From: toby@carson.u.washington.edu (Toby Bradshaw)
Subject: Re: Peregrine Diet
Organization: University of Washington, Seattle
Date: Sun, 4 Oct 1992 21:05:18 GMT

>TB> Anything's possible, but "DDT-resistant population"?  I
>TB> don't think so.  The heritability of "resistance" to DDT is
>TB> unknown, and the selection intensity is unknown, too.  Both
>TB> these factors must be known to calculate the genetic gain in
>TB> "resistance".
>
>I have seen no genetic studies to support either my contention or
>yours.

I made no contention except that the relevant data needed to
establish the genetic basis of "resistance", if it exists,
are unavailable.  Hence, there is no basis for believing
that genetic resistance to DDT exists, except the weak
statement below (for which there are many alternative explanations).
For mosquitoes, the biochemical mechanisms of resistance
are known in considerable detail.  Since American kestrels
were widely used to experimentally study DDT effects on raptors,
any genetic data would come from that program.

>However, during the DDT era, there were many nesting
>Peregrine pairs which hatched young successfully while others
>failed. Many of those "successful" young were lost to poaching.
>If these Peregrines possessed a trait which made them relatively
>immune to DDT, it seems logical that trait would be passed on
>through natural selection.

I'm sure you realize that this is not evidence for genetic
resistance; only a suggestion that genetics is one of
several possibilities.  As for poaching, and egg collecting,
and "unscrupulous falconers" (they exist, like unscrupulous
people in all walks of life) causing the decline of peregrines:
the same pressures existed in Great Britain for centuries
with no significant decline until DDT use became widespread.

>An interesting bit of data from the recovery effort in California
>is that there has been no significant increase in average
>eggshell thickness since the banning of DDT.  Peregrine eggs are
>as thin-shelled now as they were in the late 60's.  I suspect
>these birds are picking up DDT on their wintering grounds in
>Middle America.  Perhaps the birds currently breeding in
>California have developed some resistance to DDT.

If they are "resistant", why are their shells thin?  The
concentration of DDT and its metabolites can be measured 
from adipose tissue -- are the levels up or down from
historic highs?  What is the level of steroid hormones in
the blood of breeding females?  As I understand it (HEARSAY
ALERT) the DDT induces cytochromes P450 in the liver, which
are somewhat nonspecific in the cyclic hydrocarbons they
use for substrates.  Some of the induced P450s hydroxylate
steroid hormones, making them inactive and leading to eggshell
thinning as well as behavioral abnormalities during the
breeding season.

>TB> I'd like to see a citation for any evidence that there is
>TB> even genetic variation among peregrines for "resistance".
>TB> Can you provide one?
>
>Not at all.  My message was strictly speculation.  Take it with a
>grain of salt if you like.

As a geneticist, I did :)

>But I think it is logical that traits
>which promote survival or reproductive success are passed on to
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Not survival, just reproductive success. 

>future generations.  Isn't that what evolution is all about?

Sure.  But evolution only has genetic variation as raw material.
No genetic variation, no evolution, period.  Maybe four eyes would
have adaptive value for woodcocks, but unless one is born that
way, and the "four eyes" phenotype has a genetic basis,
there's no variation on the "two eyes" theme, and no
target for natural selection to favor "four eyes".  

Toby Bradshaw
Department of Biochemistry and College of Forest Resources
University of Washington, Seattle
toby@u.washington.edu



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