Complex sentence: The IMSA GT Series evolved into the American Le Mans Series, which ran its first season in 1999; the European races eventually became the closely related Le Mans Series, both of which mix prototypes and GTs.
Split into simpler sentences: The IMSA GT Series evolved into the American Le Mans Series, which ran its first season in 1999. The European races eventually became the closely related Le Mans Series, both of which mix prototypes and GTs.