Re-defining evolution

Andrew Petto (AJPETTO@MACC.WISC.EDU)
Wed, 13 Mar 1996 12:47:00 CDT

Yike! No wonder the antievolutionists are having such a field day! We can't
even seem to agree on what we agree on.

Well, I have thought about this, and I think that the problem we are having with
agreeing on definitions is that we are applying the word "evolution" to
different definitions. What I am trying to convey to my students is something
about the pattern of variation in time and space in forms of life that are
related by descent to common ancestors via processes that allow differential
survival and reproduction of these variants. It is more of a
historical/phylogenetic process -- a larger theoretical perspective.

What a number of other people have written -- Ron Kephart (comments below),
Danny Yee, and Vance Geiger among them -- uses the term "evolution" to refer to
the mechanism by which we understand that these variations *must* occur --
genetic change.

I guess my complaint is that focusing on the mechanism gives us a chance to
observe/study/theorize about how this mechanism can produce the larger
historical and phylogenetic patterns that we see in the world around us, but the
contribution that each of these genetic changes can make to "evolution" in the
broader historical sense is unknown -- as Danny Yee pointed out, though in
opposition, not in support of my assertion that the actual connection between
specific genetic variations that we observe and historical/phylogenetic patterns
are usually impossible to determine until long after the fact.

So, I don't think we disagree so much on the contribution of genetic change to
evolutionary patterns as we do on whether it is enough to observe the genetic
change and declare that that is all we need to see to declare that evolution has
occurred. That's probably why we kick around terms like "microevolution" and
"macroevolution" to identify which end of the continuum we are discussing.


Now, in particular with respect to Ron's comments:

1) I am your physical anthropologust colleague; and I am really working hard to
make these relationships (pattern and process, temporal and geographical) real
and understandable to general education students. 2) My post never said
anything about selective benefit, though I do agree that *something* has to
happen to allow the new genes to become fixed in a population and then to spread
or to increase in frequency locally (or both). 3) THe classic example of
sickle-cell trait *is* a case of clear selective advantage in a temporally and
geographically defined population. One would assume that this mutation would
have disappeared according to all the laws of recombination, segregation, and
independent assortment unless it *did* confer a selective advantage. So we all
agree that this mutation became fixed as an adaptation and that the associated
changes in gene frequencies have set the stage for a process of significant
diversification of this genome from that of the rest of the species -- or maybe
not. However, the implication of this posting is that if malaria went away, so
would the gene -- eventually, just as the peppered moth went from light to dark
and back again (and maybe again?). Changes in local variation due to local
conditions, but not more than a ripple in the whole gene pool to which this
population is connected in other areas.

ENough of the arguing, I can agree that genetic change is the mechanism by which
evolution must occur and that when we see it we are seeing the operation of this
mechanism. I hope others can agree that the historical/phylogenetic perspective
of speciation and diversification of taxa is the foundation that brought us to
the point of asking how genetic change works to produce the sorts of variation
that are the basis of the changes that we have observed in the fossil record.

Sort of like Newton's and Einstein's physics. THey both describe many of the
same phenomena, but at different points of entry, levels of causation and
interaction, speeds, masses, etc. They are not different, or even in conflict,
just focused on a different part of the problem. Is this not what we have here,
too?

Anj
Andrew J. Petto, Editor, National Center for Science Ed.
c/o Dept. of Anthropology, University of Wisconsin
1180 Observatory Drive, MADISON WI 53706-1393 ajpetto@macc.wisc.edu
voice: 608/259-2926; fax:608/258-2415
NCSE email: ajp3265@madison.tec.wi.us
++++++++++++++++++++++++++++++++++++++++++
Ron Kephart wrote:

With all due respect, and after consulting my physical anthropologist colleague,
I have to stand by my original definition: evolution is change in gene
frequencies. The change may or may not lead to speciation, and may or may not
be the result of patterned, non-random forces such as natural or sexual
selection. There are some who argue that most evolution is non-patterned, the
result of random mutations taking hold in a population even tho they carry no
selective benefit.

One classic example of evolution without speciation is the sickle-cell trait,
the gene frequencies of which vary thru time depending on the prevalence of
malaria. Without malaria, the frequency is very low because all persons with
the gene are at a disadvantage compared to those without it. Enter malaria, and
persons with one copy of the gene are at an advantage over both non-carriers and
carriers of two copies. Take away malaria, and non-carriers regain their
original advantage. This is evolution, even tho no new species appear.