Well, modern medicine + economy + social pressure resulted in RADICAL change in fitness function for human population. It's very, very different.

So it's quite likely that modern population is not fit according to old criteria.

> It's also very important to remember that this operates over hundreds of millennia.

That's not true at all. People can make new breeds of dogs and cats in just a few generations. You can literally SEE how a change of fitness function affects the phenotype.

> You'd need to look back into deep prehistory to find changes to humans attributable to natural selection.

There are many studies which describe genetic changes within latest 10,000 years or less. E.g. paper "1,000 ancient genomes uncover 10,000 years of natural selection in Europe": "We identified 25 genetic loci with rapid changes 21 in frequency during these periods". You can find many similar papers if you do a search

One of studies identified changes in loci associated with Y. pestis immunity during the Black Death (i.e. something like a century). Black Death mortality is similar in scale to early childhood mortality 150 years ago.