Adaptor proteins are proteins with a specific recognition domain on one end, and a general signalling domain on the other end. The recognition domain allows the identification of a specific phosphorylated receptor. The signalling molecule, which recognises the general signalling domain, is then able to activate the signalling pathway. Adaptor proteins are able to recognise phosphorylated receptors, due to the conformation the phosphorylation induces. This allows specific identification.
By having modularity in signalling pathway, it is possible to develop new pathways through evolution (using alternative splicing and fusion proteins).