Answer:
The difference in the number of gene copies of salivary amylase between the Hadza and the Biaka pygmies could be attributed to their different dietary adaptations. Salivary amylase is an enzyme that helps break down starch in the mouth, allowing for more efficient digestion of starchy foods.
The Hadza, as foragers, have historically relied on a diverse diet that includes a significant amount of starchy tubers and fibrous plant materials. This consistent consumption of starchy foods over many generations could have led to selective pressure for the Hadza to have more copies of the salivary amylase gene, enabling them to better digest and extract nutrients from these starchy resources.
On the other hand, the Biaka pygmies, who have traditionally practiced forest-based hunter-gatherer lifestyles, may have had less reliance on starchy foods in their diet compared to the Hadza. As a result, there may have been less selective pressure for the Biaka pygmies to maintain higher gene copies of salivary amylase.
It's important to note that this is a simplified explanation, and there are likely multiple factors at play in the genetic differences between these populations. Additional studies and analyses would be needed to fully understand the evolutionary and environmental factors contributing to the observed differences in salivary amylase gene copies.