RE: Yoyoism (was Re: Endothermic Crocs in Nature)

[Tim Williams <twilliams_alpha@hotmail.com>]

Dino Guy Ralph wrote:

> Many phasmids (stick insects) appear to be neoflightless, and secondary 
> flightedness has apparently evolved independently in four distinct phasmid 
> groups since this initial neoflightless event.  In fact, a couple of 
> phasmid clades apparently lost their wings, regained their wings, and lost 
> their wings yet again.

Such yoyo-ing also occurs in the evolutionary hisrory of tree-kangaroos 
(_Dendrolagus_).  The
kangaroo lineage ("macropods") is thought to have arisen from arboreal 
stock. The stock was probably akin to modern phalangeroids (Australian 
'possums', phalangers and gliders) which are arboreal.  The most primitive 
extant macropod (_Hypsiprymnodon_) still retains a prehensile tail, and an 
the opposable digit on the hind foot - relicts of an arboreal origin (even 
though _Hypsiprymnodon_ itself is fully terrestrial).

Here's the yoyo-ing part: One group of macropods, the tree-kangaroos, are 
arboreal (although they are clumsy in trees, and will venture down to the 
ground when they need to).  Tree-kangaroos evolved from terrestrial 
macropods close to the rock-wallabies, so they therefore represent a 
reversal to arborearilty.  However, there is one species of tree-kangaroo, 
the dingiso of New Guinea (_Dendrolagus mbaiso_), that spends most of its 
time on the ground.  This shift to terrestriality is believed to be DERIVED 
relative to other tree-kangaroos.  Thus, the dingiso is a secondarily 
terrestrial tree-kangaroo.  This means that the line went from arboreal to 
terrestrial to arboreal to terrestrial.

P.S. In previous posts I used the word 'neoflightless' in a much more 
limited sense than GSP, who uses it as a synonym of secondarily flightless.  
I was using the word 'neoflightless' in a more limited sense, to mean 
'abandoning flight soon after it was first evolved'.  Sorry for any 
confusion.

Tim