However, I'd like to play devil's advocate with a thought experiment. Specifically, I'd like to address your fundamental assumption of equally distributed evolutionary pressure. It's true of our ancient history (which covers most of human existence), but it feels premature to ignore the potential impact of modern human societies over the last few millenia.
Imagine two identical human groups, one which spends 1000 or so years in an urban or quasi-urban setting, and one which spends the same time as hunter gatherers. Seems like some traits would be self-selected more heavily in one group vs the other based purely on environmental pressure. I.e. someone gifted with an above-average capacity for abstract modeling, but with - say - severe myopia, would find it easier to procreate in an environment where their skills are valued and their physical limitations negated, and vice versa.
Human history is long, but reproductive pressure can have a discernible impact on a population even over the relatively short time scale covered by human civilization. It's an imperfect analogy, but for instance new dog breeds only require a handful of generations (<10) to be both physically and behaviorally discernible. Why should reproductive pressures applied by different social environments, stretched over millenia, not have an impact on human populations?
I disagree with several points made here. Firstly, I would question your reasoning behind the prerequisite of the population being "small", because in fact larger societies face different and more complex challenges than small ones, which in turns favors the ability to navigate the challenges of large human groups. More to the point, environmental pressure is applied regardless of a group's size.
Secondly, an isolated population does not eliminate the vast majority of humanity - far from it. While there's always some form of mobility between societies, until very recently that was a relatively rare phenomenon. By and large people lived and died within artillery distance of where they were born.
> including any racist notions like "asian" or "black."
This is actually the first time I've heard those designations described as 'racist', but that's somewhat orthogonal and more of a passing observation
> In your dog metaphor, it's like looking at a dachshund and then saying "all dogs with short hair are also have short legs."
I don't follow this extension to my analogy, which is probably a reflection of the problems of using analogies as a discussion aid in the first place
> Between the occasional plague and the occasional famine, you have times of war and times of peace. You have changing aesthetics, shifting cultures, values gained and lost.
Yes, but at its core an urban, even if premodern, existence places emphasis on different skill sets than a hunter gatherer lifestyle. This goes back to my point above about the social challenges of navigating large human groups - aesthetics can change, but figuring out how to grapple with complex social dynamics, as opposed to taming nature, is a constant (and one closely correlated with the evolution of human intelligence to boot)
Species can and do evolve different adaptations to the same evolutionary pressures. Bret isn't convincing.
Physical challenges are unevenly distributed. Some places are very hot and others are very cold. Some places are high elevation. Bodies have adapted to these differences to produce the physical differences across races. The brain solves for the challenge of life and life is difficult everywhere. What the brain adapted to is evenly distributed.