Charles Darwin‘s theory of evolution is based on three principles:
– Heredity
– Variation.
At the time that Darwin wrote, the principles underlying heredity and variation were poorly understood. In the 1940s 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 has arisen. Currently, it is well understood how genetic mutation occurs.
Evolutionary developmental biology studies how the dynamics of development determine the phenotypic variation arising from genetic variation and how that affects phenotypic evolution. At the same time evolutionary developmental biology also studies how development itself evolves. Thus, the origins of evolutionary developmental biology come from both an improvement in molecular biology techniques as applied to development, and the full appreciation of the limitations of classic neo-Darwinism as applied to phenotypic evolution.
Evolutionary developmental biology is not yet a unified discipline, but can be distinguished from earlier approaches to evolutionary theory by its focus on a few crucial ideas. One of these is modularity. As has been long recognized, plants and animal bodies are modular: they are organized into developmentally and anatomically distinct parts. Often these parts are repeated, such as fingers, ribs, and body segments. Evo-devo seeks the genetic and evolutionary basis for the division of the embryo into distinct modules, and for the partly independent development of such modules.
Another central idea is that some gene products function as switches whereas others act as diffusible signals. Genes specify proteins, some of which act as structural components of cells and others as enzymes that regulate various biochemical pathways within an organism. The modification of existing, or evolution of new, biochemical pathways depended on specific genetic mutations. In 1961, however, Jacques Monod, Jean-Pierre Changeux and François Jacob discovered within the bacterium Escherichia coli a gene that functioned only when “switched on” by an environmental stimulus. Later, scientists discovered specific genes in animals, including a subgroup of the genes which contain the homeobox DNA motif, called Hox genes, that act as switches for other genes, and could be induced by other gene products, morphogens, that act analogously to the external stimuli in bacteria. These discoveries drew biologists’ attention to the fact that genes can be selectively turned on and off, rather than being always active, and that highly disparate organisms may use the same genes for embryogenesis, just regulating them differently.
Another focus of evo-devo is developmental plasticity, the basis of the recognition that organismal phenotypes are not uniquely determined by their genotypes. If generation of phenotypes is conditional, and dependent on external or environmental inputs, evolution can proceed by a “phenotype-first” route, with genetic change following, rather than initiating, the formation of morphological and other phenotypic novelties.
13.11.1 History
An early version of recapitulation theory, also called the biogenetic law, or embryological parallelism, was put forward by Étienne Serres in 1824–26 as what became known as the “Meckel-Serres Law” which attempted to provide a link between comparative embryology and a “pattern of unification” in the organic world. The anatomist Richard Owen used this to support his idealist concept of species as showing the unrolling of a divine plan from an archetype, and in the 1830s attacked the transmutation of species proposed by Lamarck, Geoffroy and Grant. In the 1850s Owen began to support an evolutionary view that the history of life was the gradual unfolding of a teleological divine plan, in a continuous “ordained becoming”, with new species appearing by natural birth.
Ernst Haeckel (1866), in his endeavour to produce a synthesis of Darwin’s theory with Lamarckism and Naturphilosophie, proposed that “ontogeny recapitulates phylogeny,” that is, the development of the embryo of every species (ontogeny) fully repeats the evolutionary development of that species (phylogeny. Haeckel’s concept explained, for example, why humans, and indeed all vertebrates, have gill slits and tails early in embryonic development. His theory has since been discredited.
D’Arcy Thompson postulated that differential growth rates could produce variations in form in his 1917 book “On Growth and Form”. He showed the underlying similarities in body plans and how geometric transformations could be used to explain the variations.
Edward B. Lewis discovered homeotic genes, rooting the emerging discipline of evo-devo in molecular genetics.
13.11.2 The developmental-genetic toolkit
The developmental-genetic toolkit consists of genes whose products control the development of a multicellular organism. Differences in deployment of toolkit genes affect the body plan and the number, identity, and pattern of body parts. 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 encode for the production of transcription factors, cell adhesion proteins, 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.
Among the most important of the toolkit genes are those of the Hox gene cluster, or complex. Hox genes function in patterning the body axis. Thus, by combinatorially specifying the identity of particular body regions, Hox genes determine where limbs and other body segments will grow in a developing embryo or larva. Mutations in any one of these genes can lead to the growth of extra, typically non-functional body parts in invertebrates.
13.11.3 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, including those of the developmental genetic toolkit and other genes involved in development.
Even within a species, the occurrence of novel forms within a population does not generally correlate with levels of genetic variation sufficient to account for all morphological diversity.
A major question then, for evo-devo studies, is: If the morphological novelty we observe at the level of different clades is not always reflected in the genome, where does it come from? Apart from neo-Darwinian mechanisms such as mutation, translocation and duplication of genes, novelty may also arise by mutation-driven changes in gene regulation. The finding that much biodiversity is not due to differences in genes, but rather to alterations in gene regulation, has introduced an important new element into evolutionary theory.
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. Some 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 alterations of gene regulation or phenotype generation that are subsequently consolidated by changes at the gene level constitute another class of mechanisms for evolutionary innovation. Epigenetic changes include modification of the genetic material due to methylamine and other reversible chemical alteration as well as non-programmed remolding of the organism by physical and other environmental effects due to the inherent plasticity of developmental mechanisms.