user: Question: What is the Population of the nation that has a Member countries consisting of Existing members (1973)?
Columns: ['Member countries', 'Population', 'Area (km²)', 'GDP (billion US$)', 'GDP per capita (US$)']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Denmark', '5,021,861', '43,094', '70.032', '$59,928']
SQL:

assistant: SELECT "Population" FROM "Table" WHERE "Member countries" = 'Existing members (1973)';