It’s better if you read the thread.

The hard statement, on the other hand, is that where you have two populations that can be reliably distinguished by their gene pools, any phenotypic gap is likely to be essentially genetic in origin.

  • /u/JuliusBranson

and the rebuttal:

This is a fatal error, and anything which flows from it can be immediately discarded as unsupported.

Let’s consider chimps and humans, two species separated by ~5 million years of evolution, and which everyone would agree are genetically distinct in many, many ways. They are immediately distinguishable on both genetic and phenotypic examination, and on would be hard pressed to mistake one for the other.

One of the crucial distinctions is bicondylar angle, the angle of the femoral condyles at the knee relative to the femoral shaft. In chimps, this is near-zero because, as predominantly quadrupedal animals, the femur is held parallel to the tibia and both are vertical. As a result, the feet are widely spaced which gives the stability when quadrupedal, but makes them unstable when they walk bipedally, causing them to sway from side to side. In humans, the femur angles inwards while the tibia remains vertical, bringing the feet together under the body and reducing the sideways sway during locomotion. Correspondingly, the offset angle between the femur midshaft and knee articulation is ~0 degrees in chimps but about 12 degrees (give or take) in humans.

I would like to pause to note that I’m not just pulling up some obscure bit of anatomy here. The biggest reason Lucy the Australopithecus (and the partial knee remains discovered nearby a few years earlier) shook the entire physical anthropology world was because the fossils show a human-like bicondylar angle, and therefore evidence of upright walking. Lucy’s knees are up their with Ostrom’s re-descriptions of Deinonychus as an active, endothermic predator and Walcott’s discovery of the Burgess Shale fauna as the greatest paleontological discoveries of all time.

Surely such a massively important anatomical difference, that’s been in place for millions of years and underlies upright walking, one of the defining events of our evolution, is genetic, right?

Nope.

In fact, it’s mostly the product of “phenotypic plasticity”, the ability of organisms to modify their anatomy, biochemistry, and, yes, nervous system to suit their environments. Habitual bipedalism in chimps and other primates leads to non-heritable, phenotypically plastic modifications of the femur towards human and australopith values (1, 2) (whether natural or due to human training). Conversely, humans who have never walked due to paralysis since infancy show no bicondylar angle at all, and very “chimp-like” femurs (1, 2).

This represents merely a very visually striking and emotionally resonant example, as it deals with a major morphological difference in the most studied (or over-studied) evolutionary transition ever, but is FAR from exceptional. Dramatic plasticity occurs in mammals, birds, reptiles, amphibians, fish, insects, and crustaceans. This can include mechanical and biochemical responses to the environment as well as social and behavioral changes. It interacts with genetic evolution in bizarre and sometimes counter-intuitive ways. In some cases, information does seem to be transmitted generationally, but also seems to “evaporate” after a few generations.

The problem I see in most HDB arguments is not that IQ isn’t highly heritable or that humans don’t have geographical genetic variation, it’s wanting to jump straight from one to another and skip all the steps in between. In fairness, some of these steps are impossible or insanely difficult without transgressing major ethical barriers. We already know IQ is highly plastic at least in the downwards direction due to factors such as malnutrition and heavy metal contamination, yet these factors are never, IMHO, given adequate consideration.

You might cry “gene denialism”, but I’d be on equally strong footing calling “plasticity denialism”.

  • /u/GeriatricZergling