You can think of a symmetry rule in embryology as a restriction or ‘constraint’. So it is, in the strict sense that the unconstrained embryology is theoretically capable of producing symmetrical forms as well as asymmetrical ones and is therefore more prolific of forms. But we shall see in this chapter that the symmetry constraint can turn out to be an enrichment: the very opposite of a restriction. The trouble with the unconstrained embryology is that it needs to run through a myriad of forms before, by luck, a symmetrical one will chance to occur. And even then, the long-awaited symmetry will be constantly menaced by future generations of mutation. If, no matter what else may vary, symmetry is nearly always going to be desirable, the {226}
Figure 7.1 Biomorphs constrained by different numbers of ‘kaleidoscopic mirrors’, and therefore showing various kinds of symmetry. {227}
constrained embryology will be much more ‘productive’, just as it is more beautiful to our eyes. Unlike the unconstrained embryology, it won't waste time throwing up asymmetrical forms which are non-starters anyway.
As a matter of fact, the great majority of animals, ourselves included, are largely, though not entirely, symmetrical in the left — right plane. Beauty itself is not important so we must ask why, in a utilitarian sense, left — right symmetry should be a desirable characteristic. There are some zoologists wedded to an eighteenth-century idea that, with respect to major architectural features like symmetry, animals are the way they are because of some almost mystical loyalty to a ‘fundamental body plan or Bauplan. (Bauplan is just the German word for blueprint. Typically, one switches languages to indicate profundity — ‘these tuba notes from the depths of the Rhine’, as Sir Peter Medawar sarcastically put it. But actually, if I may be allowed a parenthetic in-joke, there is irony in ‘blueprint’ since it suggests a ‘reductionistic’, one-to-one relation between plan and building which would in a genetic context offend the ideological sensibilities of the very people who are most fond of the word Bauplan.) I prefer the Anglo-Saxon simplicity of my colleague Dr Henry Bennet-Clark, with whom I have discussed these matters: All questions about life have the same answer (though it may not always be a helpful one): natural selection.’ No doubt the detailed benefits of left — right symmetry vary in different animal types but he makes general suggestions too, along the following lines.
Most animals are either worm-like, or are descended from worm-like ancestors. If you think about what it is like to be a worm, it makes sense to have the mouth at one end — the end that hits food first — and the anus at the other so that you can leave waste products behind instead of inadvertently eating them. This defines a front and a rear. Then, the world usually imposes a significant difference between up and down. The minimal reason for this is gravity. In particular, many animals move over a surface such as the ground or the sea bottom. It is sensible that, for all sorts of detailed reasons, the side of the animal nearest the ground should be different from the side nearest the sky. This defines a dorsal (back) and a ventral (belly) {228} side and therefore, given that we already have a front end and a rear end, we now also have a left and a right side. But why should the left and the right sides be mirror images of each other? The short answer is, why not? Unlike the front/rear asymmetry and the up/down asymmetry which have good justifications, there is no general reason to suppose that the best shape for a left side will be different from the best shape for a right side. Indeed, if there is a best way for a left side to be, it is reasonable to assume that the best right side will share the same qualities. More specifically, any major departure from left/right mirror symmetry may result in the animal going round in circles when it should be pursuing the shortest distance between two points.
Given that, for whatever reason, it is desirable that left sides and right sides should evolve together as lock-stepped mirror images, embryologies that are ‘kaleidoscopic’ with a single ‘mirror’ down the midline will have an advantage. New mutations that are any good will then automatically be reflected on both sides. What is the non-kaleidoscopic alternative? An evolving lineage might achieve first a beneficial change on, say, the left side of the body. Then it would have to wait through many generations of asymmetry for a matching rightside mutation to turn up. It is easy to see that a kaleidoscopic embryology might well have an advantage. Perhaps, therefore, there is a kind of natural selection in favour of kaleidoscopic embryologies of increasingly restrictive — yet correspondingly productive — character.
This is not to say that left — right asymmetries can never evolve. Mutations do occasionally arise that affect one side more strongly than the other. There are special reasons why asymmetric mutations are sometimes desirable, to fit the abdomens of hermit crabs into coiled shells, for instance, and natural selection does then favour them. We have already met flat-fish such as plaice, sole and flounders, in Chapter 4 (see Figure 4.7). Plaice have settled down on their originally left side, and the left eye has migrated round to the ancestrally right, now upper, side. Sole have done the same thing, except that they lie on their right side which may, though not necessarily, indicate that they evolved the habit independently. The plaice's ancestrally left surface has become the functionally lower, bottom-hugging skin and, {229} appropriately, it has become flat and silvery. The ancestrally right surface has become the functionally upper, sky-pointing one and it has correspondingly become curved in shape and camouflaged in colour. The ancestral dorsal (back) and ventral (belly) sides have become the functionally left and right sides. Their respective fins, the dorsal fin and the anal fin, normally so different, have become almost exact mirror-images, as functionally left and right fins. The rediscovered left-right symmetry of plaice and sole is, in fact, a good advertisement for the power of natural selection as opposed to continentally fundamental body plans. It would be interesting (and feasible) to discover whether mutations in plaice are automatically mirrored on the (new) left and right sides (that is, the old dorsal and ventral sides). Or are they still, following the ancestral pattern, automatically mirrored on the (old) left and right sides (now lower and upper)? Has the difference between the silvery and the camouflaged side of a plaice been won in the teeth of a hostile old kaleidoscopic embryology, or with the aid of a friendly, new kaleidoscopic embryology? Whatever the answer to these questions, it serves to illustrate the point that ‘hostile’ and ‘friendly’ (to evolution) are appropriate words to use about an embryology. Once again, dare we suggest that a kind of higher-level natural selection might act to improve the friendliness of embryologies to certain kinds of evolution?
From the perspective of this chapter, the important thing about left — right symmetry is that any one mutation exerts its effects simultaneously in two places on the animal instead of one. That is what I mean by kaleidoscopic embryology: it is as though the mutations are mirrored. But left — right symmetry is not the only kind. There are other planes in which mutational mirrors might be set. The biomorphs in Figure 7.1c are symmetrical not only in the left — right plane but in the fore-and-aft plane too. It is as though there are two mirrors set at right angles. Real creatures with this ‘two-mirror embryology’ are harder to find than left — right symmetrical ones. Venus s girdle, a ribbon-shaped plankton swimmer of the unfamiliar phylum of ctenophores or comb jellies, is a spectrally beautiful example. More commonly, kaleidoscopic embryologies can be found which conform to four-way symmetry, like the biomorphs of Figure 7.1d, {230} Many jellyfish exhibit this pattern of symmetry. Members of their phylum either swim in the sea (like jellyfish themselves) or are moored to the bottom (like sea anemones), so they are not subject to the fore-and-aft pressures that we discussed for crawling animals like worms. They have every reason to possess an upper and a lower side, but they lack the pressure for a front and rear, or for a left and right. Looked at from above, therefore, there is no particular reason for any one point of the compass to be favoured over any other and they are, indeed, ‘radially symmetrical’. The jellyfish in Figure 7.2 happens to be four-way radially symm
etrical, but other numbers of radii are common, as we shall see. The picture, like many in this chapter, was
Figure 7.2 A four-way symmetrical animal: a stalked jellyfish. Note that each of the four axes is also left — right symmetrical about itself, so most variation is actually mirrored eight times. {231}
drawn by the celebrated nineteenth-century German zoologist Ernst Haeckel, who also happened to be a brilliant illustrator.
Animals with this kind of symmetry are capable of enormous variety of form, but with a limitation which, I am again suggesting, may turn out to be not so much a limitation as a ‘kaleidoscopic’ enhancement. Random changes affect all four corners simultaneously. Since, at the same time, the units that are four times repeated are themselves often mirrored, each mutation is actually repeated eight times. This is very clear in the case of the stalked jellyfish in Figure 7.2 which has eight little tufts, two per corner. Presumably a mutation in tuft shape would manifest itself eight times. To see what radial symmetry would look like in the absence of this additional doubling up, look at the biomorphs in Figure 7.1e. It is quite hard to find real animals with this kind of ‘swastika’ or ‘Isle of Man’ symmetry, but Figure 7.3 shows the kind of thing we are looking for. It is the spermatozoon of a crayfish.
Most radially symmetrical animals, however many radii they may have, add left — right mirror symmetry within each radius. Therefore, from our point of view of counting the number of times a given mutation will be ‘reflected’, it is necessary to count the number of radii and then double it. A typical starfish, since each of its five arms is left — right symmetrical, can be said to ‘reflect’ each mutation ten times.
Haeckel was particularly fond of drawing single-celled organisms, such as the diatoms of Figure 7.4. Here we see kaleidoscopic
Figure 7.3 ‘Isle of Man symmetry’: the spermatozoon of a crayfish. {232}
Figure 7.4 Diatoms — microscopic single-celled plants — illustrating different numbers of kaleidoscopic mirrors within one group of organisms. {233}
symmetries with two, three, four, five and more ‘mirrors’, in addition to the left — right mirror within each arm. For each kind of symmetry, the embryology is such that mutations act not in one place but in some fixed number of places. For example, the five-pointed star near the top of Figure 7.4 might mutate to produce sharper points. In this case all five points would go sharper simultaneously. We wouldn't have to wait for five separate mutations. Presumably the different numbers of mirrors are themselves (much rarer) mutations of one another. Perhaps a three-pointed star might occasionally mutate into a five-pointed star, for example.
For me, the champions of all microscopic kaleidoscopes are the Radiolaria, another planktonic group to which Haeckel paid special attention (Figure 7.5). They too illustrate beautiful symmetries of various orders, equivalent to kaleidoscopes with two, three, four, five, six and more mirrors. They have tiny skeletons made of chalk with a beauty and elegance that has kaleidoscopic embryology written all over it.
The kaleidoscopic masterpiece in Figure 7.6 might have been designed by the visionary architect Buckminster Fuller (whom I was once privileged to hear, in his nineties, lecturing for a mesmerizing three hours without respite). Like his geodesic domes it relies for its strength on the structurally robust geometric form of the triangle. It is clearly the product of a kaleidoscopic embryology of a high order. Any given mutation will be reflected a very large number of times. The exact number cannot be determined from this picture. Other Radiolaria drawn by Haeckel have been used by chemical crystallog-raphers as illustrations of the regular solids known since ancient times as the octahedron (eight triangular facets), the dodecahedron (twelve pentagonal facets) and the icosahedron (twenty triangular facets). Indeed D'Arcy Thompson, whom we met in connection with snail shells, would have argued that the embryologies of these exquisite Radiolarians have more in common with the growth of crystals than with embryonic development in the normal sense.
In any case, single-celled organisms such as diatoms and Radiolarians necessarily have a very different kind of embryology from many-celled ones, and any resemblance between their kaleidoscopes will {234}
Figure 7.5 Radiolaria. More examples of different numbers of kaleidoscopic mirrors of symmetry in a group of microscopic, single-celled organisms. {235}
Figure 7.6 A large and spectacular Radiolarian skeleton.
probably be coincidental. We've already seen an example of a four-way symmetrical many-celled animal, a jellyfish. Four, or a multiple of four, is common among such medusae, and it is presumably easy to achieve by simple duplication of some process during early embryology. There are also six-way symmetrical medusae such as those from the hydroid group known as trachymedusae (Figure 7.7).
The most famous exponents of five-way symmetry are the echino-derms — that great phylum of spiny sea creatures that includes starfish, sea-urchins, brittle-stars, sea-cucumbers and sea-lilies (Figure 7.8). It's been suggested that modern five-way symmetrical echino-derms came from three-way symmetrical remote ancestors, but they have been five-way symmetrical for more than half a billion years and it is tempting to see five-way symmetry as a central part of one of {236}
Figure 7.7 Six-way symmetrical medusae. {237}
Figure 7.8 Echinoderms from various groups: (from left to right) brittle star, many-armed starfish (which has probably suffered some individual arm loss and regeneration, hence its unequal arms), sea lily, sand dollar.
those highly conserved Baupläne that continentally inspired zoologists are so fond of. Unfortunately for this idealistic view, not only is there a good minority of starfish species with arm counts other than five, but even within respectably five-pointed species mutant individuals with three, four, or six-way symmetry sometimes turn up.
On the other hand, contrary to what we might expect from our simple analysis of what it takes to be a bottom-dwelling crawler, even echinoderms that crawl are usually radially symmetrical. And they seem to take their radial symmetry seriously in the sense that they don't mind which way they walk: no one arm is privileged. At any one time a starfish will have a ‘leading arm’ but from time to time it changes to lead with a different arm. Some echinoderms have rediscovered left — right symmetry over evolutionary time. The burrowing heart urchins and sand dollars, whom the sand must subject to extreme pressure to streamline, have rediscovered a front — rear asymmetry and superimposed a superficial left — right asymmetry {238} over their shape, which is recognizably based upon that of a five-radius sea-urchin.
Echinoderms are such exquisite creatures that, when I was trying to breed lifelike biomorphs with the Blind Watchmaker program, I naturally aspired to achieve their likeness. All attempts to breed five-way symmetry were doomed. The Blind Watchmaker embryology was not kaleidoscopic in the right way. It lacked the requisite number of ‘mirrors’. In fact, as we have seen, some freak echinoderms depart from five-way symmetry and I ‘cheated’ by simulating starfish, brittlestars and urchins with even numbers of radii (Figure 7.9).
But there is no getting away from the fact — indeed it illustrates the central point of this chapter — that the present version of the Blind Watchmaker computer program is incapable of throwing up a five-way symmetrical biomorph. In order to rectify this I'd have to make a change to the program itself (a new ‘mirror’, not just a quantitative mutation to an existing gene) so as to allow a new class of kaleidoscopic mutations. If this were done, I feel confident that the ordinary, if rather time-consuming, processes of random mutation and selection would produce far better likenesses of most of the major groups of echinoderms. The original version of the program, as described in The Blind Watchmaker, was capable of producing only left — right symmetrical mutations. The present, commercially available, program's ability to produce four-way symmetrical biomorphs, and ‘Isle of Man'
Figure 7.9 Computer biomorphs can look superficially like echinoderms, but they never achieve that elusive five-way symmetry. The program itself would have t
o be rewritten for that. {239}
or ‘swastika’ biomorphs, results from a decision, on my part, to rewrite it so as to place a repertoire of ‘software mirrors’ under genetic control.
I have been talking about various kinds of symmetry as examples of kaleidoscopic embryologies. Less geometrically spectacular, but just as important in the world of real animals, is the phenomenon of segmentation. Segmentation means serial repetition as you move from front to rear of the body, usually of an ordinarily long, left — right symmetrical animal. The most obvious examples of segmented animals are annelids (earthworms, lugworms, ragworms and tubeworms) and arthropods (insects, crustaceans, millipedes, trilobites, etc.), but we vertebrates are segmented too, though in a rather different way. Just as a train is a series of trucks or carriages, each one basically like the others but differing in detail, an arthropod is a series of segments which may differ from each other in detail. A centipede is like a goods train, with all its trucks much the same as each other. You can think of other arthropods as glorified centipedes: trains with varied, special-purpose trucks and coaches (Figure 7.10).
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