From: rambaut@castle.ed.ac.uk (A E Ram aut)
Newsgroups: sci.bio
Subject: Re: Evolution defined as genetic change?
Date: 18 Dec 92 12:35:26 GMT
Organization: Edinburgh University

swf@tools3teradata.com (Stan Friesen) writes:

>In article <29517@castle.ed.ac.uk>, rambaut@castle.ed.ac.uk (A E Ram aut) writes:
>|> swf@tools3teradata.com (Stan Friesen) writes:
>|> 
>|> >No, this will not work.  For a change to count as evolutionary, it *must*
>|> >be inheritable, that is it must be genetic.  This is because non-heritable
>|> >changes are not propagated, and thus cannot be *accumulated* - which makes
>|> >the whole process rather meaningless.
>|> 
>|> Phenotype is by definition inheritable because the phenotype is the
>|> physical manifestation of the genotype an inheritable set of alleles.

>Well, that is not how I use the terms.  I use phenotype to mean "the physical
>manifestation of the organism", no reference to genes.

>The problem is that a given gentoype can produce many different specific
>phenotypes, depending on the environment.  Thus it is entirely possible for
>the physical manifestation to change without any change in genotype.

I am perfectly aware of this, but these are still the results of gene
expression. If the gene mutates, then it may affect the development of
a characteristic as it may be influenced differently by the environment
as the genetic and environmental componants interact. I agree that
totally enviromental characteristcs (such as a damaged fish fin after a
preditor attack) are not affected by evolution, these are not what you
would call phenotypes.

>A good example of this is the increase in height in North America and Europe over
>the last several centuries.  There is no evidence that there has been *any*
>genetic change with regard to height, and, yet height can be shown to have
>a very high heritability (about as high a heritability as has ever been observed).

>What is happening?  The answer is better nutrition.  This is *not* evolution,
>it is just environmental effects.  As soon as nutrition returns to medieval
>levels the mean height will drop back to the old value in the next generation.

The problem is that this is looking on a microevolutionary scale. If you
look at the human population as a whole (not just the overfed North
America and Europe) over a longer time period, then you can say
something about the evolution of height. In North America, I doubt
whether there is much selective pressure influencing height, thus the
large (and heritable) variation. If there were a selective pressure
(favouring tall people for example), then natural selection would act on
the phenotype. Populations with stunted growth (as a result of poor
nutrition) may have a swing towards genetically tall people. In
population that had good nutrition, natural selection may not act so
strongly. However, if the selective pressure was for shorter people then
it may act to produce a population of genetically shorter people (and
perhaps to reduce the link between nutrition and vertical size). This is
not really a good example because I don't think anyone would suggest
that the increase in mean height in the US has come about by evolution.

The point is that the the major mechanism of evolution can only act upon the
charactistics of the organism. Although these characteristic may be influenced
by environmental conditions, a phenotype (by genetical definition) has
an underlying genetic componant. The final trait comes about by the
interaction between these componants. The genetic componant is
heritable and changeable so can be acted upon by nat. sel. (and other
mechanisms). Even what one could normally call environmental factors can
often be affected by genes (perhaps by the genes of the mother animal -
e.g. temperature of egg incubation).


Andrew.