Development & Evolution

Recapitulation – “Biogenetic Law” Late 19 th Century concept of Ernst Haeckel : Ontogeny Recapitulates Phylogeny Evolutionary history of a species is the cause of its embryonic development, therefore, during embryonic development an organism passes through the stages of its evolution (phylogeny). Influence from 1880s – 1920s and beyond: 1) stimulated a great deal of descriptive embryology (+) 2) stimulated many speculative historical reconstructions (+/-) 3) when applied to human anatomy and behavior served as support for a wide range of sexist and racist ideas (-) Rejected among biologists by 1920s as incompatible with our understanding of inheritance (Genetics). Cultural applications continue well into the 1960s. Unfortunately some authors continue to use the term ‘recapitulation’ when they discuss similarities in embryonic development as means of identifying homologies (see Brusca p 107-8)

Heterochrony One concept to emerge from the controversy over recapitulation and genetics is that of ‘rate’ genes – genes that somehow control the rate of embryonic development and thus can effect the relative timing of embryonic events. During the 1930s and 40s some researchers argued that major evolutionary changes (macroevolution) could occur if the relative timing of events were to change during development = Heterochrony Although ignored at the time, by the 1960s and 70s the idea of heterochrony (mutations in ‘rate’ genes) was revived. Evidence was provided from comparative embryology - especially of larval forms and experimental manipulation of metamorphosis (especially amphbians). Heterochrony still used as an explanation for certain events in evolutionary history but now considered a small subset of the impact of changes in developmental regulatory mechanisms.

The Developmental Cascade and the Revival of the Bauplan As progress in genetics unfolded - the Watson-Crick DNA model, the triplet code, the ‘central dogma’ etc. – application of genetics to embryonic development led to the concept of the developmental cascade. The directional nature of development – cleavage, gastrulation, morphogenesis, organogenesis, cell differentiation – suggested a sequence of a turning on and off of genes so that cells became increasingly specialized to function at the right time in the right place. The notion was that mutations of genes that functioned early in the cascade would greater effect than mutations in those genes that function later. Selection on such early genes would be especially strong, tending to stabilize basic body forms and thus tend to channel evolutionary change into modifications of common ‘body plans’ (Bauplans)

Raff’s Developmental Hourglass I Changes in the early development of sea urchins from indirect development to direct development nevertheless yielded typical normal adults. Such evolutionary changes occurred several times. Thus early development more flexible than thought.

Raff’s Developmental Hourglass II The Phylotypic stage of development would correspond to the establishment of the taxon’s Bauplan

Raff’s Developmental Hourglass III The period of organogenesis is a time of maximum global interaction across the embryo and thus most under selection pressure – the phylotypic stage Resulting modules (e.g imaginal discs or segments) can be altered independent of rest of embryo.

Regulatory Genes as Focus for Macroevolution The work of Raff and others suggested that macroevolutionary change – including changes in bauplan – is the result of mutations in regulatory genes [genes that code for ‘transcription factors’ which control the expression of other genes] Major advances in testing this idea came from using mutant phenotypes in the fruit fly (Drosophila) and the round worm (Caenorhabditis) to ‘dissect’ embryos of these organisms. Studies of homeotic mutations in which one structure is substituted for another (leg where antenna ought to be) led to the discovery of Hox genes. A group of regulatory genes each containing a homeobox {sequence of base pairs for transcription factor}. The hox genes are found in the same order on the chromosome that the structures they regulate have in the embryo.

Drosophila Hox Genes From Freeman & Herron 2007 Evolutionary Analysis

Conservati on of Regulatory genes: Deep Homology Note: regulatory genes other than the hox group also show deep homology. E.g. pax6 gene and eye development

Conserved Hox genes: macroevolutionary change may be down stream

The Problem of Homology The Issue: Are two structures found in different species similar because they are sympliesiomorphies (from a common ancestor) or are they a product of convergence (Natural Selection leading to similar adaptations in separate lineages – “homoplasy”)? How to distinguish 1) Level of similarity – detailed description generally shows homoplasies to be more superficial than homologies (e.g. cepahlopod eye & vertebrate eye). 2) Embryonic origin – homologies are expected to develop from the same embryonic structure/process, homoplasies are less likely to have development in common. (e.g. hyrdrozoan tentacles & ectoproct lophophore) 3) Genetic basis – genes (both regulatory & structural) are expected to be the same for homologies different for homoplasies. (eye lens proteins in molluscs) 4) Cladistic Analysis – analysis of several characters may yield a better (more parsimonious) cladogram if characters are considered to be homoplasies rather than homologies. (e.g. cephalopod eye & vertebrate eye)