Good Vibrations: the Silent Symphony of Life

176 Evolutionary Anthropology CROTCHETS & QUIDDITIES Good Vibrations: The Silent Symphony of Life KENNETH WEISS A century ago a leading biologist suggested that repetitively structured biolog- evolve incrementally. What is actually ical traits resembled interference patterns used in tuning violin plates. Are such inherited? A different gene for each ideas in tune with modern biology? hair or vertebra? That made no sense to him, and it may have been a reason for the non-specific suggestion a cen- Look at the life around you. Every- of the first prominent biologists to as- tury later that evolution was a “punc- tuated” process. where, you are confronted with mod- semble objections was St George 1 ularity and repetition, from the leaves Jackson Mivart (1870), previously re- One of his ideas about this is strik- and branches on plants, to your hair spected by and on good terms with ingly modern in concept. The sugges- and the skeletal and other structures Darwin, Huxley, and the rest of the tion drew biological parallels with concepts of fields and vibrations or within you. Modularity, segmenta- biological “in” crowd. The main issue oscillations borrowed from physics. tion, and repetition are, like measures was whether adaptive natural selec- As we’ll see, Bateson used a musical and tones in music, the way in which tion could explain species variation analogy that makes some evolution- the living opus has been assembled by and evolution. Mivart so irritated Dar- ary genetic points easy to understand. the composing processes of evolution. win that he responded at great length th There were then no actual genes In his elegant book Evolution Emerg- in the 6 edition of Origin. Some of known and his views were ignored or ing, W.K. Gregory (1951) likened Mivart’s objections involved the na- even ostracized during the decades of modular (“polyisomeric”) organiza- ture of inheritance and homology in ascendance of the neodarwinian syn- tion to “the notes in an octave...evo- regard to discontinuous variation, thesis. But the basic ideas are enjoy- lution emerging has involved an infi- such as repetitive, serially homolo- ing a justified revival in a profound nite number and variety of natural gous structures. Mivart became a beˆte advance in our understanding of the polyisomeres in both space and time.” noir who was ostracized by the Dar- role of genes in the evolution and gen- This has long been known, but little winian community then becoming the eration of complex morphologies. We analyzed, in modern evolutionary mainline of biology. Mivart also tried can trace these ideas, at least fanci- terms until very recently. To continue to reconcile evolution and Catholi- fully, back to the famous Geoffroy- the musical analogy, we can view pat- cism, and was excommunicated from Cuvier debates in 1830 on the nature terned structures as a harmony of that church, too, poor fellow. But his of animal form itself, and perhaps organization, with many parts inte- theme was picked up by W.K. Brooks even push the musical analogy back to grated to form an organism, the way in the US who credited Mivart and that time as well. the violins, horns, flutes, and so on, then by William Bateson (1892, 1894, form an orchestra. In fact, old ideas 1913, see Webster, in Bateson, 1894; and new facts suggest that musical Webster and Goodwin, 1996). A FEW BASIC QUESTIONS similes are relevant to the symphony Bateson coined the term “genetics” How Fundamental Is Modular of life. and was a strong promoter of Mende- Patterning? Soon after Darwin published his Or- lian genetics, but he did not think that igin of Species, opponents of his views the darwinian evolution by gradual It’s worth stressing how very modu- began marshaling their evidence. One natural selection could generate spe- lar structure is in the construction of cies or their diversity (e.g., Bateson, life. If the canonical essence of evo- 1913). He pointed out problems in in- lution is descent with modification terpreting modular (repetitive, seri- among individuals across genera- ally homologous, or meristic) struc- tions, to me an equally important Ken Weiss is Evan Pugh Professor of An- tures: “Segmentation ...is almost principle is duplication with variation thropology and Genetics at Penn Univer- sity. universally present...greater or less within the individual. From DNA to repetition of various structures is one proteins to morphology, among plants of the chief factors in the composition and animals (even bacterial colony Evolutionary Anthropology 11:176–182 (2002) of animal forms.” (Quoted in Web- 1 DOI 10.1002/evan.10034 Bateson’s reference to Chladni figures was made Published online in Wiley InterScience ster’s preface to Bateson, 1894). But most clearly in a letter to his sister (see B. Bateson, (www.interscience.wiley.com). discrete segment numbers cannot 1928). CROTCHETS & QUIDDITIES Evolutionary Anthropology 177 a way to make the vibrations caused by sound waves visible. He covered glass, metal, and wooden plates with sand and ran a violin bow against them. The vibrations moved the sand into patterns that are known today as “Chladni’s figures.” The vibration jos- tles the powder to areas or nodes of the plate in which vibration waves cancel each other out and there’sno net motion. The idea of using a tuning fork as a source of oscillating energy suggested an interesting way to illus- trate the phenomenon. A physician named Fe´lix Savart was interested in applying Chladni’s notions to this problem, by dusting the plate with black powder, applying a tone to the plate and observing the interference waves or nodes, as shown in Figure 3. Plates of great violins have consistent patterns within, and consistent differences between them. Makers of great violins “tune” the top and back plates by shaving small amounts of Figure 1. William Bateson in youth A; and his wood here and there until appropri- interests, B. Supernumerary premolar in ate frequencies generate the stan- upper but not lower jaw of Ateles mon- dard “modes.” An important point to key. (sources: (A) http://post.queensu.ca/ ϳforsdyke/bateson1.htm# with permission note is that the same plate has dif- of Donald R. Forsdyke; (B) from Bateson, ferent patterns if the energy, loca- 1894). tion, or frequency of the source is changed. In 1952, Alan Turing (known for de- formation), from cells to ants in a col- bers to explain comparable structures ciphering the Nazi cryptographic sys- ony, there is modular organization ev- within an organism and for repeated tem during World War II and ideas on erywhere. This is built by gene dupli- regulatory sequences around genes designing programmable computers) cation to form the genome, protein expressed in similar cellular contexts. suggested that two interacting sub- polymerization or multimerization to Wavelike patterning mechanisms stances diffusing through a uniform form functional biochemical units, have been a widely observed phenom- fluid could generate wavelike interfer- and at the morphological level, begin- enon in development. ence patterns. One substance is ning with cells themselves. known as an activator, and diffuses Across the plant and animal world, from some source, inducing its own if you look at structures, organs, or How Does Modular Structure activity as well as that of the second substance, known as an inhibitor. In organ systems you will see how com- Relate to Genes? such a “reaction-diffusion” process, monly they are built of repeated Bateson’s “vibratory” theory of pat- the inhibitor reduces the level of the subunits (themselves sometimes terning was that repeated structures activator. Depending on the produc- hierarchically modified). To make were manifestations of sympathetic tion and diffusion levels of the two this happen, sets of differently pro- vibration or similar interference substances, and their interaction dy- grammed cells are produced in peri- phenomena. His imagery was that of namics, an initially uniform area or odic or episodic fashion, and become Chladni figures. These are the wave- “field” will generate wavelike patterns the precursors of each unit within the like interference patterns that form of high and low levels of the activator. structure. In this way, duplication when a source of oscillating energy with variation has allowed life to be- diffuses through a material of some come complex, and organisms to sort (Waller, 1961). The pattern is a achieve larger size and functional spe- function of the location, frequency, cialization. and energy level of the source of vi- Recognition of modularity is thor- bration. Ernst Chladni was a Leipzig (Virtual) Figure 2. Modular structure in life: oughly built into modern biology. We lawyer, musician, and amateur scien- left-right symmetry, hair, pores, teeth, cusps, routinely search for gene family mem- tist. As early as 1787, he had reported papillae on the tongue,... 178 Evolutionary Anthropology CROTCHETS & QUIDDITIES cascades occurs when activator levels cross some threshold, with the valleys the structure-free inhibition zones. In recent years, the ability to detect cell-specific levels of expression of specific gene products (proteins or mRNA) has put these ideas to direct tests. Embryonic tissue is tested for spatiotemproal distribution of signaling factor molecules diffusing from cells of origin across the tissue. Ini- tially broad patterns narrow to stripes and then periodic spots surrounded by expression of diffusible inhibitors. For example, hair and feathers de- velop in zones expressing the Fgf and Wnt signaling factors, but are inhib- ited in areas where Bmp factors are found. These gene products have patchy distribution that presages the location and spacing of future feathers, sometimes appearing first as an initial line of expressing cells (e.g., the dental lamina), that resolves into spots, and then spreads laterally to form other spots.

Load more