> By the way, I'm still interested in hearing the judgment of biologists
> regarding Denton's concept of 'directed evolution.' Denton's position is
> largely a *teleological interpretation* of the character of the universe's
> functional and formational economies, but it should, I believe, be
> evaluated also for its scientific merit, or lack thereof.
THE CONTINGENCY-vs-CONSTRAINTS CONTINUUM
Denton argues as if his concept, "directed evolution," is diametrically
opposed to "Darwinian evolution." But these two concepts are not two
monolithic entities. In fact, Denton is simply staking out one position
on a continuum of possibilities.
The question is:
To what extent did _historical_contingencies_ determine the pathway
which evolution took in the past (and will take in the future)?
That question has been debated for many decades. Since Darwin,
evolutionists have emphasized a strong role for historical contingency.
It was believed that evolution could have followed a wide variety of
pathways; the particular pathway it did follow was simply an historical
contingency. (Darwin's hypotheses about the mechanisms of biological
variations within species, along with the earliest theories of genetics,
favored a strong role for historical contingencies.)
It's long been known that there are also constraints which reduce the
importance of historical contingencies, constraints which can "direct"
evolution along certain pathways. Two of the most obvious constraints
are (1) the constraints of biochemistry (e.g. proteins can only fold in
certain ways) and (2) the constraints of the environment. Under certain
circumstances, one or both of those constraints could force evolution to
follow a singular path (or a small number of paths).
Gould argues that, historically, evolution has been highly contingent;
Denton argues that, historically, evolution has been highly constrained.
(Denton argues that evolution is so constrained that *all* of the major
features of biological history were pre-programmed into the basic laws
of nature). Obviously, Gould recognizes that there are some constraints
in evolution. Also, Denton recognizes that there are some historical
contingencies. For example, Denton believes that some of the variations
found in isolated-island species are due to historical contingencies.
THREE TYPES OF ARGUMENTS
Denton uses several types of arguments to persuade that all the major
features of biological history are constrained. (1) Denton cites many
particular examples of constraints. (2) Denton argues that the
complexity of life is evidence for constraints. (3) Denton argues "...
that the diversity of life on earth approximates to the maximal
diversity possible for carbon-based life." (This is what Denton calls
"maximal plenitude.")
(1) KNOWN EXAMPLES OF CONSTRAINTS ON EVOLUTION
Denton mentions numerous biochemical constraints on the development and
evolution of life. Many of his examples are fairly well-accepted by
biologists. The idea has been around for some time that certain
molecules (H20, CO2, DNA) are "uniquely suited" for their role in life.
The "one-gene / one-trait" model of genetics has been discarded. The
complex ways in which genes interact with each other imposes constraints
on evolution. Also, fundamental laws of physics impose certain
constraints. For example, they set limits on how large creatures with
exoskeletons can become, or on how large flying species can become and
still be able to fly.
Obviously, the environment can impose negative constraints on the kinds
of lifeforms that can exist. The environment can also impose a
"positive" constraint in the sense of providing specific environmental
niches for species to fill, thereby requiring specific adaptations.
(And so, for example, we have flying birds with wings, flying mammals
with wings, flying insects with wings, and even some fish with vaguely
wing-like fins who fancy short bits of flying now and then.)
Denton has an impressive list of physical and biochemical constraints.
They strongly suggest that the fundamental laws of nature are "fine
tuned" for life. However, a list of particulars does not, by itself,
prove the general case. A list of *some* constraints on evolution does
not prove that *all* major features of evolutionary history are
constrained. Denton acknowledges this. In order to make his case that
all of evolutionary history is directed, Denton adds the argument from
complexity.
(2) ARGUING FROM COMPLEXITY TO CONSTRAINTS
Denton uses biological complexity to argue that evolution must be highly
constrained (pre-programmed) rather than contingent. According to
Denton, contingent-evolution can only produce variations on a theme;
contingent-evolution cannot produce genuine novelty and certainly not
complexity. The *only* way complexity can evolve, according to Denton,
is via constrained-evolution. If a complex biochemical system requires
multiple interacting genes in order to function properly, Denton argues,
that feature could not have evolved unless a very specific evolutionary
pathway is pre-programmed into the laws of nature. Thus, Denton
concludes, the fact that complexity exists at all demonstrates that
evolution is constrained and pre-programmed.
I believe that Denton's complexity-implies-constraints argument is false
as a general argument about *all* types of evolving systems. Computer
simulations can produce the self-assembly of novel complexity both in
highly constrained simulations and in highly contingent simulations.
Might Denton's complexity-implies-constraints argument be valid when
restricted to biochemistry and biological evolution? Perhaps. We
currently lack the data to trace the evolutionary history of complex
biochemical systems. Once we have such data, Denton's claim could be
falsified if we could show that many complex biochemical systems could
have evolved by a variety of pathways. Alternatively, Denton's claim
could be falsified if Michael Behe's hypothesis is correct: that for
some complex biochemical systems there are *no* evolutionary pathways.
Suppose it turns out --- as Denton would predict --- that every complex
biochemical system has only one (or a few) evolutionary pathways which
could produce it. This would argue in favor of Denton's claim, but it
would not prove it. Even if the evolution of every complex system
requires following a particular, constrained evolutionary pathway, it
could be that the selection of *which* particular complex systems
happened to evolve was a contingent event. In other words, it is
possible that there is a huge variety of possible complex biochemical
systems --- each of which can only evolve along highly constrained
pathways --- but, out of all possible systems, only a few contingently-
chosen complex biochemical systems have *actually* evolved. The only
way to evaluate this possibility, scientifically, is to tackle Denton's
third argument: "maximal plenitude."
(3) MAXIMAL PLENITUDE AND CONSTRAINTS
Denton argues "... that the diversity of life on earth approximates to
the maximal diversity possible for carbon-based life." This is a very
difficult hypothesis to prove scientifically, difficult but perhaps not
impossible. "Darwinians" (as Denton uses the term) believe that the
space of all possible lifeforms, and of all possible evolutionary
pathways, is huge. Darwinians believe that the particular pathway of
evolution on earth was contingently chosen. But perhaps the space of
all possible lifeforms, and of all possible evolutionary pathways, is
sparse. Suppose that earth's biological history has already followed
all possible "major pathways" of evolution, producing all possible major
types of lifeforms. If we could prove this hypothesis, then Denton
would be scientifically correct. Evolutionary history would be, in all
its major features, pre-programmed by the fundamental laws of nature.
EVIDENCE AGAINST DENTON?
POSSIBLE EXAMPLES OF CONTINGENCY IN BIOLOGICAL HISTORY
Some elements of biological history appear to have been historically
contingent. I'll mention two examples. (Other list members can offer
other examples.)
Consider the wide variety of eye architectures found in nature. The
variety of eye architectures has often been used as an example of
historical contingency determining later evolutionary development. But
Denton has a way to fit this into his thesis --- via maximal plenitude.
If evolution has produced not just *some* of basic varieties of eye
architectures, but *every* basic type of eye architecture, then we have
reached maximal plenitude. In that case, the evolution of eye
architecture is constrained after all, and not contingent. I think
Denton's argument is valid --- but it has yet to be shown whether or not
it is actually true.
Another oft-quoted example of (apparent) contingency in biological
history is the extinction of the dinosaurs. Dinosaurs existed for
hundreds of millions of years, then were wiped out by an ecological
crisis which left a few (comparatively humble) mammalian species alive.
These mammals, in a mere 70 million years, produced species with far
greater intelligence than anything the dinosaurs every produced in
hundreds of millions of years. If, as Denton believes, the evolution of
homo-sapiens-like creatures is inevitable, why didn't the dinosaurs do
it, and why did it take such a peculiar ecological disaster (strong
enough to wipe out the dinosaurs, but weak enough to leave some mammals)
to open the way for it? Denton doesn't tackle this particular question
in his book. I expect he'll have to tackle it. The historical role of
environmental catastrophes and mass extinctions may prove very troubling
to Denton's claim.
ONE PARTICULAR CASE:
DEVELOPMENT OF NERVOUS SYSTEMS: CONTINGENT OR CONSTRAINED?
Is there evidence that the laws of nature are designed so that
abiogenesis will inevitably produce single-celled organisms with lipid
membranes and DNA? Is there evidence that the laws of nature are
designed so that these simple organisms will eventually combine to form
multi-celled organisms? I'll let other scientists comment on those big
questions. But I will speculate on a topic I know a little more about:
nerve cells and ion channels.
All animals, except the simplest, have nerve cells. The importance of
these specialized cells is obvious. Is the evolution of nerve cells an
historically contingent event, or was it constrained? Was the evolution
of nerve cells "built in" to the laws of nature?
In order to electrically transmit information, nerve cells require quite
a few different kinds of ion channels. (Most nerve cells use several
dozen types of ion channels. It's hard to imagine a functioning nerve
cell without at least a half a dozen differently specialized ion
channels.) Many ion channels are selective for a specific ion.
Different ion channels activate and de-activate at very different
speeds. They activate and de-activate and in response to very different
voltage and chemical stimuli.
The simplest single-celled organisms need a few ion pumps and/or
channels in order to regulate their intracellular environment. So let's
take that as our starting point. Starting there, how can we get to the
wide variety of specialized ion channels necessary for nerve cells to
function? Sequence homology data suggests that modern ion channels can
be traced back to a few "ancestral" ion channels, with gene duplication
(followed by mutation) producing the increased variety of channels found
in modern species. Moreover, it takes surprisingly few strategically
placed mutations (in some cases, only one point mutation) to radically
alter a channel's ion selectivity, kinetics, ligand sensitivity, and/or
activation voltage. These data suggest to me that gene duplication and
mutation could --- indeed, would --- produce the variety of ion channels
necessary to make nerve signaling possible. The particular amino acid
sequences of modern ion channels may be highly contingent, but the
evolution of a sufficient variety to make nerve conduction possible
seems inevitable.
So if the laws of nature really are designed so that abiogenesis
produces single-celled organisms with lipid membranes and DNA and the
minimal necessary ion pumps/channels for survival, and if the laws of
nature really are designed so that these will eventually evolve into
multi-celled creatures, then --- based on the available data --- I would
be willing to speculate that the laws of nature are also designed to
produce, inevitably, creatures with nervous systems.
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I'm tempted to make some teleological and theological comments, but this
post is already too long, so I'll defer.
Loren Haarsma