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Evolution's middle floor: functional morphology and the origin of complexity Graham Budd Budd G.E. (2006) On the origin and evolution of major morphological characters. Biol. Rev. 81(4). What is structuralism? Idea being that form, in both genotype and phenotype, can’t just freely vary, but is governed by certain discoverable laws. Structuralists would normally claim that these extra laws are required to understand fully how organisms evolve. General Themes • E.g. constraint. Why have some features not changed over hundreds of millions of years? 'A developmental constraint is a bias on the production of variant phenotypes or a limitation on phenotypic variability caused by the structure, character, composition, or dynamics of the developmental system.' Maynard Smith et al. 1985 Bauplan – an idea that is “misguided and dispensable”? (G. C. Williams 1992) “The text-books would have you believe that all vertebrates have hemoglobin, but there is a diverse group of vertebrates, the Antarctic fish family Chaenichthyidae, in which hemoglobin is entirely lacking. This is merely one of many possible examples of generalizations for which exceptions must be recognised” G. C. Williams 1992, p. 88. Flavours of structuralism Self-organisers (e.g. Goodwin, Muller, Seilacher etc) Functionalists (e.g. me, Schwenk, Galis etc) As opposed to: Geneists Populationists Self-organisation Self-organization explains flocking behaviour on different levels Local interactions cause flocking behavior 1. 2. Svim in the same direction as the neighbors Don´t collide - modulate speed and direction Patterns that arise through self-organization Peculiarities of self-organizing systems 1. Many parts 2. Many interactions 3. Few and simple rules 4. No plan or directives from “above“ 5. Emergence - novel properties arise in the system 6. Often modelled with cellular automata (Conway‘s Game of life) More stripes, same distance Developmental symbiosis Squidbacteria Nutritional polyphenisms - social insects Functional rules Cichlid jaws Comparative anatomy Frazzetta on system evolution: “like making improvements to an engine while the engine is running” Why is there a problem? Both the developmental genome and the developing and adult phenotype are complex. Complexity is hard to define, but here is seen as a measure of internal structure that provides resistance to evolutionary change. Genotype-Phenotype mapping •Polyphenisms show that the same genotype can generate many different morphologies depending on environment •Yet we also know that different genotypes can generate the same morphology (phenogenetic drift; genetic code redundancy etc). •The evolutionary relationship between the two is thus likely to be complex! Complex system change Complex systems typically fail if changes are randomly made to them because of their integrated nature, even if locally the changes ”work”. In the phenotype, this can be seen in terms of functional morphology, in the genotype, in terms of pleiotropy Partial recognition of this fact in the genotype has led to an emphasis on promoter evolution etc. Modularity and interconnection Generally considered that high degrees of modularity (= many quasi-autonomous units) makes evolution “easier” as pleiotropic effects are reduced. The opposite is interconnectedness: the idea of the phylotypic stage was that it was a developmental period of low modularity, as genes had maximal global effect at this period. Developmental constraint: the zootype? Origins of modularity But the origins of modularity itself are highly controversial: An effect of self-organisation, either in genetic interactions or morphology? Or imposed, perhaps by adaptive selection? What does Cambrian evolution tell us about evolution? NOT like this!! - but rather, functional adaptation Reminder of stem groups Animals are… Complex integrated organisms that appear to show increasing specialisation and compartmentalisation through time. Study of how they evolve in the Cambrian allow us to extract more general principles… Organisms can be seen as functional networks How do we build integrated structures? QuickTime™ and a decompressor are needed to see this picture. Fundamentals of morphological evolution (?) Redundancy Preadaptation Functional asymmetry Least constraint The many faces of redundancy Multiple ways of performing the same functions will enable functional shifts, so in that sense complex systems may be more evolvable than simple ones. Not just one way of doing this though! Types of redundancy Amplification Parcellation Types of redundancy { { { Functional complex Functional degeneracy Functional take-over Preadaptation: being in the right place at the right time Almost every novelty involves a functional shift (classical example: middle ear bones of mammals used to be jaw bones of reptiles) But not everything is placed well to make make such a shift. Functional asymmetry Even in a tightly integrated system with little redundancy, there may be mutually interacting components that do not have equal dependency on each other E.g. what came first, muscles or segments (chicken and egg)? Functional asymmetry Can be represented on a diagram by arrows showing interactions between components Interactions between components are textured: they have both direction and strength Direction of dependence Least constraint In any system, some components will be less functionally constrained than others. In theory it should be these components that evolve first. Example: bilaterian systems Classical bilaterian features such as segmentation, BVS, metanephridia and the coelom are not independent, but relate to each other in an asymmetric and partly definitional sort of way. For example, a large, fully segmented animal requires a BVS in order to transport oxygen etc around the body, not rely on internal mixing. QuickTime™ and a decompressor are needed to see this picture. System dependencies Segmentation Coelom Metanephridia BVS Implied order of acquisition: coelom; BVS; then segmentation and metanephrida Least constrained components can evolve to allow redundancy to develop… L P L P …and therefore for new dependencies to develop L Example: mammalian jaw Burden is higher in the centre! Preliminary conclusions Fossil stem groups show that morphological evolution is in fact governed by certain principles. Note that in this view, “burden” is an evolutionary property that can evolve in both directions, although change in highly burdened characters requires preparation in terms of shifting of constraint as outlined before. Almost all of these types of change apply to genetic changes too (cf kernels etc!) Towards a theoretical biology Ecology, organismal structure (and physiology) and genes form three distinct but interlinked networks. Can one understand how all three relate to each other?? ecology Evolution’s middle floor… genes Genetics of micro- and macroevolution Macroevolutionary innovations (limbs, livers, etc) ???? “genetics of macroevolution”? ? Population variation Population genetics “problem of variation” Body-patterning genes? The problem: Matching up evolution in morphology and in the genome so that functionality is not compromised (Frazzetta etc) Fig. 2. Examples of putative GRN kernels E. H. Davidson et al., Science 311, 796 -800 (2006) Published by AAAS Once more! QuickTime™ and a decompressor are needed to see this picture. Relationship between developmental genotype and morphology Dev. Gen. best seen as a physical positioning system: timing less important than eventual positioning of various morphological elements. Changes in the dev. gen. can thus be correlated with positioning of morphological elements, and thus the effect on the functional map of the organism can be seen QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. Functional evaluation of homeotic mutation The developmental hierarchy? How it works in fruit flies High-level developmental changes …tend NOT to be allowed by the structuralist rules governing morphological evolution. But they have clearly occurred anyway! Suggests that the conditions have to be right (just as in morphological evolution on its own) i.e. out of the whole system, the phenotype, or at least downstream genetic components, is the least constrained component. QuickTime™ and a decompressor are needed to see this picture. Budd 2006 When might this take place? • E.g. “genetic assimilation” of Waddington: • Environmentally prompted changes can become ”fixed” in the genome by selection… Developmental plasticity Could take place at other levels… Budd 1999, Bioessays Genotype-Phenotype revisited (I) Genotype and phenotype together form a functional complex. Genes make phenotype, but selection acts on phenotype, not genotype. Hence the structuralist rules governing evolution of phenotype (strictly morphology here) take precedence over genetic change; it provides a set of important boundary conditions that genetic changes cannot cross. Genotype-Phenotype revisited (II) Phenotypic change can be sourced either from subtle down-stream changes or even from environmental change. If the latter is consistent, then there is enough time for the directive genotype to react to stabilise and canalise the resulting changes. Genotype-Phenotype revisited (III) In this view, adaptive specialisation imposes modularity on morphology. Morphology acts as a template around which genetic structures are built. In a primary sense then, developmental genes are at the whim of morphology, not the other way round, = “genes are not important in evolution” sort of conclusion. However, one can still be highly adaptive in this view, unlike the extremist structuralists. General “conclusions” Fossils are essential for reconstructing morphological evolution - the framework within which all genetic change must take place. Ecology channels selective pressures onto morphology - and only through this filter onto genes. Morphological evolution is by far, and bizarrely, the least understood component of the evolutionary building we have erected in the last 150 years…