Gene duplication followed by divergence can produce:
Gene family expansion occurs through duplication events including tandem duplication, segmental duplication and whole-genome duplication. Paralogous copies initially redundant undergo divergent selective pressures: subfunctionalization partitioning ancestral functions or neofunctionalization acquiring novel interactions. Some duplicates accumulate disabling mutations such as premature stops or frameshifts, becoming non-functional pseudogenes transcribed poorly or not translated. Globin clusters exemplify this process, containing functional embryonic, fetal and adult genes alongside pseudogenes
Ref: Ohno S., Evolution by Gene Duplication, 1970; Zhang J., Genetics 2003: Paralogs and Pseudogenes