Mark 3 - Jon Anderson (Vocal), Bill Bruford (Drums), Steve Howe (Guitar), Chris Squire (Bass), Rick Wakeman (keyboards).
Mark 4 - Jon Anderson (Vocal), Steve Howe (Guitar), Chris Squire (Bass), Rick Wakeman (keyboards) and Alan White (Drums),
They kept playing Prog Rock after the Mark 4 but their golden period was already gone.
If we said Prog Rock is a fusion of Classical and Rock music, then Yes sure is the most cross sectional/typical Prog Rock band on earth. Their music was mainly built on classical music, what's different between was ELP playing classical pieces in rock style, but the way Yes composing and their songs structure, instrumentation and virtuosity were similar to classical music.
After The Yes album (1971), the music of Yes was turning into progressive rock. Fragile (1972) was said to be their first Prog record which Rick Wakeman started to play in Yes, each member in the group contributed their own pieces and few pieces that composed by the group. As a progressive record, Fragile had shown how classical music influenced in progressive rock through Rick Wakeman and Steve Howe's pieces.
Close to the Edge released in September 1972, which I took it as their first Prog, record rather than Fragile. Because it got their first epic, 19mins Close to the Edge on the whole side A of the LP. The piece contains 4 movements, basically about a trip as recalling the ego. Also, the piece was painted graphically in the inner sleeve.
Steve Howe was a different guitarist in the Prog Rock scene; his roots were in classical and Spanish rather than rhythm & blues. The first track in the side B, And You and I, had shown how Steve Howe bought an acoustic guitar to Prog rock.
As a live record, the following Yessongs was released as a conclusion of Yes and also the progressive finest movement.
Alan White joined Yes to replace Bill Bruford who went to join King Crimson, so the Mk 4 of Yes was formed. Tales from Topographic Oceans (1974) was the most ambitious album in the Prog Rock history; it contained only 4 pieces in the 2LPs set, which each side contains only 1 piece. They were about 20mins each, were based on the Schastic Scriptures from Autobiography Of Yogi.
Yes pushed Prog Rock almost to another level by Tales from Topographic Oceans, some critical even named it as Epic Rock. But ironically, Yes and the whole Prog Rock era started to fall, as they reached certain ground that they couldn't break through, also the audiences were already got bored of the long and exaggerative Prog sound. As members moved in and out, Yes got few records released in the late 70's, but none of them can take them to a higher peak.
Nowadays, Yes is the only Prog band remains as still playing concerts, releasing records and keep their unique Yes sound to their very loyal audience.