Tag Archives: developmental plasticity

Gaps and Inconsistencies in Modern Evolutionary Thought – notes

“Here I put the flexible phenotype first, as the product of development and the object of selection, and examine the consequences for the genetic theory of evolution”

“How are complex adaptively flexible traits constructed during evolution?”

“A phenotype is a product of both genotype and environment”

“How does the systematic incorporation of genetic influence evolve?”

“In experimental genetics,, environmental sensitivity came to be considered noise rather than a subject for research, or an important factor in evolution”

Six points of confusion

  1. “The Unimodal adaption concept and the Multimodal Products of Development and Plasticity – quantitative change is only part of the story of evolution, for it does not address the question of the origin of discrete (quantitatively different) novelties…This overlooks two important properties of the phenotype and of adaptive evolution (1) the ability of organisms to facultatively switch among specializations, producing polymorphisms, polyphenisms, and different behavioral and morphological modes and (2) the common  occurrence of distinctive life-stage variants during ontogeny, with larvae, juveniles, and adults having very adaptive morphologies and behaviors…In virtually every species, juveniles have one set of morphological and behavioral adaptions, and adults another. Sexual dimorphism in structure, behavior and physiology is legion…this kind of diversification is possible because developmental divergence permits mosaic evolution in different directions within the same population”
  2. “The Cohesiveness Problem: Development as a Conservative Force versus Development as the Source of all Change-A belief in the stabilizing role of development is consistent with an equilibrium approach to evolutionary theory that begins with the Hardy-Weinberg equilibrium and treats the cause of change, such as mutation, selection, and drift, as departures from equilibrium…a language of stasis and stability has long dominated the science of change…Evolution threatens individual integration because it requires intricate readjustments of so many interconnected parts…the complexity catastrophe (the constraint of epistatic interaction on adaptive improvement) reflects the tension in biology between the complementary properties of integration and atomization, in this case with regard to the possibility of change…the need  for special explanation of multiple simultaneous directions of evolution of different complex traits within a single individual, traits produced by a single genome that is unchanging during ontogeny…innovation does not occur exclusively by terminal addition…When recapitulation of adult structures occurs during the ontogeny of an individual, it demonstrates the capacity for internal rearrangements, not terminal addition to a cohesive unchanging phenotype, because it implies that features originating in the adult have come to be expressed in earlier stages…the cohesive terminal addition view that emphasizes the mosaic or modular nature of phenotype organization and its relation to the dissociability and reorganization of traits…A simple model of phenotype structure shows how quantitative genetic variation and the continuous variation around a node, so often documented when biologists measure observable traits, are related to the evolution of multimodal or discrete phenotypes governed by switches…Ontogeny is a condition-sensitive, bifurcating process that allows and even promotes polymodal adaptation…The realization that genes affect phenotype only if they are expressed, for example, means that development must assume prominence in any intelligent discussion of genetic issues”
  3. “Proximate and Ultimate Cause-Mayr 1961…mechanisms are proximate causes and selection and evolution are ultimate causes…it was an easy step from this important point to the idea that the mechanisms of development have nothing directly to do with evolution…The problem is exacerbated because the plasticity of organisms-their ability to respond to environmental circumstances, to assess complex situations, and to learn–has always seemed a more elusive property than their morphologies or their genes…proximate mechanism represent more than just different levels of analysis or research styles. They are the causes of the variation upon which selection acts.”
  4. “The Problem of Continuous versus Discrete Variation and Change-The gradualism controversy always reappears, like a recurrent nightmare, to haunt Darwinism…the discussion really concerns variable rates of evolution, not whether or not it occurs by large or small steps…The traits and characters described for organisms…are discrete…this problem reflects a deeper one, the unpreceived tension between compartmentalism and connectedness–subdivision and integration–in biological organization itself…all traits have both modular qualities and connectedness with others–modularity and connectedness or continuity are complementary, but differently manifested and analyzed, properties of all traits…Both discreteness and connectedness are products of the switch points that structure phenotype development.Modularity is reflected in developmental reorganization via the shuffling or recombination of discrete phenotypic parts. Connectedness is reflected in the accommodation of change and the emergence of integration at different levels of organization. Switches, by integrating environmental and genomic influence, allow for quantitative fine-tuning of plasticity and gene expression”
  5. “Problematic Metaphors-epigenetic landscape…the model is incomplete…because developmental potentialities change as development proceeds…failure to deal with the dynamic effects of environmental inputs on the epigenetic pathways…genetic program…there are no such rules in the genome, only a set of templates for molecules that will become part of the phenotype…Adaptive flexibility, or behavior-as-if-by-rules, is a property of phenotypes, not genotypes, and phenotypic organization incorporates environmental directives as well as genetic ones…genotype-environment interaction…is misleading as a description of development because genes do not interact directly with the external environment during development”
  6. “The Genotype-Phenotype Problem-the practice of phrasing evolutionary explanations almost entirely in terms of genes…selection acts on phenotypes…the idea that an individual genotype has a measurable fitness that is somehow intrinsic to itself…rather than being entirely a secondary consequence of the fitness…Explanation of phenotype evolution is fundamentally a problem of explaining the evolution of phenotype development”

“the piece that is missing from a synthesis of development and neo-Darwinism is an adequate theory of phenotype organization that incorporates the influence of the environment”

“the realization that the genome has an evolutionary life of its own is one of the cardinal discoveries of molecular biology”

“three major points”

  1. “environmental induction is a major initiator of adaptive evolutionary change”
  2. “evolutionary novelties result from the reorganization of preexisting phenotypes and the incorporation of environmental elements”
  3. phenotype plasticity can facilitate evolution by the immediate accommodation and exaggeration of change”