaur, and dolphin
have tails used a lot in swimming, but the tuna has vertical tail fins
with bony rays for support, the ichthyosaur has vertical tail fins
with the vertebrae extending into the lower fin for support, and the
dolphin has horizontal muscular tail fins. The penguin, having a
rather stubby tail with feathers (a weight-saving feature of flying
birds), doesn't use the tail much and relys on flippers instead.

Consistent phylogenetic lineages are a distictive prediction of
evolutionary models. I don't know of any other model that
specifically predicts such a pattern, but it's what we observe. Of
course, "intelligent designer made it that way" is compatible with any
pattern, but it does not uniquely predict phylogenetic lineages
instead of some other pattern, so the evidence strongly favors the
claim that biological evolution is a good physical description of the
method of creation. (There's some Bayesian reasoning in there).

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
To unsubscribe, send a message to majordomo@calvin.edu with
"unsubscribe asa" (no quotes) as the body of the message.