[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Campbell's even crazier than a MANIAC? (archeopteryx climbing)

On Sep 26, 2008, at 12:17 PM, don ohmes wrote:

 Yes, clearly competitive exclusion can be powerful, but do we
really expect that the simple existence of powered flyers should
completely shut down gliders with limited powered ability?

Yes, I do. I take the lack of "gliders with limited powered ability" as implied support for this assumption.

It could just as easily be support for the hypothesis that gliding to powered transitions are very rare for mechanical reasons, though. You could be right on the money, but it just seems like such a strong blanket statement that it makes me slightly skeptical.

It is also worth noting that the 'transitional' animals you describe would be unlikely to remain in such a state for long (on evolutionary time scales).

Possibly - the stability of a specific morphology is hard to predict.

Definitely. Insects did provide a ready source of food, though.

Yup - filling and delicious.

but birds and bats certainly both appeared in the presence of pre- existing powered flyers.

I think it is a safe assumption that they were not faced w/ the competitive barriers modern-day putative transitionals would have to overcome, from the perspective of niche specialization.

Why is that? There was certainly specialist ecologies among flyers in the Mesozoic, and it's not clear how much specialist ecologies impact competitive exclusion. One could make the argument that it is actually "generalist" feeding modes, etc that would be more likely to produce exclusion. Of course, the whole specialist-to-generalist metric is a mess in and of itself...

Also, mere global presence does not imply local competition. E.g., the feathers of basal birds could have allowed exploitation of thermal regimes not suited for rhamphorhynchnids.

Definitely - two organisms are not necessarily in tight competition just because they both fly. Many volant animals face stronger competition for space and food from non-volant species (and vice versa) than they do from volant relatives. In the same way, I'm reluctant to assume that powered flight cannot originate from any modern groups simply because diverse flyers already exist. It certainly didn't stop flight evolution in the Mesozoic.

Of course, pure gliders are MUCH more numerous than incipient "ground-up" animals.

Yes, this is true (at least in the modern time slice). So we have two different types of rarity - terrestrial origins for flight are probably rare because heavy exaption is involved (i.e. rarity by ancestor constraints). Arboreal origins of powered flight are rare most likely because transitions to powered flight from an obligate gliding morphology are rare (i.e. rarity by mechanical transition). And, since these are just ends of a spectrum (and not dichotomous variables), we have a huge range of potential probabilities, all of which are basically dominated by the ecology of ancestral states. The one thing that seems to be consistent is that heterogenous environments are probably important to the origin of flight in all four cases.

My apologies, perhaps I was momentarily unclear. I define an animal leaping of a high place as 'gravity-driven', and do not restrict the term to animals that simply let go.

Oh, okay - fair enough.



Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280-0181