An armchair tick from the new C&B

Subject: An armchair tick from the new C&B
Date: Fri, 25 Jan 2008 10:06:32 +1100
> 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.

A phylogenetic tree may be constructed as a binary tree, but it
isn't an arbitrary binary tree so the worst case scenarios for
viable alternatives doesn't occur.

Pairs of birds aren't chosen at random; they are selected because
they have (or are believed to have) a more recent common ancestor
than the alternatives.  Trees that place emus and albatrosses as
near neighbours are unlikely to be valid phylogenic trees, but
those with pelicans and albatrosses may be.  Repeat for all known
relationships, and the degrees of freedom to produce alternative
trees that satisfy the known data are greatly reduced.

   Paul Taylor                           Veni, vidi, tici -
                    I came, I saw, I ticked.

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