Population , we breath , you breath CO2 is increasing , so stop breathing and die somewhere lol. (ok just kidding)
The answer is speciation by geographic isolation. When the population of a species gets separated due to reasons such as continental drift, mountains, canyons, etc. then each separated group of population may survive and adapt to their respective environment leading to speciation. These barriers do not allow the evolving populations to interbreed initially and over time when they evolve into different species they become completely reproductively isolated.
If a person is shot, and the bullet destroys the part of the brain called the medulla, which maintains basic life functions (heart rate, breathing), there is virtually no chance for surviving that injury
