[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: FW: A Perspective on Cope's Rule
Milbocker: This statement seems to be self-contradictory. I agree with the
statement smaller animals should evolve at higher rates.
On the one hand, small animals have higher metabolism and therefore more
mutations. On the other, large animals have small population sizes,
therefore more genetic drift, and therefore more mutations get fixed in the
populations. The former effect wins, but perhaps not as clearly as one would
However, evolution optimizing to a particular environmental
circumstance can act to reduce diversity by
increasing specialization, and tweaks to a body plan that is fatally
specialized do not contribute to diversity needed for survival.
Good old stabilizing selection.
Milbocker: Success is not at issue, and the sauropod body plan underwent
more changes immediately after each extinction event than during the
That's an interesting statement. Can you provide examples?
Large organisms [...] are K selected species after all
By no means automatically. Even the largest crocodiles are r-selected.
Judging from their clutch sizes, the same held for hadrosaurs and sauropods.
Milbocker: Not sure I follow your argument above. My assertion is that if
you had 100 foot sauropods and 1 foot sauropods, the poorly preserved
1 foot sauropods would be more likely to carry the genetic resource for
post-extinction next generation of sauropods.
My problem with this is simply that I can't imagine a 30 cm short sauropod,
except perhaps a hatchling in a well-guarded nest! We're talking here about
animals which were better at walking, but even worse at running than baby
sea turtles -- and lacked a turtle shell.
Milbocker: I agree for periods between extinction events, but cladistics
assumes the opposite, that convergent evolution is relatively rare, that
species "radiate" they don't converge.
I think it was Sereno who made a very succinct remark on this... it was
like... "cladistics isn't the assumption of minimisation of convergence,
it's the minimisation of assumption of convergence". Outside of (oh irony)
Sereno's cladograms, you won't find a homoplasy index below 0.6 or so,
meaning that 60 % of the character state changes are convergences or
reversals. Mine has some 6.5, and I've deliberately not used characters like
"maxillary" or "dentary teeth: present/absent", because their derived states
would either have been local autapomorphies of terminal taxa (thus telling
me nothing new) or perhaps screwed up the tree, due to the small total
amount of characters (thus giving me a few thousand more MPTs). (I'm already
having the latter effect: "premaxillary teeth: present/absent" is a part of
the strange affinity of *Yandangornis* for *Confuciusornis*, and of the less
strange but still unexpected affinity of *Boluochia* for *Gobipteryx*.)
The practice of taking a single
outgroup taxon to start a cladogram is symptomatic of this point of view.
There are cladograms with, like, 5 taxa in the outgroup.