Heritability, Vg, Ve, and Cranial Morphology in Humans and Neandertals.

1. Both your interview and your article mentioned the complex factors involved in Neandertal brain/cranial morphology.  What is the h² of facial form (or facial size) thought to be?  What does this tell you about the V_E and V_G of cranial morphology?

 Both your interview and your article mentioned the complex factors involved in Neandertal brain/cranial morphology. What is the h² of facial form (or facial size) thought to be? What does this tell you about the V­_G and V_E of cranial morphology?

When considering any trait in an organism, evolutionary biologist must try and determine how much of a trait is dependant on heritability (h²). This narrow sense ability determines a ratio or percentage that can be attributed to genetic factors (V_G­) by diving the genetic variation by the total phenotypic variation.

For cranial size, I found three different numbers for heritability in humans, and they are as follows (taken from Martinez-Abadias’s “Heritability of human cranial dimensions: comparing the evolvability of different cranial regions.”):

Height: h² = 0.34

Length: h² = 0.32

Breadth: h² = 0.28

When these numbers are analyzed as percentages, it is easy to see that cranial size in humans has low heritability. In other words, about two-thirds of variability in human skull size is due to environmental factors (V_E); genetic makeup only accounts for about one-third of variability in human skull size.

Our paper discusses a couple different environmental factors that may contribute to the variance in skull morphology, and they are climate, locomotor behaviors of the species, and genetic drift.

Neandertals were thought to live in northern Europe, an area of mostly cold climate.  However, when comparing morphology of Neandertals to humans today, they were most similar to those skulls of sub-Saharan Africa and so this theory was not exactly supported as the main environmental factor. They do make mention of nasal features corresponding to morphologies found in high-altitude organisms, which could give some (if only little) credibility to that theory.

Dental loading is also another theory considered to explain skull morphology, because the anterior teeth have more wear than posterior teeth. This dental loading would have to be considered an adaptive trait on which natural selection would have acted for many generations. On investigation of bite forces though, modeling shows that Neandertal bites were not particularly high and therefore provides a problem for supporting this theory of adaptive dental loading.

Genetic drift seems to be the most supported hypothesis in his paper, as the author ran a simulation using quantitative genetics and was unable to reject any statistical results. Further, he argues that fossil records support the genetic drift theory even more by stating that skull features do not appear all at once but rather gradually accumulate over the years.