How are new species formed?
Stage 1: A barrier to breeding.
Examples of barriers include mountains, the ocean, land (for
water creatures), rivers, deserts, and ice caps.
Smaller barriers may include altitude or depth (in water), sunny
vs. shady places, different temperatures or amounts of rainfall, and different
soil types. All of these can limit several populations of the same species from
intermixing their genes and thus prevent them from staying the same.
Long & Slichter