Question

if a single species of squirrel evolved over time into 2 species, each on opposite side of the grand canyon, what is this change mostly due to?

Answer 1

On one side of the grand canyon could be a squirrel with a birth defect witch reproduses with a  squirrel with normal genes. But the birth defect is stronger than whatever trait that may be. so the litter of squirrels mostly have this trait. and the cycle contiues