Trafalgar,
i think it depends what you mean by 'specialists'.
If I had trained as a secondary science teacher, you would probably regard me as 'specialist' - I have a very high class Science degree + a PhD in an area that overlaps with 2 of the 'school' sciences.
However, for teaching the curriculum of any of the 'school' sciences, I would in fact be teaching something I had not studied since A-level / 1st year university.
Equally someone who studies English at university, and perhaps specialises in 20th century literature during their degree. When teaching Shakespeare, they would be teaching something they thrmselves had not studies since A-level.
Or a Historian, teaching 20th century German political history despite taking no courses at university that covered anything posy 1800 - in fact, there are A-level routes that also do little or no modern history, so they could be teaching material they hadn;t touched since GCSE.
Equally a Human Geography graduate teaching the Geography syllabus, which includes physical geography.
In almost all subjects, teachers will be teaching content for which they are actually non-specialists.
Without having a very, very large number of flexible part-time teachers - a geologist to teach the relevant parts of Chemistry and Geography, a wide group of History teachers each with specialist knowledge of different parts of the syllabus etc - there will be parts of every subject where even a 'nominally qualified' teacher will be teaching subject content that they have not studied post A-level, in some cases not post GCSE, and in some cases - e.g. Politics, some aspects of the modern computing syllabus - not formally studied themselves at all.