On Wed, Jan 23, 2008 at 02:42:03PM +1100, Evan Beaver wrote:
> it's not as outrageous as it sounds. It doesn't take many items in a
> combination to make the number of possible combinations exceptionally
> large. A list with 10 possible items placed in order has 3628800
> possible combinations.
Yes, if you write down a list of 800 Australian bird species there are
rougly 10^1979 (10 to the power of 1979) different orderings (strictly
permutations not combinations) you can use.
With a couple of assumptions you can calculate that only about 10^242 of
these orderings will be consistent with the phylogeny of Australian birds.
So after a taxonomist decides on current evidence the most likely
phylogenetic tree for 800 Australian birds, they then get to choose 1
of approximately 10^242 linear sequences for these birds.
For comparison the number of atoms in the universe is usually put
at 10**80.
This exaggerates the taxonmist's task a little. Assuming a binary
phylogeny, they only to make 799 ordering decisions. Traditionally for
birds when forming the sequence they put the daughter lineage thought to
be most similar to the common ancestral form first in the sequence.
Schodde&Mason go clockwise around Australia at the outermost taxonomic
level - where presumably the previous rule wouldn't be feasible.
Andrew
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