Genetics and Evolution IB 201 06 Lecture 11 Evolution of Development Terms Recapitulation the tendency of developmental stages to represent a series of evolutionary forms Examples of such trends recognized by von Baer below and Haeckel s ontogeny recapitulates phylogeny below serial homology a relationship among repeated often differentiated parts of a single organism defined by the similarity of their developmental origin arrayed along the body axis E G the legs and mouthparts of an arthropod vertebrae etc homeotic genes genes whose products provide positional information in a multicellular embryo these genes act within cells to select their developmental fate selector genes they regulate the overall body plan and determine the number identity and pattern of body parts Mutations in homeotic genes cause the transformation of one body region or part into the likeness of another The results of these mutations lead to abnormal forms known as homeotic mutants Homeotic mutants have led to the identification of the homeotic genes which have also been coined toolkit genes homeobox Hox containing genes a family of eukaryotic genes that contain a DNA sequence called the homeobox which encodes a protein homeodomain of 60 amino acids that binds to DNA and controls gene expression during development Most homeobox genes are transcriptional regulators Hox genes a subfamily of homeotic homeobox containing genes conserved in all metazoan animals and which control anterior posterior identity by regulating transcription of many genes during development they are found in linked clusters in all bilaterians For example in Drosophila there are 8 linked Hox genes that affect the specification of particular segments in the developing embryo larva and adult MADS box genes homeotic homeobox containing genes in flowering plants specify which floral parts appear in different locations analogous to the Hox genes in Bilateria homeotic mutants the result of a mutation in homeotic genes cause the transformation of one body region or part into the likeness of another regulatory hierarchy is a developmental pattern in which the expression of a developmental gene is dependent on the order of expression of other genes This can be considered a vertical organization in which tiers of genes control lower teirs of gene These control the sequential generation of the major body axes e g anterior posterior the formation of primary and secondary fields and the patterning of individual fields an example is gap pair rule and segment polarity genes in generating the segmental organization of the larva from the undifferentiated egg in Drosophila target gene Single genes may control expression of a large group of target genes downstream A gene acted upon by a regulatory gene or a regulatory hierarchy tagmosis trend toward development of distinct regions in a segmented body such as that seen in head thorax and abdomen of an insect from an undifferentiated segmented body such as that of an onychophoran People Karl Ernst Von Baer noted in 1828 that characters common to a more inclusive taxon e g phylum Vertebrata appear earlier during development than the characters that distinguish the taxa at lower levels Von Baer s Law Example many tetrapod embryos in the early stages have gill slits a notochord segmentation and paddle like limb buds before the unique features of their class become apparent Ernst Haeckel German biologist Darwin supporter ontogeny recapitulates phylogeny the development of an organism repeats the evolutionary history of the adult forms of its ancestors biogenetic law e g gill slits of embryonic mammals resemble the gill slits of their adult ancestors the fishes By studying embryology one could discover a species phylogenetic history and use this to infer phylogenetic relationships among organisms Turns out not to hold in most cases gill slits of mammal embryo do not acquire the form found in fishes His idea of recapitulation worked by terminal addition in which new groups arise by addition of new terminal developmental stages In this way one sees the terminal stage of one organism e g a fish is an intermediate stage of a more modern organism such as a human see pdf diagram of recapitulation Johann Wolfgang von Goethe first hypothesized that the parts of a flower evolved from leaflike branching structures Richard Owen coined the term homology and presented hypothetical ancestral archetypes for vertebrates These archetypes presumably exhibited common elements that set the body plans for the structures of modern groups developed the concept of serial homology Richard Goldschmidt hopeful monsters evolution can occur in very large leaps by macromutations This was his attempt to explain the transitions in form body plan between major groups he did not accept the Darwinian model that the transitions evolved gradually under the continual control of selection upon morphology Stephen Jay Gould 1970 s voice for the reunification of embryology and evolutionary biology wrote Ontogeny and Phylogeny which stimulated new discussion and new developmental research also forwarded the idea of punctuated equilibrium evolution marked by long periods of stasis interrupted by intervals of rapid change He attempted to resolve the evolutionary problem of transition between major morphological differences in body plan by suggesting that changes in timing e g slowdowns leading to delayed maturation speeding up during development heterchrony could explain the phenomenon This was an attempt to bridge the gradual mechanism of Darwin and the macromutational leaps postulated by Goldschmidt Sean Carroll important in developing current evo devo research involved in discovery of Hox genes and their homology in Bilateria Written several books on evo devo pushing concept of tool kit genes master genes that all animals share governing the formation and patterning their bodies and body parts This research may bridge the mechanism of evolution by natural selection and the genetic steps underlying the diversification of animal designs by revealing the evolution of DNA and the regulation of its expression Further Reading Carroll S B J K Grenier and S D Weatherbee 2001 From DNA to Diversity Molecular Genetics and the Evolution of Animal Design Blackwell Science Oxford UK