Re: Cambrian explosion bit of an embarassment

William Morse wrote:

John Harshman <jharshman.diespamdie@xxxxxxxxxxx> wrote in news:ex2Og.1063
$6S3.812@xxxxxxxxxxxxxxxxxxxxxxxxxx:


William Morse wrote:


John Harshman <jharshman.diespamdie@xxxxxxxxxxx> wrote in
news:joKNg.602$vJ2.488@xxxxxxxxxxxxxxxxxxxxxxxxxx:



William Morse wrote:


OK, but I think I see what Walter Bushell and R Norman are getting
at, namely that there are two ways of measuring relatedness between
two organisms. The first (the generally accepted one) is by the
number of ancestors you have to step through to get from one organism
back through the LCA and up to the second organism. The second is by
the number of gene changes you have to step through to change one
organism into the second organism.

Neither of these is what's generally meant (in phylogenetics) by
relatedness. The first is the usage within populations, like the human
one. The second isn't relatedness according to any usage I know of.
It's called patristic distance.

Thanks, I hadn't heard that term before. It is good to know there is
already a word to describe the concept I was talking about.

To belabor the point: patristic distance is distance counted along the
branches of a phylogenetic tree. That distinguishes it from simple
pairwise distance, in which you just look at two taxa and count their
differences.


Pairwise differences and $1.00 will get you a cup of coffee. They might be
useful for doing a hash table.



In phylogenetics, what we mean by
relatedness is relative, not quantitative, and it's expressible only
in relation to three taxa: A is more closely related to B than to C if
A and B share a common ancestor more recent than the common ancestor A
and B share with C (which is necessarily the same one for both A and
B).

I understand that terms often have specific meanings in specific
disciplines that are different from their usage in common parlance.
Unfortunately it is not always clear in which realm the discussion is
taking place.

Notice that the common usage of "related" differs from the phylogenetic
usage only in that we don't count discrete generations but instead count
relative time. They're pretty close.


Since we are picking nits: I would argue that the common usage of
"related" is genealogical, in which we don't count only back to the LCA,
we also then count forward from the LCA to the individual. Thus
genealogically we differentiate between first cousins and first cousins
once removed. This differs from your definition of phylogenetic
relatedness.

Only in the ways I have already mentioned.

And in either usage, overall
similarity doesn't enter into it


So aardvarks and pangolins used to be classified with anteaters just for
fun?

Yes, if you consider ignorance to be fun.

.

Relevant Pages

  • Re: Cambrian explosion bit of an embarassment
    ... namely that there are two ways of measuring relatedness between ... back through the LCA and up to the second organism. ... A and B share a common ancestor more recent than the common ancestor A ...
    (talk.origins)
  • Re: Cambrian explosion bit of an embarassment
    ... namely that there are two ways of measuring relatedness between ... back through the LCA and up to the second organism. ... The second isn't relatedness according to any usage I know of. ... A and B share a common ancestor more recent than the common ancestor A ...
    (talk.origins)
  • Re: Cambrian explosion bit of an embarassment
    ... namely that there are two ways of measuring relatedness between ... back through the LCA and up to the second organism. ... The second isn't relatedness according to any usage I know of. ... A and B share a common ancestor more recent than the common ancestor A ...
    (talk.origins)
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