Posted: Jan 01, 2011 9:22 am
by Darwinsbulldog
An article by Eric Davidson & Douglas Erwin here: shows the relationships between changes in Gene Regularity Networks and changes in body form. Thus there is a relationship between phylogeny and form.
Development of the animal body plan is controlled by large gene regulatory networks (GRNs), and hence evolution of body plans must depend upon change in the architecture of developmental GRNs. However, these networks are composed of diverse components that evolve at different rates and in different ways. Because of the hierarchical organization of developmental GRNs, some kinds of change affect terminal properties of the body plan such as occur in speciation, whereas others affect major aspects of body plan morphology. A notable feature of the paleontological record of animal evolution is the establishment by the Early “Cambrian of virtually all phylum-level body plans. We identify a class of GRN component, the kernels” of the network, which, because of their developmental role and their particular internal structure, are most impervious to change. Conservation of phyletic body plans may have been due to the retention since pre-Cambrian time of GRN kernels, which underlie development of major body parts. /0709/A1_Gene_Regulatory_Networks_and_the_Evolution_of_Animal_Body_Plans.pdf

What D and E define as evolutionarily stable GRN "kernels" do not change over vast periods of evolutionary time, but "plug-ins" can change quite readily at species level and below. This means that various signaling pathways can influence end-of-development regulation of the structural genes. [Structural genes are those which produce "building proteins", such as muscle proteins, connective tissue etc in particular parts of tissues of the body.
Thus the bug, insect or indeed anything else can be selected on it's basis as a mimic of another creature [as in Bayesian Mimicry] or to resemble a twig. Obviously a bug that gets eaten because it is not well camouflaged will not be able to pass it's genes on. Those that do have developmental changes that make it a little better at camouflage will have a chance at passing it's modified structure on to the next generation. If this modified GRN subsystem [which defines the new shape during development] becomes fixed in the population due to it's higher fitness value, then it will become a permanent change.

Hox genes and their friends define the four-dimensional geometry of the developing embryo. Heterochronic changes of Hox gene expression of particular tissues or parts can lead to quite profound-looking changes in the animal's body shape.
These changes might occur initially as frequency dependent polymorphism.

In Dung Beetles, for example, there is sexual dimorphism in horn expression. Males have horns and females don't. Thus the sex determination pathways feed into the developmental networks that control gene expression for horns. But there is also hormonal pathways and insulin signaling pathways that can influence horn development also. So we can get size dimorphism between males based on insuin levels, which are in turn, determined by the size of the dung-ball that the mother beetle lays her egg in. Big balls of shit produce horned males, and smaller balls of poo produce hornless males:-

Emlen, D. J. and H. F. Nijhout (1999). "Hormonal control of male horn length dimorphism in the dung beetle Onthophagus taurus (Coleoptera: Scarabaeidae)." Journal of Insect Physiology 45(1): 45-53.
Male dung beetles (Onthophagus taurus) facultatively produce a pair of horns that extend from the base of the head: males growing larger than a threshold body size develop long horns, whereas males that do not achieve this size grow only rudimentary horns or no horns at all. Here we characterize the postembryonic development of these beetles, and begin to explore the hormonal regulation of horn growth. Using radioimmune assays to compare the ecdysteroid titers of horned males, hornless males, and females, we identify a small pulse of ecdysteroid which is present in both hornless males and females, but not in horned males. In addition, we identify a brief period near the end of the final (third) larval instar when topical applications of the juvenile hormone analog methoprene can switch the morphology of developing males. Small, normally hornless, males receiving methoprene during this sensitive period were induced to produce horns in 80% of the cases. We summarize this information in two models for the hormonal control of male dimorphism in horn length. ... .%2099.pdf

Thus these bugs are adaptable to variable food availability. So long as the food availability is cyclic, this polymorphism will persist. But speciation can occur if one form is favoured over another. See this paper on lizard polymorphism:-

Corl, A., A. R. Davis, et al. (2010). "Selective loss of polymorphic mating types is associated with rapid phenotypic evolution during morphic speciation." Proceedings of the National Academy of Sciences 107(9): 4254-4259.
Polymorphism may play an important role in speciation because new species could originate from the distinctive morphs observed in polymorphic populations. However, much remains to be understood about the process by which morphs found new species. To detail the steps of this mode of speciation, we studied the geographic variation and evolutionary history of a throat color polymorphism that distinguishes the “rock-paper-scissors” mating strategies of the side-blotched lizard, Uta stansburiana. We found that the polymorphism is geographically widespread and has been maintained for millions of years. However, there are many populations with reduced numbers of throat color morphs. Phylogenetic reconstruction showed that the polymorphism is ancestral, but it has been independently lost eight times, often giving rise to morphologically distinct subspecies/species. Changes to the polymorphism likely involved selection because the allele for one particular male strategy, the “sneaker” morph, has been lost in all cases. Polymorphism loss was associated with accelerated evolution of male size, female size, and sexual dimorphism, which suggests that polymorphism loss can promote rapid divergence among populations and aid species formation.

Of course, I have said very little about camouflage and mimicry directly, because I think my example of dung beetle horn dimorphism illustrates how flexible gene regulatory networks are in producing drastic changes in morphology within a single species, or indeed, a single sex by receiving inputs from sex and hormonal signaling pathways.
The literature on "camouflage" and "mimicry" is vast, and some basic google searches turns up lots of hits. Ad "Hox" to the search and one will get entries like this one:-

Wittkopp, P. J. and P. Beldade (2009). "Development and evolution of insect pigmentation: Genetic mechanisms and the potential consequences of pleiotropy." Seminars in Cell & Developmental Biology 20(1): 65-71.
Insect pigmentation is a premier model system in evolutionary and developmental biology. It has been at the heart of classical studies as well as recent breakthroughs. In insects, pigments are produced by epidermal cells through a developmental process that includes pigment patterning and synthesis. Many aspects of this process also impact other phenotypes, including behavior and immunity. This review discusses recent work on the development and evolution of insect pigmentation, with a focus on pleiotropy and its effects on color pattern diversification.

Or this:- ... ne.0004035

Rubinoff, D. and J. J. Le Roux (2008). "Evidence of Repeated and Independent Saltational Evolution in a Peculiar Genus of Sphinx Moths (Proserpinus: Sphingidae)." PLoS ONE 3(12): e4035.
Background: Saltational evolution in which a particular lineage undergoes relatively rapid, significant, and unparalleled change as compared with its closest relatives is rarely invoked as an alternative model to the dominant paradigm of gradualistic evolution. Identifying saltational events is an important first-step in assessing the importance of this discontinuous model in generating evolutionary novelty. We offer evidence for three independent instances of saltational evolution in a charismatic moth genus with only eight species.Methodology/Principal FindingsMaximum parsimony, maximum likelihood and Bayesian search criteria offered congruent, well supported phylogenies based on 1,965 base pairs of DNA sequence using the mitochondrial gene <italic>cytochrome oxidase subunit I, and the nuclear genes <italic>elongation factor-1 alpha</italic> and <italic>wingless</italic>. Using a comparative methods approach, we examined three taxa exhibiting novelty in the form of Batesian mimicry, host plant shift, and dramatic physiological differences in light of the phylogenetic data. All three traits appear to have evolved relatively rapidly and independently in three different species of <italic>Proserpinus. Each saltational species exhibits a markedly different and discrete example of discontinuous trait evolution while remaining canalized for other typical traits shared by the rest of the genus. All three saltational taxa show insignificantly different levels of overall genetic change as compared with their congeners, implying that their divergence is targeted to particular traits and not genome-wide.Conclusions/Significance Such rapid evolution of novel traits in individual species suggests that the pace of evolution can be quick, dramatic, and isolated—even on the species level. These results may be applicable to other groups in which specific taxa have generated pronounced evolutionary novelty. Genetic mechanisms and methods for assessing such relatively rapid changes are postulated.

While there is no doubt that some evolution is gradual and achieved in small steps, each of which must confer fitness and is historically contingent on previous innovations, it is also clear that saltational stepping [large changes to morphology and even speciation] can occur via relatively minor genetic change. In the light of such successful rather than merely hopeful monsters that can be produced via the seemingly paradoxical properties in Gene Regulatory Networks of both high conservation and great evolvability and flexibility to respond to environmental challenges via signaling pathway inputs into those GRN's, then the evolution of mimicry seems like child's play.
That GRN's can produce such stability and innovation is based, as Davidson and Erwin suggest, is because GRN's are nested and hierarchical. Changes very early in development are almost always fatal, but late changes in development are not.