We've heard the age-old debate of "nature vs. nurture" - whether a person or dog is innately "born" with certain characteristics, or if it's a product of its environment or training. What we don't usually consider, however, is the influence nurture can have on genetic evolution.
Epigenetics is the idea that genetic evolution can be influenced by environmental stimuli. This means that a dog is predisposed to be of a certain size and weight, given his parents' size and weight, but can possibly be influenced by nutrition, exercise, and early stressors or environmental factors.
It is well-proven that weight-lifters and athletes have denser bone structure to support the heavier muscle on their frame, and muscle itself (male and female) contains testosterone. So an athlete who is well-nourished and well-muscled may in fact be a more "strapping" specimen than its parents, especially if this given spurt of athleticism and muscle/bone development was at adolescence when it was determining the overall adult self and sexual contribution to the gene-pool. By stressing or changing the environment of this particular dog in some way, mutations were introduced to allow the dog to adapt or change and thrive for the environment requiring it to be more muscular and/or athletic. In this way, evolution has occurred, and his or her offspring will become legacy to this gene mutation.
So consider a dog who has a polygenetic trait (a trait consisting of multiple contributing genes) influencing its temperament. It's generally low-key, but it gets sold to a high-stress environment where there is a lot of tension and negative stressors. Another dog it lives with fights with her daily, there are always fireworks going off across the street, the owners and family are dysfunctional and yell at each other a lot... it's a pretty miserable environment for the poor dog to be in. So the constant strain and stress she's under creates a chemical imbalance that influences a stronger fight or flight expression in her genes. This bitch is then bred and has a litter of puppies - who not only inherit these mutated genes, but also experience at key developmental times this environment and are influenced by mom's constant anxiety. It's not a good start, and so they end up having even more anxiety - pretty soon your low-key, nice dog has developed a legacy of mutated genes that compound themselves, lending to heightened fear, aggression, and anxiety - because in the wild, these traits would be beneficial to keeping the dog alive.
So when you consider getting a dog from a pet-store or puppy-mill, consider you, too, are negatively evolving a system by rewarding mutations in a system that neglects the needs of the individual adults and puppies, pasts and futures of their lines by selling them through impersonal means and assembly-line point-of-sale.
Now consider what you can do to positively influence a dog's evolution and genetics through an environment that promotes health and happiness, and challenges yet doesn't defeat. It is important to consider the quality of nutrition, sleep, conditioning and mental well-being of your parents as much as the puppies and the homes they enter.
A responsible breeder is not simply using a dog as a baby-factory - this is their family-member and companion, and she should be well-taken care of, loved and give this impression. She is their pet for 99% of the time, and only active as the mother of puppies for ~4 months. How well she is loved, cared for, and treated can influence gene expression at a critical time when a puppy's personality is developing for life - because while certainly temperament is inherited, her influence will also be paramount. You'll want a loving mother with a temperament to match what you want in your puppy, as they will learn by her example and be forever changed by her experiences.
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