Selective breeding means many of the dog breeds that are around today no longer look anything like what dogs used to look like. Since the first dog was domesticated some 30,000 years ago, people have purposely bred certain types together in order to select for very particular traits. Over thousands of years, this has led to a huge variety in the species, with breeds differing from each other in significant ways.
This process only accelerated in the 20th century as more sophisticated methods of selective breeding were discovered, and modern medicine allowed for breeders to inspect dogs at the genetic level. This means that dog breeds 100 years ago are vastly different to their contemporary counterparts. In some cases, this breeding has led to dogs that just appear different, but in others it has caused severe problems to the animals' health. These images of dog breeds then and now provide clear evidence of the impact of selective breeding.