EVOLUTIONARY DEVELOPMENTAL BIOLOGY

Evolutionary developmental biology (evolution of development or informally, evo-devo) is a field of biology that compares the developmental processes of different animals and plants in an attempt to determine the ancestral relationship between organisms and how developmental processes evolved. Although interest in the relationship between ontogeny and phylogeny extends back to the nineteeth century, the contemporary field of evo-devo has gained impetus from the discovery of genes regulating embryonic development in model organisms. General hypotheses remain hard to test because organisms differ so much in form and shape^[1].

DOUBLE WINGS: An ultrabithorax mutant fly has a complete duplication of the body segment that carries wings.

Introduction

Charles Darwin´s theory of evolution is based on three principles: natural selection, heredity, and variation. At the time that Darwin wrote, the principles underlying heredity and variation were poorly understood. In the 1940s, however, biologists incorporated Gregor Mendel´s principles of genetics to explain both, resulting in the modern synthesis. It was not until the 1980s and 1990s, however, when more comparative molecular sequence data between different kinds of organisms was amassed and detailed, that an understanding of the molecular basis of the developmental mechanisms which are encoded by those genes has become clear. Evolutionary developmental biology has arisen in response to these data, and by incorporating molecular genetics and embryology into the the principles of modern synthesis;

Evolutionary developmental biology hinges on two ideas. First, as has been long recognized, animal bodies are modular: they are organized into developmentally and anatomically distinct parts. Often these parts are repeated, such as fingers, ribs, and Arthropod body segments. Evo-devo has discovered the genetic and evolutionary basis for the division of the zygote into distinct modules, and for the independent embryonic development of each module. A common method employed is to isolate single gene mutations that have a noticeable effect on development, leading to the discovery of a genetic toolkit for development. Furthermore, this toolkit is highly conserved across the Metazoan lineage.

The second idea is that some genes function as switches. Genes provide templates for, and produce, polypeptides that compose enzymes that in turn produce and regulate various biochemical pathways within an organism. Most biologists working within the modern synthesis assumed that any given organism was the product of all its component genes. The modification of existing, or evolution of new, biochemical pathways (and, ultimately, the evolution of new species of organisms) depended on specific genetic mutations. In 1961, however, Jacques Monod and François Jacob discovered within Escherichia coli a gene that functioned only when "switched on" by environmental stimulus. Later, scientists discovered specific genes, such as Homeobox (or HOX) genes that act as switches for other genes (HOX and other "switch" genes themselves may be activated through the release of proteins from the mother into the embrionic environment). This discovery drew biologists´ attention to the fact that genes can be selectivly turned on and off, rather than being always active, and that many distinct organisms (for example, both fruit flies and human beings) have the same genes vital for correct embryogenesis.

In addition to giving support to Darwin´s claim that all organisms are descended from a common ancestor, this finding suggested that the crucial distinction between different species (even different orders or phyla) is not due to differences in the gene composition, but rather the difference in spatial and temporal expression of conserved genes. This view led to the conclusion that large evolutionary changes in body morphology are largely due to changes in gene regulation rather than the evolution of new genes. In short, the action of natural selection on, and mutation of, HOX and other "switch" genes may play a major role in evolution.

History

The importance of embryonic development in the understanding of evolution was recognized by Charles Darwin in The Origin of Species:

We can see why characters derived from the embryo should be of equal importance with those derived from the adult, for a natural classification of course includes all ages.

Ernst Haeckel (1866), in response to Darwin's newly published theory, proposed that ontogeny recapitulates phylogeny: the development of the embryo of every species repeats the evolutionary development of that species fully. This theory has been discredited in its absolute form. However, it served as a backdrop for a renewed interest in the evolution of development after the modern evolutionary synthesis was firmly established.

The origins of the modern understanding of evo-devo are multiple: Stephen J. Gould, in Ontogeny and Phylogeny (1977) argued for the importance of heterochrony (changes in timing of development) as a mechanism for evolutionary change; the discovery of homeobox (Hox) genes by Lewis (1978) rooted the emerging discipline of evo-devo in molecular genetics. In 2000^[2] and 2005^[3] two journals devoted issues to the emerging field of evo-devo.

The developmental-genetic toolkit

The developmental-genetic toolkit consists of genes whose products control the development of a multicellular organism. Mutations in toolkit genes affect the body plan and the number, identity, and pattern of body parts. The toolkit is highly conserved across animal phyla. Only a small fraction of the genes in the genome are involved in development. The majority of toolkit genes are components of signaling pathways and include transcription factors, cell adhesion, cell surface receptor proteins, and secreted morphogens. Their function is highly correlated with their spatial and temporal expression patterns. One of the major goals of evo-devo is to catalogue all genes (their identity, product, function, and interaction) in the toolkit.

Development and the origin of novelty

Among the more surprising and, perhaps, counterintuitive results of recent research in evolutionary developmental biology is that the diversity of body plans and morphology in organisms across many phyla are not necessarily reflected in diversity at the level of the sequences of genes involved in the regulation of development. Indeed, as Gerhart and Kirschner (1997) have noted, there is an apparent paradox: "where we most expect to find variation, we find conservation, a lack of change".

Even within a species, the occurrence of novel forms within a population do not point to the preexistence of genetic variation sufficient to account for all morphological diversity. For example, there is significant variation in limb morphologies amongst salamanders and the differences in segment number in centipedes, even when the genetic variation is low.

A major question then, for evo-devo studies, is: Where does the novelty come from? If the morphological novelty we observe at the level of the different clades is not always reflected in the genome, where does it come from? The discovery that much biodiversity is not due to differences in genes, but rather to alterations in gene regulation, has made an important contribution to the resolution of this question.^[4]. Diverse organisms have highly conserved developmental genes, but highly divergent regulatory mechanisms for these genes.

Novelty may arise by several means, including gene duplication, mutation-driven changes in gene regulation, and epigenetic alterations in gene regulation or morphogenesis that are later consolidated by changes at the gene level. Gene duplication allows fixation of a particular cellular or biochemical function at one locus, leaving the duplicated locus free to fulfill a new function. In contrast, changes in gene regulation are "second-order" effects of genes, resulting from the interaction and timing of activity of gene networks, as distinct from the functioning of the individual genes in the network.

The discovery of the homeotic Hox gene family in vertebrates in the 1980s allowed researchers in developmental biology to empirically assess the relative roles of gene duplication and gene regulation with respect to their importance in the evolution of morphological diversity. Several biologists, including Sean B. Carroll of the University of Wisconsin suggest that "changes in the cis-regulatory systems of genes" are more significant than "changes in gene number or protein function" (Carroll 2000).

These researchers argue that the combinatorial nature of transcriptional regulation allows a rich substrate for morphological diversity, since variations in the level, pattern, or timing of gene expression may provide more variation for natural selection to act upon than changes in the gene product alone.

Epigenetic changes include modification of the genetic material due to methylation and other reversible chemical alteration (Jablonka and Lamb 1995) as well as nonprogrammed remolding of the organism by physical and other environmental effects due to the inherent plasticity of developmental mechanisms (West-Eberhard 2003). The biologists Stuart A. Newman and Gerd B. Müller (see articles in Müller and Newman, 2003) have suggested that organisms early in the history of multicellular life were more susceptible to this second category of epigenetic determination than are modern organisms.

See also

• Animal evolution
• Evolution of multicellularity
• Ontogeny
• Ontogeny recapitulates phylogeny
• List of gene families
• Important publications in evolutionary developmental biology

Notes

References

• Sean B. Carroll (2000). "Endless forms: the evolution of gene regulation and morphological diversity". Cell 101: 577-80.
• Sean B. Carroll (2005). Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom. Blackwell Science.
• Sean B. Carroll Jennifer K. Grenier, Scott D. Weatherbee (2004). From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design, 2nd ed., Norton.
• John Gerhart and Marc Kirschner (1997). Cells, Embryos and Evolution. Blackwell Science.
• Eva Jablonka and Marion J. Lamb (1995). Epigenetic Inheritance and Evolution: The Lamarckian Dimension. Oxford University Press.
• Marc Kirschner and John Gerhart (2005). The Plausibility of Life: Resolving Darwin´s Dilemma. Yale Univeristy Press.
• Monod J., Changeux J.P., Jacob F. (1963). "Allosteric proteins and cellular control systems". Journal of Molecular Biology 6: 306-329..
• Gerd B. Müller and Stuart A. Newman (Eds.) (2003). Origination of Organismal Form: Beyond the Gene in Developmental and Evolutionary Biology. MIT Press.
• Mary Jane West-Eberhard (2003). Developmental Plasticity and Evolution. Oxford University Press.

Further reading

• Leo W. Buss (1987). The Evolution of Individuality. Princeton University Press.
• Brian Goodwin (1994). How the Leopard Changed its Spots. Phoenix Giants.
• Stephen Jay Gould (1977). Ontogeny and Phylogeny. Harvard University Press.
• Alessandro Minelli (2003). The Development of Animal Form: Ontogeny, Morphology, and Evolution. Cambridge University Press.
• H. Allen Orr, "Turned on: A revolution in the field of evolution?", The New Yorker, 10/24/2005. Discussion of Carroll, Endless Forms Most Beautiful
• Rudolf A. Raff (1996). The Shape of Life: Genes, Development, and the Evolution of Animal Form. The University of Chicago Press.

External links

• [3] A 2000 issue of the Proceedings of the National Academy of Science (PNAS) devoted to "evo-devo" consisting of an editorial, several reviews, and several research articles.
• [4] A 2005 issue of the Journal of Experimental Zoology Part B: Molecular and Developmental Evolution devoted to evolutionary innovation and morphological novelty.
• Scott F. Gilbert, The morphogenesis of evolutionary developmental biology