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Re: tiny-armed theropods



On Sat, Oct 22, 2011 at 11:27 AM, Robert Schenck <schenck.rob@gmail.com> wrote:
>
> My understanding is that cladistics shows how organisms are related to
> one another, whereas phenetics, the assumption was that it shows
> relations, but in reality it was really just showing morphological
> similarity.

Actually, that wasn't even the assumption. As I understand it,
phenetics was about despairing of basing anything on relationships and
simply classifying by similarity instead. (It ran into trouble when it
turned out there aren't any useful, objective, general measures of
similarity.)

> If you put me, my brother, my great grand children and my cousin into
> a cladistic and phenetic analysis, the cladistic analysis would show
> the correct relationship, while the phenetic analysis would possibly
> only show how similar we all look.

On this fine a scale a cladistic analysis is very unlikely to have
useful results, Assuming it did, it would still show you as the sister
group to your great-grandchildren rather than the ancestor (although
you'd have no autapomorphies, so potential ancestry could be
inferred).

Your general point is correct, though.

>> Total time divergence between I. bernissartensis and M. bucklandii:
>> (240 - 125) + (240 - 165) = 190Ma.
>> Total time divergence between P. domesticus and M. bucklandii: (175 -
>> 0) + (175 - 165) = 185Ma.
>
> This method seems quite sensible and contradicts my expectations. I
> like it. I was thinking of M. and I. as close cousins and P. as a
> great-great grandchild (for some arbitrarily large number of greats),
> looks like my error is in how 'close' those cousins are.

Well, keep in mind this is only one metric. I would guess that average
generation times are much shorter along the branch from the
megalosauroid-neotetanuran ancestor to _P. domesticus_ than along any
of the other branches. So if you were using number of generations as a
metric, it's quite possible _M. bucklandii_ and _I. bernissartensis_
would be closer than _M. bucklandii_ and _P. domesticus_.

This sort of difficulty is why we usually refer to the amount of
shared ancestry when we talk about closeness of relation--it's much
easier to compare.

-- 
T. Michael Keesey
http://tmkeesey.net/