What is an organism?:A discussion
|Fig1.The Life cycle of Acetabularia acetabulum,showing the different stages of morphogenesis including the production of the transient verticils (whorls of laterals).|
This suggestion comes from modelling studies. We observed
that once the parameters in the equations describing growth and morphogenesis
were set so that localized tip formation could occur, subsequent changes
of form happened spontaneously giving a sequence of whorls. Genes can be
regarded as determining parameter values, so they play an important role
in setting up the dynamics. But then the organization of the system takes
over and leads to whorls without further changes of parameter, morphogenesis
apparently leading to a generic form in what Gould ( 1991 ) calls morphospace,
the space of potential forms. Whorls of order to which Acetabularia
belongs, the Dasycladales, have been around for at least 570 million years.
In most species the whorled laterals act as gametophores, where gametes are
produced. Only in the Acetabulariaceae are they supplanted by caps
as the gametophores, though whorls continue to be made. What this suggests
is that such high-level taxonomic characters are robust consequences of
morphogenesis typical of a particular type of organism, due to their basic
organisation (described in a particular type of morphogenetic field). So,
the Acetabulariaceae have whorls not because they are useful (adapted)
but because they are typical of the organization of the order to which they
belong. Genetic differences can result in variations on this theme; but beyond
a certain range of genetic change the whole morphogenetic sequence changes
and a different generic form arises. This then defines another taxonomic
group. Evolution at this level is transformation between generic forms.
|Fig2. An algae with three whorls and the beginnings of a cap.|
So, my conclusion is that organisms are not survival machines constructed by selfish genes, machines whose behaviour is dictated by natural selection. They are self- generating wholes, complex systems that obey principles of dynamic order described by field equations. These dynamic forms use their DNA to specify parameters of the process so the parameters replicate along with the life cycle and result in repeatability of the morphogenetic process. The whole reproductive cycle must be stable if the species is to survive. So we accommodate all of the basic properties of genes and the exigencies of natural selection, but within the context of a theory of the organism and its , life cycle as the fundamental entity in biology. As defined, organisms are not machines, they are not reducible to genes and gene products, and their properties are not explained by natural selection. They must be understood as self-generating, reproducing structures of a distinct kind whose transformations define the range of living forms available for evolution.
RICHARD DAWKINS : ORGANISMS MAY BE WHOLES BUT ONLY GENES ARE REPLICATORS
What is a motor car? A motor car is a means of personal transportation.
No it isn't; a motor car is a metal box with padded seats, a wheel at each corner and an internal combustion engine.
No no, you're both wrong; a motor car is the product of a factory where it is assembled from parts separately manufactured, usually on a production line, to a design drawn on a drawing board.
Nonsense, a motor car isn't any of those things. A motor car is a status symbol ... the curse of the age ... a boon to the country doctor ... the single greatest contributor to atmospheric pollution ... where most American teenagers lose their virginity ... a lethal weapon in the wrong hands ... a horseless carriage, the twentieth century descendant of the coach and four ... a death-trap on ice ....
I hope it is clear that the only thing seriously wrong with each of these answers is the No , that begins it. A motor car can be more than one thing at a time. In particular, we can recognize my first three answers as the functional answer, the structural answer and the developmental answer. All three are true. All three are important. None can, in any sense whatsoever, be regarded as a substitute for either of the others. They are three distinct truths, simultaneously true about the same object.
The same applies to the question, "What is an organism?" Goodwin may be right that organisms are "self-generating wholes, complex systems that obey principles of dynamic order described by field equations." But I am simultaneously right that organisms are survival machines constructed by selfish genes that have weathered natural selection. Brian Goodwin is an embryologist. The questions he asks are development questions. These are important questions, and his answers are clever and possibly correct answers. But there are other questions that are simultaneously interesting and worthwhile- If Goodwin's answer to his developmental question is right, this may imply that other answers to the same question are wrong. But it does not imply that answers to different questions are wrong. For the sake of argument, let' s accept Goodwin' s characterization of organisms as self- generating wholes governed by field equations. Suppose Goodwin is right that the organism develops as "a functional and a structural unity, in which the parts exist for and by means of one another" as opposed to a system, like a car in which the parts "pre-exist before being assembled into a functioning unity." Well and good, but, as we shall see, some of us might feel the need to ask why natural selection has favoured the one kind of embryology over the other. The explanation will be a Darwinian explanation. According to our modem understanding of Darwinism, that means an explanation in which genes fostering the one embryology survive better than genes fostering the other.
But what, in my world view , is so special about genes anyway? If Goodwin is right that they play only a minor role in determining the forms of organisms, minor in comparison to the principles of dynamic order, why thrust genes to centre stage? The reason is that genes have a unique role to play in functional explanations. Genes do two quite different things. They affect embryonic development within an individual, and they replicate themselves down the lineages of different individuals. Replicating entities are sifted by natural selection and, if they are successful in the sifting, they potentially go on replicating forever. Given spontaneous variation due to mutation, the world becomes full of successful genetic replicators at the expense of unsuccessful ones. But what makes some genetic replicators successful and others not? What do the winners have that the losers lack? This is where the other main activity of genes comes in, their embryological influence. Genes are successful, or unsuccessful, because of their effects on embryonic development. Unlike the blueprints of a car, genes have to ride inside the vehicles whose development they influence. They have to live with their mistakes. In their mistakes, even! If the vehicle falls over a cliff, so do they . If the vehicle is devoured by a leopard, so are they. As the generations go by, therefore, the world automatically becomes full of copies of genes whose products don't fall over cliffs and are not eaten by leopards. Every creature born is descended from an unbroken line of successful ancestors, and they inherit what it takes to be successful.
The business of surviving to reproduce becomes extremely complicated, especially with enemies simultaneously evolving ever more ingenious tricks of predation, parasitism, escape. Not surprisingly, in the course of these arms races, genes have evolved ever more complicated mechanisms for staying alive and reproducing: survival machines.
And why do we say that these survival machines work "for" the organism's genes, rather than for, say, its teeth or ankles? After all, if the organism's ankles fail and it stumbles in the path of a leopard, the ankles perish along with the genes. Ankles, too, have to live with their mistakes. Why isn't the organism a survival machine for ankles? The answer is simple. Genes are replicators. Ankles are not. You may be tempted to think that ankles are replicators, because .they reappear in every generation. But your ankles are not replicas of your parents' ankles. This is not only because you have two parents. It is also because changes in ankles, unlike changes in genes affecting ankles, are not inherited. If they were, babies would be born with ready-sprained ankles covered with varicose veins. But "sprain" a gene, for example with X-rays or a mutagenic chemical, and the baby really will inherit the altered form. What is special about genes in evolutionary explanations is that, amid the welter of complexity that enters into the development of a body, genes and only genes (setting aside the inheritance of cultural tradition) have the potential for being replicated down the generations with errors that are themselves replicated. That is why bodies are survival machines for genes.
Brian Goodwin does have an interesting point to make. Just as the whirlpool in the bath-waste settles down into one of two stable spiral patterns, and the only freedom for variation is whether it is clockwise or anticlockwise, so body form may be similarly constrained. There may be a limited set of alternative stable states, and genetic mutation may be free only to kick embryonic development from one to another of them. Everything that is interesting and beautiful about a whirlpool comes from the physics of vortices, and all that your foot can do when you get out of the bath is nudge it towards the clockwise or the anticlockwise mode. In the same way, it might be that all that is interesting and beautiful about living organisms comes from field equations, not from differences between genes, which can only nudge the organism's form into the animate equivalent of a clockwise or anticlockwise spiral.
But Goodwin, like his predecessor D'Arcy Thompson ( 1942), is wrong to think that his views are in any way anti-Darwinian. Even if Goodwin's vision were true in its extreme form it would still not undermine the statement that adaptations, to the extent that they are for the benefit of something, must be for the benefit of self- replicating entities-genes. Field equations, unlike genes, do not exhibit heredity. Whirlpools don't beget lineages of daughter whirlpools that inherit their particular attributes- If they did, they would be true replicators and might well engender an evolution of their own- In any case,I am pretty sceptical of Goodwinian embryology in its extreme form. This is because living organisms are so beautifully and elaborately "designed." No doubt embryology does impose some constraints on what mutation and selection are allowed to get away with: doubtless there are some "attractors in morphospace." But it seems intuitively clear that organisms must be miles away from the "whirlpool" end of the spectrum, simply because they are so functionally elegant. Organisms are finely tuned to survive, honed to the last detail. If they were subject to the sort of constraints that hem whirlpools in, organisms wouldn't be so exquisitely good at what they do. Just open any page of, say, Cott's (1940) Adaptive Coloration or Wickler's (1968) Mimicry, Hansell's (1984) Animal Architecture or, for that matter, Darwin's ( 1882) Orchids, and ask yourself if this sort of thing could be achieved by highly constrained, Goodwinian embryology.
But now here is a more constructive overture towards Brian Goodwin. I have represented "whirlpool embryology" as a constrained embryology. Let me change the metaphor to show what a difference it makes: let's consider "kaleidoscopic embryology". Because of the arrangement of mirrors or prisms in a kaleidoscope, random heaps of coloured chips cannot help looking pretty. This is thanks to the constraint of radial symmetry. Random mutations-knocks on the barrel that change the positions of the chips-set off intricate and elegant changes at all corners of the radically symmetrical pattern. We can say that the shapes are constrained by the mirrors to be radically symmetrical, but "constrained" sounds too negative. We can also regard the mirrors as positively designed to make the patterns more elaborate and captivating, perhaps via symmetry in various planes (in which case the metaphor of the kaleidoscopic mirror is especially apt), or perhaps via segmentation or other kinds of controlled repetition (Dawkins, in preparation).
In my essay The Evolution of Evolvability (Dawkins 1989), I have suggested that natural selection has not only favoured genes that make organisms good at surviving and reproducing. It has also favoured embryologies that are , "good at evolving." To suggest that mutations could anticipate future changes in the environment is rightly regarded as heresy (Williams, 1966). But it is not out of the question that some form of selection could pick out those embryologies that - perhaps through being kaleidoscopic - have proved evolutionarily fruitful. It is still right to regard each new mutation as random with respect to function. But mutations have to work by altering the existing embryology. And not all embryologies are equally pregnant with evolutionary responsiveness, equally luxuriant in mutational possibility. Perhaps natural selection has fostered "kaleidoscopic" embryologies because of their evolutionary plenitude. If organisms develop "as self-generating wholes, complex systems that obey principles of dynamic order described by field equations," it is because natural selection has favoured this kind of embryology in the past. Perhaps it has done so not just because such embryologies make organisms that are good at surviving and reproducing. Perhaps, I am suggesting, this kind of embryology is favoured because it consistently unleashes cascades of potent mutations-it is good at evolving. Instead of belittling Darwinism, I think that Brian Goodwin should see his version of embryonic development as one of Darwinism's greatest achievements and the key to the blossoming of further great achievements.
BRIAN GOODWIN: GENES MAY BE REPLICATORS BUT IT IS ORGANISMS THAT EVOLVE
Richard Dawkins' modified position is so reasonable that I am tempted to say that we are agreed in principle but disagree over details and emphasis. A transformed Darwinism in which organisms are recognized as dynamic wholes with distinctive principles of organization that underlie their capacity to develop, reproduce, and evolve is an enormous step away from the reductionism of selfish genes and their survival in gangs that cooperate just to leave more copies of themselves. It brings biology into line with mainstream contemporary studies of complex systems such as fractals and chaos in which it is recognized that, underlying what appears to be kaleidoscopic variety of form and behavior there are principles of order and organization that make the diversity intelligible.
This is the position I am arguing for: that organisms, despite their remarkable diversity of form, belong to a class of organized dynamical system with deep principles of order that cannot be understood in terms simply of gene activities, nor be explained as the result of natural selection. I have no desire to ignore or belittle the importance of genes, but they have to be understood in context, which is the dynamic form we call an organism. Gene activities define molecular composition, which specifies parameters such as kinetic constants of reactions via enzymes, binding constants of calcium- regulating proteins, and the viscous and elastic constants of cytoplasm; natural selection relates to the stability of life cycles. Important as these factors are, even together they are not sufficient to explain the properties of organisms and their diversity as members of the particular class of dynamical system we call living.
There are ways of studying and classifying the members of this set, which define different species, by experimental and theoretical methods. I described one approach briefly in relation to experimental, mathematical and computer modelling of the species Acetabularia acetabulum, giving an explanation for structures that are not easily explained by natural selection but are readily understood in terms of the developmental dynamics of this organism. If Dawkins' "kaleidoscopic embryology" means acknowledging the study of whole organisms and their transformations (developmental and evolutionary) as a basic and fundamental biological enterprise, with genes specifying essential factors (parameter values and other aspects of the dynamics) of life cycles but not themselves sufficient to explain orgasmic form and behavior, then we're agreed in principle. However, let' s not fudge the issue: this description of organisms inverts the emphasis from that in Neo-Darwinism, which focuses on inheritance and natural selection and largely ignores the generative principles of organisms and their life cycles (dynamics). In the biology I'm proposing, nothing of value in Neo-Darwinism is lost, but whole organisms as life cycles are the irreducible entities that evolve, with genes and natural selection as aspects of these dynamic cycles and their stability in particular environments. For a comprehensive evolutionary theory it is necessary to go further than this, into the dynamics of ecosystems, but these should be constructed also out of interacting organisms (life cycles) and their environments, not out of genes.
Richard Dawkins' final remarks make it clear that he stops short of an organism-based biology, continuing to see natural selection as the explanatory principle of evolution, not the dynamics of organisms and their interactions. Natural selection has favoured embryologies that are , "good at evolving," he says. But this is inevitable: those systems that generate more stable dynamic processes will be more numerous, whether in biology, chemistry, or physics. It is just the stability argument, used at another level: a system with the potential of producing many different stable states rather than just a few will have more dynamical "offspring" that "survive." Hence it will be better at evolving. But we need to describe the actual dynamics of such biological systems, the developmental dynamics of life cycles and other aspects of biological organization, to understand their intrinsic order and what organizational properties underlie differences of potential for evolution. A recent and very interesting conjecture is that there is a dynamic attractor in complex systems "at the edge of chaos," which is the "best" place for evolving systems to be (Kauffman, 1993).
There is a problem in Dawkins' argument that I must consider, relating to adaptation- He sees adaptation as the result of the selection of genes by nature. The problem is that how we think about adaptation is also a result of selection, the selection of examples of adaptation by Darwinian biologists. While there are plenty of instances of remarkable adaptations, from the striking mimicry by one butterfly species of another's wing pigment patterns to the perfection of the eyes of cats and owls, there are as many non-adaptations that seem to cause no survival problems. I have already mentioned the whorls on Acetabularia. In addition, there are wasps of several families that spend most of their time under water, using their wings to swim. These wasps look just like their land-dwelling relatives. No adaptations here. [That can't be true.In order to breathe underwater there must be some adaptation from an air breathing variety. -LB]
Similarly, flowering plants have taken over most of the earth, equipped with superior morphological and reproductive mechanisms, yet ferns, horsetails, clubmosses, and their relatives with the relatively primitive structures of 200-million-year-old ancestors "compete" successfully.
[Such "competition" is misunderstood if niches are accounted for.If there is no threat to a niche or an organism develops a good strategy in the first instance there is little need to update it.Certain species of crab still look similar to those though to have existed in prehistory.The Nautilus seems unchanged when compared to fossil versions.If an organism has found a quiet cul-de-sac,there is little requirement for change.Competition happens where there is dispute over a niche.-LB]
It looks more like anything goes that works well enough, rather than constant fine-tuning of species to habitats, the expectation of competition and adaptations whereby species are constantly climbing hills in fitness landscapes to out-do their competitors.
Finally some of the Neo-Darwinists' favourite examples can be explained in developmental terms. "Adaptations" such as butterfly mimicry can most readily be explained by constraints on development, which have the consequence that only a limited number of pigment patterns is possible. So it is not difficult for one species to develop a pattern similar to that of another. The phenomenon of pseudomimicry greatly strengthens this view . In this case, species that inhabit totally different regions-such as Puerto Rico and Indonesia- nevertheless have similar pigment patterns. What advantage could this possibly give them? Understanding the principles according to which pigment patterns develop on wings as studied by F. Nijhout (1991), provides ways of explaining why both types of mimicry are not only possible, but probable. Again, we have to look to the organism for answers to all of these questions. When we have done this systematically , biology is going to look like a rather different science from its present form.
Offprint 1 From Perspectives in Ethology, Vol. 2, Behavioural Design, edited by N. S. Thompson.
Cassirer, E. ( 1981 ) Kant's Life and Thought, Yale
University Press, p. 336.