Well, I'll try my best.
Basically, it's like this. The Theory is that Pokemon Evolution is always in Motion;
As Pokemon gain newer Evolutions as time goes by, first Stage evolutions tend to fall off,
getting replaced by later Evolutions or becoming extinct (much like real-life evolution theory.)
In the case of
Nidoran, Nidoran is once thought to have been a single species,
But over time, their each respected species diverged, specializing along gender.
This is prior to there being a
Nidorino/
Nidorina 2rd stage Evolution,
whichcame about as a result of the gender specialization and
Nidoking/
Nidoqueen arose when Moonstones were introduced.
As a result, the original species of Nidoran that was composed of both Genders now no longer exists,
Only Nidoran (male) and Nidoran (female) remains.
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