From: colby@bu-bio.bu.edu (Chris Colby)
Newsgroups: talk.origins
Subject: Re: evolution and natural selection
Date: 1 Oct 92 23:21:48 GMT
Organization: animal -- coelomate -- deuterostome

In article <BvGn3G.AC8.2@cs.cmu.edu> kck+@cs.cmu.edu (Karl Kluge) writes:

>Since neither Darwin or Wallace knew anything about genes or alleles, I will
>admit to a certain discomfort with this definition. It seems to me that the
>core of evolution is common ancestry. 

Evolution refers to both a process (the definition I give defines this)
and a historical fact (common ancestry). It would be better if there
were two different terms for this as it causes confusion.

>> natural selection: differential reproductive success of (pre-existing)
>> genotypes in a population

>Similarly, natural selection would be a fairly useless concept if which
>genotypes showed greater reproductive success was random. Why not define it
>as "(pre-existing) genotypes which are more fit (in an objective,
>engineering sense) increase in frequency in a population."

Two reasons 1.) fitness is just a description, not an explanation
and 2.) selection can also result in allele frequencies not changing
even when alleles differing in fitness exist -- balancing selection and
frequency dependent selection are two cases of this.
 
>> fitness: average reproductive success of a genotype in a population
>> (usually expressed relative to the most fit genotype in the population
>> which is arbitrarily assigned a fitness of one.)

>This is how population genetic defines the term, and it is vulnerable to the
>creationist criticism of circularity.

>> When fitness differences exist, natural selection can cause a change
>> in allele frequencies (evolution). 

>I.e. by plug-and-chug of the above definitions, "When differences in average
>reproductive success of the genotypes in a population exist(*), differential
>reproductive success leads to a change in genotype frequencies in the
>population." 

(I've added the (*) -- I explain below)

A very good point, but fitness differences are caused by (heritable)
biological differences among organisms. For the purposes of pop gen,
it doesn't matter what the differences are, only a measure of their
magnitude is needed (i.e. relative fitness). Different biological
traits could lead to organisms with the same fitness in a population.
Once fitness differences exist it is desirable to find their cause,
but the term fitness need not include the source of the difference.
Fitness is just a term to quantify the magnitude of the difference in
reproductive success, not an attempt to explain its source.

Add in "due to phenotypic differences with a heritable component"
where I put the (*) to "rescue" the statement from circularity.
Also change "leads to a change" to "may lead to a change" for reasons
given above.

Endler's book "Natural selection in the Wild" (1986, Princeton Press)
has an explicit discussion of this very point (on page 33).

>I don't mean to be nasty, Chris, but BFD.

I hope the above answers your question. As far as populations genetics
is concerned, it is a damn good thing that plugging in definitions
results in a "1=1" type statement. But, I agree that for any real
world application, I should have included the link to reality --
e.g. the cause of fitness differences.

As long as you bring up good points (and give me an excuse to
yammer on about evolution as a result), feel free to be as nasty
as you want to be 8-) 

>Karl

Chris Colby 	---	 email: colby@bu-bio.bu.edu	---
"'My boy,' he said, 'you are descended from a long line of determined,
resourceful, microscopic tadpoles--champions every one.'"
 	--Kurt Vonnegut from "Galapagos"