Hallo Wayne W !!!
Shortly,the Yakovlev's WW2 Yak fighter series was started by I-26 ( later called Yak-1) prototype.This aircraft was built as the answer to the Kreml's requests for the new fighter according to the Russian experience gathered in Spain War. I-26 would have been a "mixture" of Supermarine Spitfire and Messerschmitt Bf-109 ,the best fighters in western Europe.Equipped with M-105 engine was flown the first time in 1939.In December 1940 the names of the Russian planes were changed.From that time all names of aircrafts started with the first letters of the constructors' surnames.So the I-26 became YAK-1 ( Yakovlev-1).During the WW2 which started for Russian on 22'nd June of 1941, the Yakovlev's Design Bureau (OKB no. 115) developped the serie that at the end of WW2 consisted of Yak-1,Yak-1M,Yak-7,Yak-7A,Yak-7B,Yak-7W,Yak-7UTI,Yak-7D,Yak-3 and Yak-9 serie.All the planes were very similar to each other.The difference were in engines, armament, dimensions.Yakovlev used for their construction non deficit materials ( wood etc...).The exception was Yak-3 which was of metal. That's all mate.