I just want to note this study by Mark Christie and colleagues
We used a multigenerational pedigree analysis to demonstrate that domestication selection can explain the precipitous decline in fitness observed in hatchery steelhead released into the Hood River in Oregon. After returning from the ocean, wild-born and first-generation hatchery fish were used as broodstock in the hatchery, and their offspring were released into the wild as smolts. First-generation hatchery fish had nearly double the lifetime reproductive success (measured as the number of returning adult offspring) when spawned in captivity compared with wild fish spawned under identical conditions, which is a clear demonstration of adaptation to captivity. We also documented a tradeoff among the wild-born broodstock: Those with the greatest fitness in a captive environment produced offspring that performed the worst in the wild. Specifically, captive-born individuals with five (the median) or more returning siblings (i.e., offspring of successful broodstock) averaged 0.62 returning offspring in the wild, whereas captive-born individuals with less than five siblings averaged 2.05 returning offspring in the wild. These results demonstrate that a single generation in captivity can result in a substantial response to selection on traits that are beneficial in captivity but severely maladaptive in the wild.
We have few cases of new or recent domestication, so this kind of experiment is hard to do in other contexts. Also, in this case the selection is "natural-looking", imposed by the captive environment in some way, instead of directly applied by culling undesirable individuals. In most cases of mammal domestication, the wild relatives are either now vanishingly rare, or have been potentially influenced by introgression from the domesticated population. But I think it's reasonable to hypothesize that the additive variation in behavioral traits in wild populations is large enough to have allowed early mammalian domesticates like dogs and horses to adapt to captivity almost as fast as the salmon. Notice that the key element here is high reproduction in captivity, and in the salmon that trait covaries negatively with success in the wild.
Domestication may not have been a "hump" that humans brought wild animal populations over; it may have been a valley that trapped once-wild animals into dependence on humans.