Evolutionary Constraints, Natural Bioengineering, and ‘Irreducibility’
Many prior biopolyverse posts have concerned evolutionary themes, either directly or indirectly. In the present offering, we consider in more detail factors which may limit what evolutionary processes can ‘deliver’. More to the point, are there biological structures which we can conceive, but which could never be produced through evolution, even in principle?
It has been alleged by proponents of so-called ‘Intelligent Design’ (ID) that some features of observable biology are so complex that no intermediate precursor forms can be envisaged in a feasible evolutionary pathway. Of course, the hidden (or not so hidden) agenda with such people is the premise that if natural biological configurations of sufficient complexity exist such that they are truly ‘irreducible’, then one must look to some form of divine intervention to kick things along. In fact, all such ‘irreducibly complex’ examples proffered by such parties have been convincingly demolished by numerous workers with more than a passing familiarity with the mechanism of evolution.
These robust refutations in themselves cannot prove that there is no such thing as a truly evolutionarily irreducible structure in principle. What is needed, then, is not to attempt to find illusory non-evolvable biological examples in the observable biosphere, but to identify holes in the existing functional and structural repertoire as manifested by all living organisms collectively. Biological ‘absences’ could result from two broad possible scenarios: features which are possible, but not present simply due to the contingent nature of evolutionary pathways, and features which have not appeared because there is no feasible route by which they could arise. (Perhaps a third possibility would exist for ID enthusiasts, whereby God had inscrutably chosen not to create any truly irreducible biological prodigies). Of course, deciding between the ‘absent but possible’ and ‘absent and never feasible’ alternatives is not always going to be simple, if indeed it ever is.
The Greatest Show On Any Planet
Richard Dawkins has called it the Greatest Show on Earth. Sean Carroll used words of Darwin himself, “endless forms most beautiful”. These and many other authors have been struck by the incredibly diverse array of living creatures found in a huge variety of terrestrial environments. With the great insights triggered by the labors of Darwin and Wallace, all of this biological wonder can be seen as having been shaped and molded by the blind and cumulative hand of natural selection. And once understood, selective processes can be seen to operate in a universal sense, from single molecules to the most complex arrangements of matter, as long as each entity possesses the means for its own replication. It is for this reason that Darwinian evolution has been proposed as a universal hallmark of life anywhere, whatever form its replicative essence may take. While there may be few things which are truly universal in a biological sense (see a previous post for the view that molecular alphabets are one such case in point), it is hard to escape the conclusion that change through evolution and life go hand-in-hand, no matter what form such life may take.
So where do the ‘endless’ outpourings of biological design innovations ever reach some kind of end-point? There is a classic example that can be considered at this point.
It has often been claimed that a truly human invention unrepresented in nature is the wheel, and this absence has been proposed as a possible true case of ‘irreducible complexity’. At the molecular level, however, wheel-like structures have been documented. Three such cases are known, all rotary molecular motors: the bacterial flagellum, and two component molecular motors of ATP synthase. Remarkable as the latter structures are, it is of course the macroscopic level that people have had in mind when contemplating the apparently wheel-less natural world.
It will be instructive to make a brief diversion to consider what constraints might operate for a biological wheel design on a macroscale, and their general implications for the selection of complex systems. We can refer to a hypothetical macroscopic wheel-organ in a biological organism as a ‘macrobiowheel’, to distinguish it from true molecular-level rotary wheel-like systems. Although beyond the molecular scale, such an organ need not be large, and could in principle be associated with any multicellular animal. Such a postulated biological wheel structure could be used for locomotion in either terrestrial or aquatic environments, using rolling or propeller motion, respectively.
First there is a pseudo-example which should be noted. The animal phylum Rotifera encompasses the set of multicellular though microscopic ‘wheel animalcules’, rotifers, which superficially are characterized by a wheel-like locomotory organ in their aquatic environments. In fact, these ‘wheels’ are an illusory effect created by the sweeping motion of rings of cilia, and thus need not be considered further for the present purposes. Wheels of biological origin that can be unambiguously confirmed with the naked eye (or even a simple microscope) are thus conspicuous by their absence. Is this mere contingency, or strict necessity?
Re-inventing the Wheel
Let’s consider what would be required to construct a macrobiowheel. Firstly, one would have to define what physical features are required – is the wheel structure analogous to bone or other biological organs composed of hard inorganic materials? The problem of how blood vessels and nerves could cross the gap between an axle and a wheel hub has been raised as a seeming insurmountable constraint – but with some imagination potential solutions could be conceived. For example, the axle and bearings could be bathed in a very narrow fluid-filled gap, where vessels on the other side of the gap take up nutrients and transport them to the rest of the living wheel structure (a heart-like pump within the wheel might be required to ensure the efficiency of this, depending on the size of the hypothetical animal). Transmission of nerve signals might be more problematic; perhaps the macrobiowheel could be insensate, although this would presumably be a disadvantage. Conceivably, the same fluid-filled gap could also act as a ‘giant synapse’ for nerve transmission, such that perception of the state of the wheel structure is received as a continuous whole, without discrimination as to specific local wheel regions. (This would thus alert an organism to a problem with its macrobiowheel organ without specifying which particular part is involved; a better arrangement than no information at all). Another possibility is the use of perturbations in local electric fields as a ‘remote’ sensing device, as used by a variety of animals, including the Australian platypus. The rotational motion for the ‘drive axle’ might be obtained from successive linear muscle-powered movements of structures coupled to the axle by gear-like projections.
No doubt much more could be said on this particular theme, but that will be unnecessary. The issue here is not to indulge in wild speculation, but to make the point that it is uncertain whether a biowheel of any scale at the macro-level is an impossibility purely from a biological systems viewpoint alone. So perhaps we could be so bold as to claim that with sufficient ingenuity of design, a true macrobiowheel could be assembled in a functional manner. But having acknowledged this, the formal possibility that a macrobiowheel could exist is not at all the same thing as the question of whether a feasible pathway could be envisaged for such a structure to emerge in terrestrial animals by natural selection. The potential problems to be addressed are (1) too large a jump in evolutionary ‘design space’ (across a fitness landscape) is required; (2) [along with (1)] no selective advantage of intermediate forms is apparent; (3) [along with (1) and (2)] the energy requirements for the system may be unfavorable compared with alternate designs such as conventional vertebrate limbs (consider the problem as noted above of the non-linkage of the macrobiowheel circulatory system from the rest of the organism).
The first problem, the ‘design space jump’ conundrum, implicitly states that a macromutation producing a functional macrobiowheel would be a practical impossibility. In the brief speculation as to how such a biological wheel might be constructed, it is quite clear that multiple novel processes would be required; the macrobiowheel would need to be supported by multiple novel subsystems. Where a macromutation producing any one such subsystem is exceedingly improbable, the chances of the entire package emerging at once is effectively zero. So it is one thing to design a complete and optimized macrobiowheel; to propose a pathway for evolutionary acquisition of this exotic feature we must also rationalize ‘intermediate’ structures with positive fitness attributes for the organism. Thus even if one of the postulated macromutations should amazingly appear, it would be useless for an evolutionary pathway leading to macrobiowheels unless a fitness advantage is conferred. (As always, natural selection cannot anticipate any potential advantage down the line, but adaptations selected for one function may be co-opted for other functions later in evolutionary time). A depiction of the constraints on evolution of macrobiowheels is presented in Fig. 1 below.
Fig. 1. Representations of fitness landscapes for evolution of a locomotion system for a multicellular organism. Here the vertical axes denote relative fitness of expressed phenotypes; different peaks represent distinct genotypes. In all cases, dotted lines indicate ‘moves’ to novel genotypes that are highly improbable (gray) or proscribed through transition to a state of reduced fitness (red). A. In this landscape it is assumed that a macrobiowheel is inherently biologically possible. In other words, for present purposes it is taken that there exists a genotype from which a macrobiowheel can be expressed as a functional phenotype. Yet such a genotype may not be accessible through evolutionary processes. The conclusion of A is that even though a biological construct corresponding to a macrobiowheel is possible, it cannot feasibly arise naturally, since it is effectively impossible to cross an intervening fitness ‘valley’ in a single jump (A to X; gray dotted line), and transition to intermediate forms cannot occur through their lowered fitness relative to any feasible starting point (A to B or C (gray-shaded); red dotted line). In turn, transitions from B or C to peak X (purple dotted lines) cannot take place. It is also implicit in this schema that no other feasible pathway to configurations B or C exist. Thus, configuration (genotype) X is a true case of unattainable or ‘irreducible’ complexity. B, Depiction of a conventional evolutionary pathway whereby the same starting point as in (A) transitions to an improved locomotory arrangement through intermediate forms of fitness benefits.
So, ‘true irreducibility’ can result in principle from inability to create intermediate steps, universal pre-commitment to alternative design, or finally by an absolute incapacity to biologically support the proposed function. Also, the likelihood of a biological innovation acting as a fitness advantage is fundamentally dependent on the nature of the environment. Thus, with respect to our macrobiowheel musings, it has been pointed out that an absence of roads might counter any tendency for wheel-based locomotion to arise. It is not clear, though, whether an organism dwelling in an environment characterized by flat plains might benefit from wheel mobility, and in any case this issue is not relevant to macroscopic aquatic organisms and hypothetical wheel-like ‘biopropellers’ driven by rotary motion (as opposed to micro-scale real rotary bacterial flagella).
A Very Indirect Biological Route to Crossing Fitness Valleys
In a previous post concerning synthetic biology, it has already been noted that human ambitions for tinkering with biological molecules need not suffer from the same types of limitations which circumscribe the natural world. ….. So if a macrobiowheel is compatible with biological systems at all, humans with advanced biotechnologies could then in principle design and construct such a system. Such circumstances are schematically depicted in Fig. 2.
Fig. 2. Potential role of human intervention in the generation of ‘unevolvable’ biological systems, as exemplified here with macrobiowheels. Here the natural fitness landscape of Fig. 1 (orange trace) has superimposed upon it peaks corresponding to biological constructs of human origin. Since the human synthetic biological approach circumvents loss of low-fitness forms through reproductive competition*, ‘intermediate’ forms all are depicted here as having equal fitness. Thus, by human agency, intermediate forms B and C can be used as synthetic stepping stones towards the final (macrobiowheel) product, despite their non-viability under natural conditions (Fig. 1). Alternatively, should it be feasible at both the design and synthetic levels, ‘direct’ assembly of a genome expressing the macrobiowheel structure might be attainable (direct arrow to the ‘X’ peak).
*Note that this presupposes that completely rational design could be instituted, although in reality artificial evolutionary processes might be used to achieve the desired results. But certainly no third-party competitors would be involved here.
Construction of a macrobiowheel would serve to validate the hypothesis that such an entity is biologically possible. Also, demonstration of a final functional wheel-organ would greatly facilitate analysis of what pathways would have to followed if an equivalent structure was to evolve naturally. This would then consolidate the viewpoint that a true macrobiowheel is indeed biologically irreducibly complex. But since other structures and pathways might still exist, it would not serve as formal proof of the irreducibility stance in this case.
The ‘human agency’ inset of Fig. 2 has itself evolved from biological origins, just as for any other selectable attribute. Therefore, from a broad viewpoint, a biological development (human intelligence) can in itself afford an unprecedented pathway for the crossing of fitness valleys which otherwise would be naturally insurmountable. So whether we are speaking of exotica such as macrobiowheels or any other biological structures with truly ‘irreducible complexity’, then their existence could in principle be realized at some future time through the agency of advanced human synthetic biology. And given the current pace of scientific change, such times may arrive much sooner than many might believe.
Finally, we leave this theme with a relevant biopoly(verse) offering:
Biological paths may reveal
What evolution can thus make real
Yet beyond such constraints
And purist complaints
Could we make a true bio-based wheel?
References & Details
(In order of citation, giving some key references where appropriate, but not an exhaustive coverage of the literature).
‘….proponents of so-called ‘Intelligent Design….’ The ‘poster boy’ of ID is quite probably Michael Behe, of LeHigh University and the Discovery Institute. He is the author of Darwin’s Black Box – The Biochemical Challenge to Evolution (Free Press, 1996), and more recently The Edge of Evolution – The Search for the Limits of Darwinism (Simon & Schuster 2008).
‘…..all such ‘irreducibly complex’ examples proffered by such parties have been convincingly demolished…’ See Zuckerkandl 2006; also a National Academy of Sciences publication by a group of eminent biologists.
‘……a third possibility would exist for ID enthusiasts…..’ A personal perspective: A religious fundamentalist once asked me why there are no three-legged animals; he seemed to somehow think that their absence was evidence against evolution. Of course, the shoe is definitely on the other foot in this respect. If God created low-fitness animal forms that prevailed (among which tripedal animals would likely be included) , or fabulous creatures without any conceivable evolutionary precursors, then that in itself would be counted as ID evidence.
‘ Richard Dawkins has called it the Greatest Show on Earth.’ This refers to his book, The Greatest Show on Earth: The Evidence for Evolution. Free Press (2010).
“ Sean Carroll used words of Darwin himself, “endless forms most beautiful”. The renowned developmental biologist Sean Carroll published a popular book entitled Endless Forms Most Beautiful – The New Science of Evo Devo, which gives a wonderful overview of the field of evolutionary development, or how the development of multicellular organisms from single cells to adult forms has been shaped by evolution. Darwin referred to “endless forms most beautiful” in the final section of The Origin of Species.
‘….the blind and cumulative hand of natural selection.’ This is not to say that the complete structure of biological entities, from genome to adult phenotype, is entirely a product of classical natural selection, but the latter process is of prime significance. For a very informative discussion of some of these issues, and the influence of non-adaptive factors in evolution, see Lynch 2007.
‘……Darwinian evolution has been proposed as a universal hallmark of life anywhere….’ For a cogent discussion of the NASA ‘evolutionary’ definition and related issues, see Benner 2010.
‘……the wheel, and this absence has been proposed as a possible true case of ‘irreducible complexity’ ‘ See Richard Dawkins’ The God Delusion, Bantam Press (2006).
‘….animal phylum Rotifera……..’ See Baqui et al. (2000) for their rotifer site, which provides much general information and further references.
‘…….how blood vessels and nerves could cross the gap between an axle and a wheel hub has been raised as a seeming insurmountable constraint……’ | ‘….an absence of roads might counter any tendency for wheel-based locomotion to arise…..’ See again Dawkins’ The God Delusion, Bantam Press (2006).
‘……..the use of perturbations in local electric fields as a ‘remote’ sensing device, as used by a variety of animals, including the Australian platypus.’ For more background on electroreception, especially in the platypus, see Pettigrew 1999, and Pedraja et al. 2014.
‘……could exist is not at all the same thing as the question of whether a feasible pathway could be envisaged for such a structure to emerge ……. by natural selection.’ For an extension of this theme at the functional RNA level, see Dunn 2011.
‘ Fig. 1. Representations of fitness landscapes…..’ Further discussion of evolutionary problems in surmounting fitness valleys can be found in Dunn 2009. The title of Dawkins’ book Climbing Mount Improbable (1997; W. W. Norton & Co) is in itself a fine metaphor for how cumulative selectable change can result in exquisite evolutionary ‘designs’, which of course is the major theme of the book.
‘……advanced human synthetic biology….’ The ongoing role of synthetic biology in testing a variety of possible biological scenarios was also discussed in a previous post under the umbrella term of ‘Kon-Tiki’ experiments.
Next Post: April.