  Blazar subclasses ranging from flat-spectrum radio quasars (FSRQs) through
low-frequency-peaked BL Lac objects (LBLs) to high-frequency-peaked BL Lac
objects (HBLs) exhibit a sequence of increasing spectral hardness with
decreasing luminosity that cannot be explained solely by orientation effects.
Using an analytic model for the synchrotron, synchrotron-self-Compton, and
Compton-scattered external radiation from blazar jets, we propose an
evolutionary scenario that links these blazar subclasses in terms of a
reduction of the black-hole accretion power with time. As the circumnuclear
material accretes to fuel the central engine, less gas and dust is left to
scatter accretion-disk radiation and produce an external Compton-scattered
component in blazar spectra. This evolutionary trend produces the sequence FSRQ
-> LBL -> HBL. Such a scenario may also link radio-loud AGNs with ultraluminous
infrared galaxies and optical QSOs, if the latter constitute the high
Eddington-ratio epoch of supermassive black hole growth, as suggested by the
observed anti-correlation between radio and soft X-ray activity in some
Galactic black-hole candidates.
