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From page 258 of Life, Itself, Robert Rosen wrote:
"Evolution is concerned with "explaining" the
characteristics of present generations retrospectively, in terms of the
characteristics of temporally remote ones. Heredity is concerned with what
passes between one generation and the next. Thus, if we view evolutionary
processes as a kind of time integral of hereditary ones, in which a "generation"
provides the individual time-step by which evolutionary chronicles are indexed,
it is important to have a tangible bridge between today's generation and
yesterday's, the immediately preceding one. This, roughly, is where genetics
enters the picture.
Let us begin with a few historical
comments.
Gregor Mendel was perhaps the first to concern himself
with how the forms and behaviors of parents are related to those of their
offspring. That is, he was comparing phenotypes, embodied in particular
phenotypic characters, between one generation and the next. It seems to
me, although as far as I know Mendel did not say so, that he was much influenced
by the chemistry of his time. In [that] chemistry, a small number of
different kinds of atoms, the chemical elements, could enter into an unlimited
number of compounds. These compounds could be of utterly diverse
character, or "phenotype"; they could be solid or fluid, crystalline or
amorphous, red or yellow. In any case, the underlying atoms could be retrieved
unchanged from any such combination into which they had entered, and recombined
to yield new compounds, with new "phenotypes." Chemical reaction, in its
turn, could be regarded as a way of liberating the constituent atoms and
recombining them, and thus turning given "phenotypes" into new ones. This
clearly constitutes a powerful metaphor for what goes on between parents and the
production of offspring. Note carefully that this viewpoint does not, in itself,
constitute a reduction of heredity to chemistry; the relation between
them is a metaphoric one, one of alternate realization. But it is powerful
nevertheless.
Let us explore this metaphor a little further. In
particular, let us ask whether the atomization of chemistry in any way
"atomizes" the phenotypes manifested by individual chemical compounds in bulk.
For instance: are there "atoms" for crystallinity, or solidity, or color, or any
other such morphological characters? Even in chemistry, it is not nearly as
simple as that; the atomization of a molecule does not, in that sense, at all
entail a corresponding "atomization" of its phenotypic characters. Nor does it
entail any kind of 1 to 1 mapping between these characters and those of the
atoms, or even the molecules, which underlie them. Put in another language,
which we developed earlier, the phenotype cannot be fractionated into
"atoms" as a constituent molecule can; nor can the actual atoms be fractionated
into separate phenotypic characters for which they are responsible. We will come
back to this, or at least to its hereditary analogues, in a
moment.
Returning to Mendel: he published his evidence for
atom-like, particulate factors (as he called them) in heredity in 1866.
As is well known, his papers sank without a trace. The fact is that no one then
cared much about heredity per se; it was not until the connection
between heredity and evolution, about which everybody cared, was slowly
perceived, that Mendel's ideas were retrieved from oblivion. It was only then
that the study of heredity, soon to be rechristened genetics, became a
crucial part of biological main stream. By that time, Mendel himself was long
dead.
Implicit in Mendel's ideas was a duality between the
phenotypic characters, manifested in the forms and behaviors of
individual organisms within a generation, and the particulate factors
which pass between genera-- from parent to offspring. This is the
genotype-phenotype dualism, which was first stated by August Weismann in his
influential book Das Keimplasm, published in 1874, and to which I have
already alluded. He proposed, in apparent ignorance of Mendel's work, a duality
between what he called soma and what he called germplasm.
Roughly speaking, the somatic part pertains to what is mortal in biology, with
what we today identify with phenotype. Germplasm, on the other hand, pertains to
what is immortal, flowing from generation to generation from the beginning of
life on the planet. Thus the soma must be recreated anew in each generation, in
such a way as to facilitate the flow of the precious germplasm. This was
Weismann's conception of evolution itself.
We have already alluded to this dualism in the preceding
section, where we identified organism with soma, and with life itself. Weismann,
on the other hand, was the first to propose identifying life with germplasm, and
with its flow. He thus, if indirectly, provided the crucial ingredient for the
outlook embodied in contemporary biology.
But, as we have seen, Mendel himself was concerned
primarily with this soma, with phenotype. he characterized his particulate
factors in terms of their functional roles in this regard; i.e., in
terms of how they manifested themselves in somatic forms and their generation.
That was how his factors were recognized, and in turn, that is what the factors
were for.
A basic transformation of the situation occurred roughly
between the years 1900-1910. During these years, the identification came to be
made between the hypothetical Mendelian "linkage groups" and the tangible,
material, cytological structures called chromosomes. Accordingly, the Mendelian
factors themselves, the genes, must be materially incarnated somehow in the
structure of these bodies. So instead of resting content with characterizing
these genes in functional terms, through their modulation of form and
morphogenesis, we could rather imagine characterizing them intrinsically, in
independent structural terms. That would, if accomplished leave only
the problem of accounting for the genes' functional, morphogenetic activities in
terms of this basic, intrinsic structure.
All this has a very contemporary ring; indeed, we are
almost at the molecular biology of today. And of course, it is all fully in
accord with the basic machine metaphor; Chromosomes could be fractionated from
cells, and genes from chromosomes, purely as material structures, as independent
material systems, without loss of information.
At the same time, the functional relation between genotype
and phenotype was becoming progressively more complicated. Mendel thought that
each of his functional factors could be associated with a fixed, demarcated
phenotypic character; the one-gene-one-character hypothesis. As we have already
suggested, it is not that simple; the dualism between genotype and phenotype
does not extend to any kind of 1 to 1 mapping between them. Geneticists early
discovered phenomena of pleiotropy, in which the "same gene" is
involved in may phenotypic characters; and inversely, of polygeny, in
which the same character is associated with many different genes. On top of
this, there was growing evidence, soon to become an absolute necessity, for
"cross-talk" among the genes themselves, directly or indirectly. All this was
evidence of nonfractionability, but being uncongenial to a machine
metaphor, it was regarded as a mere technical inconvenience, to be subsumed into
a bigger machine.
All of this, it will be noted, pertains primarily to
Weismannian soma. What about evolution, which was regarded as the primary issue?
Here, the genotype-phenotype dualism manifests itself in another form. Namely,
the operation of Darwinian selection pertains to phenotypes; to soma. On the
other hand, evolution itself pertains to the flow of genotypes; to germplasm.
Thus we must regard selection on the phenotypes as a modulator of this flow of
genes from one generation to the next. This is where the notion of
fitness comes in.
As presently viewed, fitness involves a decision made by
natural selection about a particular soma, a particular phenotype. It is a
decision that can be imputed to the associated genotype. A low fitness
rating translates operationally into a disadvantage in populating the next
generation; in leaving offspring. Thus a low somatic fitness serves as a filter,
which prevents the associated genotype from reaching the next generation. Thus
the gene flow is modulated, and the "gene pool" will manifest itself in
somatically fitter individuals than we started with.
On the other hand, there are no "genes" for fitness
either. Indeed, it cannot be regarded as a somatic feature in the usual sense at
all. Fitness can only be defined operationally and retrospectively as far as
individual organisms are concerned. In particular, it cannot be fractionated
from anything. It is in fact a very mysterious concept; evolution in the
Darwinian sense would be unthinkable without it, but it has always given
evolutionists the greatest difficulty.
As we have seen, the introduction of the concept of
genotype amounted to posing a new causal category for talking about
somatic effects. That is, we could now answer a question of the form "why this
somatic character?" with the answer "because these genotypic
factors."
Biology today is furthermore completely committed to the
idea that phenotype is a chemical concept. In other words,
biological forms, and the morphogenetic processes which generate them, are
ultimately chemical, or biochemical in nature. This, it will be seen, is a far
cry from asserting an analogy of the kind sketched, above. This is
a reduction.
On the face of it, it is an astonishing claim. It is not
adduced on the grounds of great success in faithfully translating anatomical,
embryological, or physiological processes into a syntactic chemical language.
Quite the contrary, in fact, is true. It is adduced, rather, primarily on the
grounds that otherwise we simply could not answer "why"? questions about these
processes with a "because..." framed exclusively in terms of intrinsic,
fractionable (i.e., chemical) structure. That is, unless we identify
phenotype with biochemistry, we can no longer claim that the functional genetic
factors originally posited by Mendel can be identified with fractionable pieces
of chromosomal structure. Indeed; the viability of the entire machine metaphor
in somatic biology currently rests precisely here; on the identification of
phenotype with biochemistry. If it fails, then mechanism fails; but as we have
stressed many times before, the alternative is not vitalism, it is
complexity"
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