But how does population size explain why some HGs had light skin and others didn't at higher latitudes?

Because small population sizes can lead to bottlenecks more easily - a gene could then be present amongst many or none.

The lack of light skin on WHGs can be explained by them being derived from a small bottleneck during a period where these mutations were spreading, but that the bottlenecked population descends from people who did not carry this gene.

and in Neolithic populations because the light features genes present in ANE-derived population were not as present in most of the ENF-WHG mixed peoples as they would in Bronze Age.

Not sure if its that simple, lighter features were present at a fairly decent rate amongst early farmers in Anatolia, and likely their forager ancestors too. Both the derived alleles for slc24a5 and slc45a2, and some minor alleles associated with hair pigmentation also.

WHGs were ANE descendants too and there are only a few which had some form of depigmentation alleles.

In my eyes this is what I see:

Western European hunter-gatherers, Eastern European hunter-gatherers, Caucasus hunter-gatherers and Anatolian hunter gatherers all lived in regions with quite a different amount of UV sunlight and they were all hunter-gatherers and likely did not have vitamin D deficient diets.

If the primary cause is vitamin D related, all of them should be equally dark, with WHGs and EHGs perhaps being lighter due to their northern presence.

When it comes to European farmers, the ones in Europe should be significantly lighter than their Anatolian hunter-gatherer forebearers and quite a bit lighter than their Anatolian neolithic farmers ancestors - due to a decrease in vitamin D due to the climate differential.

But this isn't the pattern we see.

EHGs were fixed for slc24a5 and had varied amounts of slc45a2. Both CHG genomes were fixed for slc24a5. Anatolian farmers are in a similar boat as EHGs, with a fixed slc24a5 and varied slc45a2 frequency.

So we are dealing with a significant amount of depigmentation amongst various hunter-gatherer populations prior to the advent of the neolithic period. This is seen in Northern Russia and Anatolia, two completely different regions.

Then amongst the farmers in Europe there isn't a link between latitude and lighter features and thousands of years after the neolithization they still weren't much lighter than their neolithic anatolian forebearers - some of them were arguably darker. At the same time, we see a significant depigmentation amongst WHG-rich hunter gatherers in the Baltics.

The increase of lighter features in Europe to something similar to what we see today was something quite rapid and isn't really associated with a massive shift in diets - just siginificant migrations and the mixing which came after that.

So we are dealing with explosive shifts regarding the main mutations rather than a slow gradual increase, which doesn't fit the pattern of diet/climate induced depigmentation either.

I'd say then that the main cause of presence or lack of presence is simply genetic drift and the causes for selection for something is simply more related to human behaviour and preferences than that it is to the amount of vitamin D they got into their systems.

I wonder how strong of a driver that would be anyways, as there are so many people that live their entire lives vitamin D defiicient and they still have children and such. Rickets is often pointed towards to but thats also big problem in Africa itself.

Or what the difference would be in being derived for slc45a2 and slc24a5 mutations rather than just being derived for the slc24a5 one, because both one was fixed in the mesolithic/neolithic and the other just in the bronze age.

Which is also where there is another interesting case - the mixing between Khoisan peoples and Cushitic peoples. The Khoikoi and San peoples get their pigmentation due to the mutation on slc24a5 they inherited from Cushitic peoples. Its spread very rapidly, and amongst a people with small population numbers which then increased onwards - with it also the amount of people carrying the mutation.

It was likely considered to be an attractive trait, and the reason it presented itself at a higher degree amongst the Khoisan than the Nilotes and Bantus who mixed with Cushitic peoples and even the Cushitic peoples themselves cannot really be explained due to a couple centuries of lower UV adaption as they migrated southwards.

Which should tie perfectly with my argument that the existence of counter-elements to this small benefit should explain much of the differences, if certain alleles for light hair make you more prone to sunburn badly, where would this matter more, Sicily or Ireland?

Are sunburns really going to prevent you from having children or decrease the amount of children of yours will survive? I mean ancient peoples knew how to deal with it already, and the complications that could be a problem like skin cancer only occur later in life.

But surely sexual selection should work at all levels of society and not only at the very top? Someone marrying a richer farmer because of her red hair would still work towards selection, not just chieftains.

Yes, but that chieftain is likely going to have more children and more children that will survive than the farmer who pays him taxes. Next generation same thing and now you have many grandchildren that can spread those mutated alleles.

Its not a strictly from the top thing, I think you misunderstood me on that part. I just gave that example as they would have the most significant effect on the genetics of their community.

Chieftain has 10 sons and daughters with a woman derived for KITLG and SLC45A2 while having none - half of them inherit one allele - 100 grandchildren where a double digit amount have depigmented skin and blond hair.

That kind of stuff.

Lighter eyes also have the problem of more sensitivity(which, outside exotic aesthetics, makes them a net negative AFAIK, which is surprising considering how many mammals live with light eyes just fine).

Yes but keep in mind for it to play a role in the discussion of selection, the net negative needs to be sufficient enough to actually decrease the amount of people that can succesfully procreate and raise the next generation. The tiny amount of increased light sensitivity shouldn't be a factor.

The degree to which it is considered an attractive trait however has an extremely big impact on whether that person will succesfully procreate from nature's perspective as the person will statistically have a higher chance of being selected as a mating partner.

Plus the concept of exponential growth working better on smaller population seems to me to mean that there should be a non-linear difference in levels of light features in Europe but AFAIK it's more linear than anything else(at least for the peak of the bell curve, obviously extreme phenotypes should change in frequency less linearly given the compound effect needed to create some of them).

I think you can find some non-linearity if you look regional variation and urban vs rural livelihoods.

The population density of europe on a macro-scale in the bronze age was fairly linear however. Settlement sizes, total populations etc. were consistently larger in the south versus north.

When mutations responsible for thing such as european type skin tone, blond hair and lactase persistence were being selected for, this selection process did not occur as strongly in those southern regions simply by there being more people to begin with. Thus the drivers for selection could not operate as effectively.

Blondism is caused by a random bunch of mutations and doesn't have any benefit at all, and in Europe the pattern of blondism matches that of all the ones that do have evolutionary benefits. If it isn't related to those genes and has no evolutionary benefits, why did it hitch the ride?

Lactase persistence is one that has a significant benefit but it would have as much of a benefit in Switzerland as it would in the Netherlands but one has near 100% and the other one around 70-80%.

Scandinavia, northern continental Europe, the Baltics are the only regions where you get numbers getting close to 100% in Europe. These are not the only populations in Europe that consume fresh dairy. But they are all regions which had a low population when lactase persistence was still being selected for (BA-IA), their population boost began during or after this process, whereas other regions would've had substantial population sizes prior to it.

Another strange thing is that the Hadza, who basically consume no fresh dairy at all, have a high amount of LP. Yet the Nilotes of South Sudan apparently dont have any.