Re: [asa] New fruit fly threat in Southern California

From: wjp <>
Date: Sun Aug 16 2009 - 09:34:11 EDT

My purpose here is to make more evident the process whereby
"common descent" is inferred.

Consider two events, E1 and E2.
Let E1 be that a species S1 has a certain pheremone character.
E2 is that another species S2 has a pheremone character that
is similar to S1.

Consider now the joint probability P(E1&E2) and consider the

P(E1&E2)/(P(E1)P(E2)) = P(E2|E1)/P(E2)

If you judge events E1 and E2 to be independent, then the ratio
is equal to 1.
Whereas, if you judge events E2 to be entailed by E1, the ratio
is equal to 1/P(E2). If you judge P(E2) to be small, then
the ratio will be large. However, if you judge E2 to be likely,
then there will be little difference between correlated events
and independent ones.

It is in the case of pheremones that we judge P(E2) to be small.
In this case, if we judge the conditional probability P(E2|E1)
to be large, then we will conclude that the events E1 and E2
are highly correlated.

The point of this analysis is to suggest that what we discern
in such assessments of probabilities is not "common descent,"
but correlation, and correlation is not equivalent to
"common descent," unless we construe "common descent" much more
broadly than is commonly done.


> On Wed, 5 Aug 2009 17:32:48 -0500, David Campbell <>
> wrote:
>>>> One of David's examples below provides another example of how
>> evolutionary
>>>> theorizing can *look* as if it contributes substantially to biological
>>>> theory or practice while *in fact* being merely a redundant
>> interpretive
>>>> gloss.
>> On the contrary, evolutionary models provide a reason for supposing
>> that species that are presumed to be close relatives based on one set
>> of data are likely to be similar in other features. (It would be
>> possible to develop a non-evolutionary model with a similar
>> prediction, though I can't immediately come up with one that isn't
>> reducible to "the designer made things to look as though they
>> evolved.")
>> If the similarities were merely common constraints imposed by
>> necessity on separately designed organisms, then there would be no
>> reason to suppose that other features, not subject to the same
>> constraints, would show the same resemblances. For example, a
>> phylogenetic analysis of the flies based on cox1 genes (a
>> mitochondrial protein) has no known reason to correspond to
>> similarities in pheromones, except common ancestry. In fact, it would
>> generally be advantageous for different species of similar flies that
>> live near each other to have different pheromones to avoid confusion.
>> --
>> Dr. David Campbell
>> 425 Scientific Collections
>> University of Alabama
>> "I think of my happy condition, surrounded by acres of clams"

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Received on Sun Aug 16 09:35:11 2009

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