This serious work by Kirschner and Gerhart (K&G) is notable as it is a de facto take-down by eminent biologists at the most prestigious academic institutions of Darwin's theory. The bottom line notion implicit in K&G is that Darwin's theory will not work without major modifications being made to it. In its classic form, as stated by Darwin and as it has been understood for most of its history, the theory cannot plausibly account for novelty in life. Novelty would seem to be something a theory of evolution would account for. Forget about the complex eye that skeptics feel the theory incapable of accounting for. The eye-spot, that patch of light sensitive pigment thought to be the earliest forerunner of the complex eye, that patch that might have appeared on the worm-like ancestor of all bilaterians, its inception may not be explained by the theory. How the eye-spot morphed into the camera eye is fair game for the theory; not how the eye-spot got there in the first place. To try to determine how is, for Darwin, the equivalent of trying to explain the beginning of life. But if the eye-spot is off limits, why wouldn't the later-appearing neural tissue that connects itself to the eye-spot also be? Darwin did not want to tackle the birth of things, as he must have felt that explaining the evolution of all forms of life from their primitive beginnings was task enough. And with this, modern biology is in agreement, as its approach to evolution is defined in the often expressed mantra: nothing from scratch; everything from precursors.
The authors' grievance with Darwin goes beyond the issue of novelty. They find Darwin deficient in explaining the occurrence of any variation that would subserve the adaptive interests of the organism. Consider the theory, consisting of two overarchng notions, that of phenotypic variaton and that of selection. Heritable variants whose phenotypic expression boosts an organism's fitness (i.e., reproductive capability, broadly conceived) are maintained in a population. This is called selection. When this happens in the absence of man's intervention, it is known as natural selection (NS). Over time such selected heritable variants can accumulate in a population and produce large phenotypic change. This is called evolution. That's not all. Phenotypic variation has three key characteristics: (1) Variants are phenotypically small so that change produced by a single occurrence of selection is also small, thus meeting Darwin's requirement of being "insensibly fine," and the accumulation of many such changes has the appearance of being gradual, (2) Variants are random, like the outcome of a coin toss, the roll of dice, or the selection of a number in a lottery, and (3) Variants are unlimited or unconstrained--the possibilities are perhaps infinite; so the mega-lottery is the truest analogue of the variation situation. The problem for Darwin derives from the conception of variation as being blind and seemingly unfettered. Is it plausible, not to say possible, that unfettered, random variation receiving direction from only NS produced the diversity of life that we know? K&G think not.
NS can give direction once a, say, useful variant has occurred. After an eye-spot or a nerve (whether full-blown or incipient) has appeared, NS can come into play. But is it plausible that the useful variant, the incipient eye-spot or nerve, will occur, spontaneously, out of the blue, de novo, given any length of time, if variation is completely unconstrained. The authors think not. Of course, no one wants any part of having to explain the spontaneous appearance of something from nothing! But can evolution be explained without having to deal with the sudden appearance of something seemingly from nothing?
If variation (as in mutation) is completely free (and Darwin seems not to explicitly or implicitly rule this out), it is limitless or infinite. The probability of getting any one variant out of infinitely many possible ones by chance would be effectively zero. Let's return to the example of the eye-spot. Suppose that an eye-spot has appeared. Now what is needed is an optic nerve, correctly placed. Without constraint or bias or direction, what is the probability of such materializing? Commonsense tells one, "zero."
Again, Darwin does not talk of constraint on variation. Does this mean, recurring to our example, that if the eye-spot is in place the optic nerve is no more likely to appear near the eye-spot than it is near the ear or gut? If there is constraint, what is its nature? And quickly, an example from Origin. Lungs evolved by transitioning from a swim-bladder of a fish. Somehow the swim-bladder becomes divided by highly vascularized partitions. Were the chances of vascularization the same for every part of the organism, say, an ear lobe (sic), as for the interior of the swim-bladder? Probably not, but then why not?
It is obvious that there is constraint on variation--wings on lions just do not occur outside of the sphynx. It is not possible for anything to appear as a variation. And if there were not constraint or bias on variation, nothing would have evolved--life would have died at its inception (the reviewer opines). Constraint is widely recognized in biology where the notion is that in evolution nothing (with the possible exception of life) has been created from scratch. Instead one thing builds on that which already exists. A gene heretofore involved in eye production is not going to mutate into one concerned with limb production. Here is an excerpt that captures the authors' view on constraint on variation:
Evolution has been compared to a biased random walk because of the bias introduced
by selection, but we say phenotypic variation is itself biased. Rather
than staggering like a drunken sailor, evolution marches. . . taking forceful
steps [while] avoiding many lethal obstacles.
But constraint is a dangerous notion for science. What is the exact nature of the constraint? Let's be sure that with our use of it we are not giving admission to design or purpose or intelligence in disguise! One has to be careful that any such invasion is not inadvertently allowed. The authors have presented us this work because they believe they have a scientifically based explanation of the constraint of variation that makes evolution possible and plausible. They call their theory facilitated variation. I am going to comment on two of its proposals. This should suffice to give an idea of the theory and the reservations the reviewer has with it.
The first of these gives a major role in evolution to what the authors call somatic adaptation. This is adaptation that is not genetically based (if one can imagine such a thing). This is the same kind of thing we are dealing with in Lamarck's theory of the inheritance of acquired characteristics. It is illustrated by my doing strength training by lifting weights. My genome does not compel me to do this. It is adaptive behavior which somehow is free of the influence of the genes. What can happen when I reach my weight-lifting limit is that mutation can easily occur that stabilizes genetically those extreme efforts of mine to lift maximally that were somatic adaptations and hence not genetic. So this is clearly a proposal for giving direction to variation--genetic variation piggybacks onto somatic (nongenetic) variation. It is hard to do justice to a proposal as "tricky" as this one is.
The proposal is based on a dichotomy wherein characteristics of organisms are, in all or none fashion, either genetic or nongenetic. Thus, in the course of the discussion, the characteristic of some reptiles to have the sex of the individual determined by ambient temperature is said by K&G to be wholly environmentally determined. As if this could not be a characteristic evolved thanks to selection and thus a heritable one! But the idea being introduced here, that somatic (not heritable) variation becomes a target for the production of heritable variation and thus gives direction to variation, has problems of its own, without the issue of heredity and environment. How this comes about is not made clear (to the reviewer) and thus the notion's addition to Darwin's theory does not help the plausibility of that theory.
On the same matter, K&G make the discovery of somatic adaptability, plasticity and the like. They find more interesting and more important phenotypes that consist of a range of reactions, such as the ability to react adaptively to a range of temperatures, oxygen levels, nutrition levels and so on., as opposed to a fixed phenotype. However, by raising the issue of adaptability the authors thereby raise new questions that are difficult for Darwin. In discussions of natural selection, Darwinians talk about the sculpting of characteristics by selection. But what kind of sculpting is that which produces a huge range of possible phenotypes...a strange kind of modeling clay you have there. Yes, and human intelligence, a capacity for infinite phenotypes...another example of being sculpted, no doubt!
Darwin sometimes saw a world of infinite possibilities; there was no end to what organisms could evolve into. In actual fact, thanks to some of the most impressive work ever by biologists during the past several decades, we know that things are quite otherwise. Yes, there is a tremendous amount of variation and thus tremendous diversity in the forms of life. But it is far from being the case that anything is possible.
Almost as amazing as the diversity of life, is the commonality in diverse forms of life. The most basic molecular and cellular processes of life are shared by all forms of life, from bacteria to yeast to plants to humans. Three billion years ago, life first appeared in a bacterium-like organism. The creature that is the ancestor of all life had the same DNA, RNA, genetic code for making proteins, metabolism (based on the breakdown of sugars) as is possessed by all forms of modern life, humans, fungi, plants, and bacteria. Another suite of core processes appeared two billion years ago with the appearance of the eukaryote--sexual reproduction, nucleus, organelles, cytoskeleton. And more recently (600 million years ago), there appeared the first bilateral animal which is the ancestor of all bilaterians--arthropods, annelids and chordates, which last includes the vertebrates. There was in place in this tiny worm-like animal a body plan, conserved to this day, which included features such as bilaterality, a mouth in the head region, a throughgut leading to an anus, eyes or eye-spots in the head region, a heart, a capacity to build limbs. All of the listed characteristics, present in all present day bilaterians, are said to have been conserved.
The conserved processes are in aggregate another of the processes that gives direction or constraint to variation and thus expedites evolution. The most basic cell or molecular processes, DNA, protein, metabolism, are going to be the same as that found in bacteria through humans. Conserved body plans dictate the placement of organs such as limbs. Thus the placement of bilaterian limbs (arms, legs, wings, fins) is a constant for all bilaterians. Limbs will not appear on backs or heads. It is and will continue to be the same for quadrapeds, fish. reptiles, birds, primates. Mutation can certainly produce change, but only within the constraints imposed by the conserved core processes.
One of the most important recent (past fifty years) discoveries in the present regard is that of regulatory genes. These are genes that do not code for protein in the usual sense. They are not directly involved in the first line activities of the organism. Instead they regulate or supervise such activities by controlling when and where "working" genes turn on and thus how genes combine their activities. Instead of building eyes or limbs they oversee this activity. In the case of mutant forms of these genes, development will go awry. Over the 600 million years these genes have been in place, there has been no need to start from scratch in evolving a wing or an eye or a heart. And thereby, evolution has been greatly expedited
The body building genes and other regulatory genes are wondrous things. But they come at a price. They need to be explained, but they are not easy to explain. These genes exist, but how do we explain these wonders? There are generic eye building, limb building, heart building genes and genes that can lay down a grid coordinate system that allows for the correct placement of every cell in the embryo. Somehow in the molecular machinery (genes, proteins, etc.) there is implicit the generic concepts of organ of sight, heart and limb. The supervising gene for eye building is read properly by genes that do the actual building, whether the genes are building a mouse's or a fly's eye, very different eyes. The building of two entirely different structures is turned on by one gene that may be able to do the same for any eye you throw at it. Understand there is no brain here, no nervous system; only "dumb" molecules. Is no one in biology going to get up and proclaim this business marvelous, if not miraculous?
Further, come up with a scenario for how these wonders evolved! What did the phenotype look like at the very start? If the final product is a grid map of the body, what did the intermediate forms of the phenotype look like? Yes, and how was fitness boosted by "insensibly fine" (Darwin) changes in the phenotype? And what did the organism do before it was able to lay down a map that it could use for putting the right cells in their right locations? If there is skepticism about the evolution of an eye, how much more will there be when it comes to the evolution of super-regulatory genes?
The aim of the authors' effort is to breathe new life into Darwin's theory by making it plausible. My opinion is that the present effort has not had its intended effect.
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R. Downing
Good, but could be better
Reviewed in the United Kingdom 🇬🇧 on December 22, 2006
