Evolution, development and molecules

The evolution of complex biological systems is a major area of interest in modern science. It is generally agreed that, in a hierachical system, compleltely novel, emergent properties arise at each level. It has also been argued that the properties of one level in a hierachical system cannot be extrapolated downwards to other levels. Take, for example, the case of the brain. Thoughts and ideas are a property of the brain, considered as a whole organ; but the molecules that make up the neurons of the brain are not, themselves, considered to be capable of thinking. By contrast, when considering the evolution of embryonic development, there may well be some kind of “downwards causation” in which the properties of a higher level in the hierarchy can indeed influence the lower levels in that same hierarchy. I consider these ideas with reference to the evolution of vertebrate development, with special reference to the evolution of limbs and other aspects of the body plan. I argue that embryonic development is indeed a hierarchical biological system, in which the fertilised egg passes through various successive levels of complexity until it reaches the adult stage. But in this case, unlike the case of the brain, I argue that the higher levels in the hierarchy do indeed influence the lower levels. Thus, selection on the adult phenotype may lead to selection for different patters of gene expression in young embryos – a form of downwards causation.