Right now, some individuals are better off in their locale because natural selection has favored them. A favored animal may pair with an animal adapted to a different environment. Their offspring would probably have a gene combination poorly adapted to either situation. The kids are bad off. The real world of species prevents such unrestricted sex.
Evolution segregates the total genetic variability into discrete packages.
We call these packages, species . Nature separates one species from another by reproductive barriers and thus prevents the production of too great a number of bad gene combinations . The reproductive barriers vary from one species to another. Chance determines what kind of device a species uses. The species concept is complicated.