Evolutionary developmental biology
Evolutionary developmental biology, commonly called Evo-devo, is the discipline of biology that studies developmental processes in different species and across taxa in order to better understand the developmental processes themselves, evolutionary relationships between organisms, and the molecular processes underlying phenotypic variation and change. Evo-devo is often considered to be a sub-discipline of developmental biology, as it examines how changes in developmental processes are responsible for physical differences between species.
Evo-devo is relatively new as a field in its own right. It was founded in the late 1970s and early 1980s after early studies in comparative genomics revealed the unexpected genetic similarities between different species of complex organisms (mostly animals). Early discoveries in Evo-devo resulted in a major paradigm shift in evolutionary thought; establishing that changes in patterns of gene expression, rather than changes in the protein-coding sequences of the genes themselves, are the major underlying cause of morphological variation.
Although now thoroughly discredited, recapitulation theory was an early attempt to examine development in an evolutionary context. This theory, summarized by the famous phrase "ontogeny recapitulates phylogeny", was quickly demonstrated to be largely incorrect. However, the hypothesis that similarities and differences in early embryonic development can be used to infer evolutionary relationships and the mechanisms giving rise to physical similarities and differences between species has been consistently well supported by experimental evidence for almost a century.
With the rise of molecular biology and genetics in the second half of the twentieth century, it became possible to investigate the molecular and biochemical mechanisms governing development. This led to early studies comparing the molecular regulation of development in various species.
One surprising finding to come out of these early studies was the high degree of conservation (that is similarities) of the molecular mechanisms governing development in seemingly distantly-related clades. This finding catalyzed a major paradigm shift in the field of evolutionary biology, revealing, for the first time, the molecular causes of phenotypic variation and change.
The development of a single cell into a large, complex organism with well-organized tissues is an elaborate process requiring intricate regulation. Cells have to divide, migrate, apoptos, differentiate, and undergo a host of other processes with clockwork precision in close coordination with other cells. A defect in early development is universally lethal to a complex organism; similarly, even defects late in development are often profoundly detrimental to an organism's ability to survive in its environment.
These simple observations posed a significant paradox in evolutionary thought. If evolution is driven by the force of natural selection acting on phenotypic variations between individuals, then how do these variations arise without irreversibly disrupting intricate developmental processes? The answer to this question is deceptively simple; these variations arise by reorganizing existing developmental processes while leaving their machinery (that is, genes and molecular signalling pathways) largely intact.
The evidence for this explanation is abundant. The genes responsible for regulating embryogenesis are very highly conserved among animals; they do not change significantly in sequence or function, even between clades that appear to be relatively distantly related. For instance, the Hox genes (which are needed to organize body plan) are expressed in an almost identical pattern in a fruit fly embryo that they are in a mouse (or human) embryo. The differences in developmental regulation that do exist between clades tend to be simple variations in the expression pattern of molecular developmental regulators, rather than structural variations in the regulators themselves.
- Stern, David. (2010). Evolution, development, and the predictable genome.
- Gilbert, SF. (2010). Developmental biology, 9th edition