Synaptic Modification
Once the basic variegated anatomical structure of the brain is laid down during early development, it is more or less fixed. But given the numerous and diverse collection of available circuitry, there are bound to be functionally equivalent albeit anatomically non-isomorphic neuronal groups capable of responding to certain sensory input. This creates a competitive environment where circuit groups proficient in their responses to certain inputs are "chosen" through the enhancement of the synaptic efficacies of the selected network. This leads to an increased probability that the same network will respond to similar or identical signals at a future time. This occurs through the strengthening of neuron-to-neuron synapses. And these adjustments allow for neural plasticity along a fairly quick timetable.
Read more about this topic: Neural Darwinism