Evolution proposes that when random genetic variations or mutations occur, those that are beneficial (aid the organism in survival and reproduction) remain, and the mutated gene is passed to the next generation. Differences in various dog breeds would be an example of microevolution, whereas a reptile evolving into gorilla demonstrates macroevolution.
Microevolution isn’t disputed, but the mechanism for change isn’t always clear. For example, a large variation of dog types have been created from a single ancestor by selective breeding, which doesn’t involve Darwinian random genetic mutations. Modifications come by intelligently combining dog’s genetic information through creative breeding, with no changes to DNA introduced.