How species form

Divergence and barriers.

Splitting

When populations stop interbreeding, their differences accumulate independently until they can no longer produce fertile offspring. Geographic separation is the common cause.

Species boundaries are fuzzy, because the process is gradual.
Ring species show intermediate stages happening today.

Speed

Change can be slow over millions of years or fast when pressure is intense, as with antibiotic resistance. There is no fixed rate, only fixed rules.