Tactical Change in the Roman Army

Today we will be thinking about tactics, and especially infantry tactics in the Roman period. In the more modern era, technology has often caused tactics to change, which in turn has given impetus to make new techological advances, but in the ancient world this was rarely the case. In a broad sense, the types of weapons used by the Romans in the Punic Wars were not so very different to those employed in the fifth century AD - certainly not compared to say smoothbore muskets to rifles, to breech loaders to magazine rifles etc. So if technology was less of a factor, how and why did Roman tactics change?