On Thu, Jan 24, 2008 at 12:21:05PM +1100, Peter Shute wrote:
> > Concerning the recent thread regarding the taxonomic order of
> > Australian birds it should be noted that C&B discuss this at
> > some length in the Introduction under the heading "Taxanomic
> > methods". They say "...tree topologies are like mobiles....
>
> Mobiles? Like you hang from the roof?
Yes. Its a textbook analogy. Imagine a mobile composed of horizontal
rods. Each rod is suspended from a string in the centre and has strings
dangling from its left and right ends holding up another rod or leading
to a bird species. So at dangling the bottom of the mobile would be the
~800 species of Australian bird.
If you want to reduce the tree (mobile) into a sequence, there are two
choices for the orientation of every rod. If there are 800 birds then
the mobile will contain 799 rods. As a consequence there are 2^799 =
3334007216439927137039925895360628898572379161157954080198128905882018618908816035760716100435777145371464955296716620222944400827059682540181678026165415023047578789757007279231539142955907012364482508067943300990845374018738230645581938688
possible sequences consistent with the phylogenetic tree. If this is
what C&B mean by several alternatives, they are masters of the
understatement.
Traditionally for birds the rods of the mobile are oriented so the
lineage considered most similar to their common ancestor hangs from the
left arm and hence is placed before the lineage hanging from the right
arm in the sequence. This gives you a unique sequence. The arrangement
of some pairs of lineages (rod orientations) will be pretty arbitrary
but this doesn't matter as all we want is a standard sequence so
we can all use the same ordering when we list bird species.
Alan as I understand it Schodde&Mason list ultrataxa at the outermost
level (what others might call subspecies) in geographical order going
clockwise around Australia. This is presumably more practical than
trying to estimate a phyogenetic tree down to this level - but better
read S&M's own words.
Andrew
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