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book reviews, parsimony and some loose ends



Recently T. A. Curtis asked about Fastovsky and Weishampel's _The
Evolution and Extinction of Dinosaurs_.  I thought I should point out
that Dale Russell wrote a review of it for _Science_ (Sept. 27th
issue, I think, but it may have been Sept. 20th).  The review is quite
positive, though he does suggest improvements, mainly because he
expects new editions to come out.  There are a couple of other book
reviews of interest in that issue, so you might want to look for it.

In response to my query about the consistency of George's arguments
vis a vis parsimony, George claims:

> If points A and B are particularly distant in time or morphospace,
> then a straight-line interpolation or a cladistic analysis is not
> necessarily the tool to elucidate what happened in between A and B.
>
> Once you have some of the details _in between_ A and B, however,
> such as A --> A1 --> A2 --> ... --> Ax --> B, the finer resolution
> increases the likelihood that a straight-line interpolation between
> A(i) --> A(i+1) approximates reality between those two detail
> stages. The more details we have, the more evolutionary stages we
> can envision, and the closer the straight-line path through
> morphospace approximates reality.

How can you not see the problem here, George?  By your own admission,
the fossils necessary to fill in the details of your "BCF" idea have
not (and probably never will be) found.  It's thus disingenuous for
you to claim that your view is better supported than any alternatives.
How many millions of years separate "the common ancestor of all the
archosaurs" and "any particular modern bird--say, the robin"?  How
many of those A(i)'s can you fill in for us in order to convince us
that parsimony is aptly applied here and not in the places where you
complain about its application?

And in an argument with LN Jeff, George writes:

> I don't see how quadrupeds could use their forelimbs as grasping
> organs in prey capture, supposedly leading eventually to bipedality;

While I wouldn't argue that they're on their way to bipedality, the
above quote makes me wonder if George has ever seen a big cat hunt.

On the other side of the fence, LN Jeff makes an analogy:

} To give an analogy, one small branch of mammals, the primates,
} re-evolved color vision.

As Tom has repeatedly tried to tell us, mammals didn't lose color
vision.  We lost some facets of it, but the basal condition among
mammals appears to be dichromatic color vision (the most relevant
article on the topic is a review written by Jerry Jacobs about three
years ago; I'll dig it up if you're interested).  

} Note that I am not saying that assuming color vision in all
} DINOSAURS is illogical.  Both living reptiles and birds have color
} vision, so a phylogenetic bracket can be made.

As I've argued here before, inferences about color vision in dinosaurs
can be made much more strongly than Jeff is implying.  Analyses of the
sequences of the genes which code for visual pigments can show fairly
clearly which genes amongst various groups are homologues of each
other.  It's a fast-moving field which I don't have time to keep up
with, but from what I know of it it seems to me that the only way that
dinosaurs could have *not* had good color vision would have been if
they'd lost visual pigments as mammals apparently did.  Jeff's analogy
was ok, but unlike feathers (made up of proteins which are used for
purposes other than the construction of feathers) the low-level
aspects of color vision are mediated by proteins that are not used for
anything else (so far as is known).  We can thus make pretty strong
inferences about color vision in dinosaurs based on the molecular
biology of living organisms.

Which reminds me that before I went away, Bonnie Blackwell asked:

] 1.  If mammals lost the two colour receptors that both reptiles and
] birds had because they did not need them being nocturnal animals, do
] owls and other night birds show reduced numbers of colour receptors
] or reduced efficiency in the ones they have?

I don't know as that owl photoreceptors have yet been tested with this
in mind.  For some reason bird retinas have not received as much
attention as fish and mammal retinas.  So far as I know, chicken and
pigeon retinas are the only ones that have been studied extensively,
though a reasonable amount of work has gone into studying the high
acuity areas of some hawks.  Like I said above I don't keep up here,
but I suspect fairly strongly that the number and type of cones found
in owls hasn't yet been characterized.

As for the more general question about animals in environments that
are deprived of light, it's pretty much universally true that cones
(the photoreceptors which mediate color vision) become fewer in number
and less varied in type as you move to niches with less and less
light.  It's pretty easy to talk about this sort of thing with respect
to fish because the deeper they live the less light is available to
them.  Fish that live near the surface generally have three or four
types of cone.  Fish living deeper underwater (or in water that's
murkier) may have only one or two types of cone.  Functional retinas
that have no cones are rare, though apparently some bottom-dwelling
rays fit that description.  Some deep sea fish have cheated by
harvesting bacteria that produce light in organs that the fish can
turn on and off like flashlights.  Those fish tend to have pigments
"designed" to be sensitive to their own emissions (and hence the
emissions of their conspecifics), and thus they may have two types of
cone where you'd expect them to have one or none.  [Gross
generalization alert!] But other than that, you can tell a lot about
where a fish lives by looking at its cones (and vice versa).

] 2.  In humans, many males are colour blind, but is that due to
] impaired colour receptors genes (or the lack of one or two due them
] being on the X chromosome) or is it due to another biochemical
] pathway that screws up development or use of those colour receptors.

I probably shouldn't be answering this here since I'm supposedly mad
at Bonnie for asking a non-dinosaur question, but... :-)

The genetics of human color vision is another fast moving topic I'm
only partially following.  However, I think it's pretty safe to say
that the reason males are more likely to be color blind than females
is merely because two of our visual pigment genes are located on the X
chromosome.  Women therefore have two chances to get each of those
pigments expressed "correctly", whereas men have only one.  That's a
bit of an oversimplification because many of us actually have many
copies of each of those genes on our X chromosomes.  In those cases,
each X chromosome has more than one sequence which might express the
correct pigment.  However, that complication notwithstanding I don't
think anything other than the X-linkage is necessary to explain what
you're asking about.

And finally, Rob Meyerson wrote:

| Nick Longrich has demonstrated quite effectively that diplodicids
| and stegosaurs carry most of their weight on their hind limbs (and
| kind of threw me for a loop ... for a while).  This demonstration is
| then used to suggest the idea that these animals reared while
| feeding, either habitually or consistantly.  I suggest this is not
| necessarily the case.

Might I submit, Rob, that if you really want to impress us then
instead of digging in your heels when met with unexpected data you
would just admit that your previously expressed certainty was
unwarranted?  Are you trying to understand how sauropods lived, or are
you merely trying to demonstrate that you were right when you made
your first utterance on the subject?  I fear that's the sort of
behavior we're all prone to express.  But we should try to avoid it,
IMHO. 

--
Mickey Rowe     (mrowe@indiana.edu)

"I will not fear.  Fear is the mind-killer.  Fear is the little death
 that brings total obliteration.  I will allow my fear to pass over
 me and through me.  When it has gone I will turn the inward eye to
 follow fear's path.  Then only *I* will remain."

[That's just to see if Darren reads books too!]