In a nutshell, you have to undergo specialist training in order to become a consultant. Ie all consultants are specialists. This applies to both doctors and dentists.
As of 2016, about 40% of doctors (5047) are specialists in Singapore. I suspect that the number of specialists will fall, as our population ages and we need more doctors within primary care (ie GPs).
In fact, there are also a group of GPs who undergo additional and specialised training under the Family Medicine Programme to provide even better care within the primary sector.
Are there doctors who choose not to become consultants?
Yes, there are.
If so, how is their career progression like, what ranks do they have?
Within the public sector, doctors can continue working as Medical Officers, or choose to work as RPs (resident physicians) who have a pay grade similar to a registrar in specialty training.
The pay for RPs probably maxes out at about Associate Consultant level.
Often, doctors who don’t undergo specialty training also leave for the private sector.
Cheers