To: All Msg #157, Sep0193 01:11PM Subject: genes and evolution (for Joe) Joe Morlan (Joe.M
From: Chris Colby
To: All Msg #157, Sep-01-93 01:11PM
Subject: genes and evolution (for Joe)
Organization: animal -- coelomate -- deuterostome
From: firstname.lastname@example.org (Chris Colby)
Joe Morlan (Joe.Morlan@f216.n914.z8.rbbs-net.ORG) wrote:
: I think the real reason that modern biologists define evolution
: in terms of changes in genetic variation, is because the field
: has been dominated by geneticists since the advent of the "modern
This is correct.
: The modern definition can be paraphrased "gene pools
: are not immutable." Is this progress? IMO modern biologists
: have confused a mechanism of evolution, with evolution itself.
Is it progress? Only in the same way discovering atoms and
arranging them in the periodic table was progress for chemistry.
The emergence of the "modern synthesis" was a great leap forward.
This brought the focus of evolution to where it should be --
heritable characteristics. In other words, the study of elements
that transcend the lifetime of individual organisms. Your assertion
(in a previous post) that you don't need to understand genetics to
understand evolution was so outrageously stupid that I couldn't
even bring myself to post a rebuttal. Allow me now to briefly explain
why genetics is central to evolutionary theory.
First, lets start with the painfully obvious. Genes replicate
themselves, usually exactly. Genes are capable of being in-
herited for many, many generations without alterations.
Organisms, and the phenotypic traits that compose them, are
not replicated with that fidelity. This is especially true in
sexual populations where offspring are the expression of two
half genotypes (haplotypes) in the context of the environment.
The environment plays a large role determining the phenotype of
an individual organism. In contrast, the environment plays
almost no role in determining the genotype of an individual
organism. Unless the environment is highly mutagenic, the alleles
in any individual remain the same and are simply passed on.
The rate at which they are passed on, of course, is highly dependent
on the environment. But, the information content of any allele
is immune to almost all influences from the environment.
Thus, the phenotypic characteristics of a population can change due
to genetics or environment. A sun and fitness craze
could hit a population of (oh lets say) Scandinavians and the
frequency of tanned and muscled guys named Sven or Ole could sky-
rocket. A few years later, the government could warn that the ozone
is gone and the sun is bad, and that people should stay inside, read
Ibsen and listen to Grieg. The frequency of pale, skinny Norskies
would then rise. However, these changes would leave no record of
themselves in the biology of the descendants.
Changes in the frequency of heritable elements (genes) do leave their
mark on future populations. If the frequency of an allele conferring
resistance to a deadly disease increases to a high frequency following a
severe plague, it stays at a high frequency once the plague has gone.
In addition, the frequency of many other traits may have changed
"permanently" as well because they were linked to the beneficial
allele. For example, if the original subset of people carrying
the resistance allele all had blue eyes, the frequency of blue
eyes would increase in the population as a side effect. This
change in frequency would also remain once the plague was over.
Genetic changes have a permanence that phenotypic changes due
solely to environmental conditions don't.
In addition to this, the fact that alleles are inherited as
discrete, unchanging (except in the rare instance of mutation)
entities allows them to be counted. Simple (and subsequently
more complex) models can be derived to account for changes or stability
in gene frequencies. Building quantitative models does a lot
toward moving a field from stamp collecting and just-so stories
to a real, productive branch of science.
Furthermore, genes contain traces of their ancestry. The genome
of any organism is a palimpsest of information. It has been
rewritten and rearranged continually to confer reproductive advantage
to it ancestors. In systematics, any useful trait must be heritable.
Traits that are completely environmentally determined give no
information of ancestry. Systematists for years have used pheno-
typic traits, because that is all thatwas available. And,
it has for the most part worked. But genes have a few distinct
advantages over phenotypic traits. For one thing, you always know
a gene is heritable.
What about macroevolutionary changes? Well, at the very least we
can say that microevolutionary change contributed to it. Some
biologists would say that the models of genetic change within
populations have the potential to completely describe the changes
involved in macroevolutionary transitions. Others say they are
a component, but other mechanism must also be invoked. So, how
important changes within a gene pool in macroevolution are is
controversial -- but you cannot deny that it did play _a_ role.
I would say that, if you compare the number of
different generations that lineages remain as an intact gene
pool versus how many times they speciate (and the opportunity
for higher level mechanisms is present) -- the conclusion that
the bulk of macroevolutionary change is mostly cumulative micro-
evolution change is a safe bet.
In chemistry class, the prof tells his students about chemical
reactions using the model of atoms as basic units of matter.
They can be combined into molecules and molecules can like-
wise be broken up into atoms. Knowing the characteristics of
individual atoms is a first step in understanding the chemistry
of larger molecules. And, more importantly, it allows a vast
amount of knowledge about molecules to be (at least partially)
condensed into a simple framework. A chem prof _could_ teach a
class where you memorized reactions, but never mentioned atoms.
The students might learn what happens when you heat chemical A
and chemical B, but they would be missing a very central point
to chemistry. Specifically, given two chemical names and the
conditions which they are to be combined in, the students would
have no hope of predicting the outcome.
You are missing an extremely central point to evolution (and hence
biology) if you don't understand the role of genetics in evolution.
We could memorize that this form followed this form and so on, but
where is the explanatory or predictive power?
I see that you have recommended (to Mickey I think) Mayr's "Animal
Species and Evolution". Guess what? I read (O.K. skimmed) it a
few years ago; not a bad book. But, the field didn't die after
Mayr wrote the book. At the t.o. party, your name came up and I
proffered the opinion that you were proof of the maxim that a
little (emphasis on little) knowledge is a dangerous thing.
How about reading Futuyma and considering, just for a second, that
the entire combined field of evolutionary biology composed of
scientists might have a few insights into evolution that a bird
watcher who has read one book by Mayr might not have? Either
that or you might consider adding HTE to your .sig file 8-)
E-Mail Fredric L. Rice / The Skeptic Tank