A 1940s psychologist named Robert Tryon wondered if rats could be bred to complete a maze more competently, and after seven generations of selective breeding he succeeded. That classic experiment is still shaping our thinking about the age-old question of nature versus nurture when it comes to human intelligence and behavior.
The real problem with eugenics and selective breeding is that you are by necessity limiting your gene pool and therefore genetic diversity. While you are able to breed for select desired traits you often also get traits that you don’t want.
Even if you can keep from getting any undesired traits your new breed will lack the genetic diversity of the old breed. This means that an environmental or biological factor (like a virus) has a higher chance of being able to wipe out the whole of the new breed. Genetically diverse populations are more resilient against extinction because their genetic diversity makes them more adaptable as a group.