CREATE TABLE table_71507 ("Member countries" CLOB, "Population" CLOB, "Area  ( km\\u00b2 ) " CLOB, "GDP  ( billion US$ ) " CLOB, "GDP per capita  ( US$ ) " CLOB)

What is the Population of the nation that has a Member countries consisting of existing members (1973)?
SELECT "Population" FROM table_71507 WHERE "Member countries" = 'existing members (1973)'
SELECT "Population" FROM "table_71507" WHERE "Member countries" = 'existing members (1973)'