Discuss the roles of development, learning and evolution in the construction of the nervous system.
Evolution, development and learning all affect the organisation of nervous systems and all are inextricably linked together. The importance of evolution can be observed easily simply by looking at the differences that occur in nature, from the relatively simple systems in invertebrates through to what is perhaps the pinnacle of complexity in the primates. Natural selection will act upon the behaviours asserted by the animals and the underlying neural circuitry will be selected as a consequence. New structures that arise as a result of such selection will be present in some animals as opposed to others as developmental systems will have been affected due to the appearance of novel homeobox genes. As a result, individuals of phyla will have similar basic neural organisations, differing only due to polymorphism which also provides the raw material for natural selection.
There is also a third mechanism which acts to modify this general circuitry, and this is learning. Such a mechanism is viable due to many epigenetic processes which can act to revise synaptic connectivity in response to experience, thus allowing individuality. Learning is linked to development as this is the period during which the system is most amenable to such processes, in addition to learning having a genetic basis, which also links it to evolution. Thus each of the factors plays an important role in the organisation of neural circuits, albeit to different degrees, the first two, for example are likely to contribute more to the disparity between higher taxonomic groupings, whereas the latter will assert its effects primarily in variation within species. In this essay I shall discuss each of the factors and their contributions to the differences observed in the nervous systems of animals.