What is the Population of the nation that has a Member countries consisting of existing members (1973)?
CREATE TABLE "1973_enlargement" (
  "member_countries" text,
  "population" text,
  "area_km" text,
  "gdp_billion_us" text,
  "gdp_per_capita_us" text
);
SELECT "population" FROM "1973_enlargement" WHERE "member_countries"='existing members (1973)';