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Teleology (was: Why do trees change color in the fall?)

Toby Bradshaw toby at stein.u.washington.edu
Mon Aug 23 18:04:46 EST 1993

In article <CC8AFI.M13 at sugar.neosoft.com>,
Cameron Laird <claird at NeoSoft.com> wrote:
>There has been plenty of attention to efficient causes,
>but little (to my surprise, considering the usual evolu-
>tionary bent sci.bio takes) to final ones.  My favorite
>just-so story involving fall foliage is that it's a co-
>evolved trait, one that's supposed to signal "EAT HERE"

Doesn't seem to explain fall color in aspen, though :)

>to the great migratory flocks of seed-dispersers.  A
>cousin of mine did some slick calculations demonstrating
>that there hasn't been enough time since the last glaci-
>ations to have allowed for the fixation of such a
>phenotype; I haven't been able to work around his ob-
>jection yet.  Anyone know the latest from the specialists
>in this sort of thing?

Two things.  The genetic control of fall phenology is strong,
as a trip through the aspen clones in the Rockies will show,
but less than that of spring phenology for which broad sense
heritabilities are in the 0.95 range.  As for the inability to
fix an allele for phenology (I think that's what's implied
above -- feel free to correct me), such a thing would spread
like wildfire if the trait is oligogenic.  The onset of dormancy
is triggered by photoperiod, so it's not hard to imagine that
this could be oligogenic.  Spring phenology certainly is (I
should be writing that QTL m'script instead of this :).  Trees
are generally conservative, but retention of photosynthetically
active foliage in the fall is important for radial growth, hence
probably under some significant selection in natural popns.

Toby Bradshaw                       |
Department of Biochemistry          |  Will make genetic linkage maps
and College of Forest Resources     |            for food.
University of Washington, Seattle   |
toby at u.washington.edu               |

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