Final answer:
The Roman Senate evolved from an advisory body to the king into a powerful legislative institution in the Republic, then saw its power decline in the late Republic and under the Empire, becoming largely symbolic after the reforms of Emperor Diocletian.
Step-by-step explanation:
The make-up of the Roman Senate evolved significantly over time. During the kingdom phase, it was primarily an advisory council to the king, but as Rome transitioned into the Republic, the Senate became more powerful, with elders holding high prestige, as they were the only ones eligible to become senators. This period saw the Senate exercise real legislative power, control over finances for war and public building, and a strong influence on foreign policy. However, during the late Republic, its influence began to wane, and after Augustus founded the Empire, the Senate's power diminished greatly. Under Diocletian's reforms in the third century CE, the Senate eventually became largely ceremonial and far less relevant to the central administration of the empire.