You know, the neurophysiological background of high intellectual abilities is level of complexity of brain cortex and subcortical structures. The development of neurons forming them occurs when we use intensively our mnemonic and higher cognitive functions, e.g. memorizing vast amounts of new information, mastering and developing some intricate compound concepts. When a chain of neurons is activated it increases the size of each neuron cell, the speed of thought electromagnetic impulses distribution and the number of connections between the neurons through synapses.
However different brain activities need different substratum for passing and stimulate development of different brain departments. People of different occupations activate their brain for solving different kinds of tasks and that's why there is no universal concept of intellect that would be universal, e.g. could be measured and compared in any cases. There are practical intellect, mathematical intellect, abstract