Subspecies were a result of evolution. New subspecies, as well as new species, meant that Light's undertaking of cataloguing life became an endless task. (TV: Ghost Light)

Horlak's Silurian subspecies carried Raldoma Syndrome which, while harmless to themselves, was deadly to the other 99% of Silurians. (COMIC: The Lost Dimension)

The Tenth Doctor described the new humans as "a whole new subspecies". (TV: New Earth)

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