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Re: Big-brained birds evolve faster
I'm curious: could the same thing be said about forelimb morphology?
I would think that most would say "Of course the wings of birds
allowed them to enter niches unavailable to other organisms, leading
to greater diversity." If this is true, why is the same claim for
large brains more controversial?
Well, for starters, I'm not sure I'd agree with the wings statement:
wings should promote survivorship, which promotes range expansion and
population size. While this can produce high diversity simply by
promoting interactions with new environments (via mobility), I would
argue that it is variation in wing structure that is more likely to
promote diversity. There are particular flight dynamics that seem to
generate species diversity most readily (specifically, those planforms
and behavioral guilds that promote isolation).
In any case, part of the problem is that the actual effect of a large
brain is a bit more fuzzy, as is the source of selection for large
brains. I'm not so much skeptical of the result that diversity and
brain size can go together for certain groups, but more that this must
involve a cognitive effect. It could just as easily be sensory and
control related (though cognitive ability is certainly plausible, too).
Michael Habib, M.S.
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205