In the case of Howea palms, what factor served as a selection pressure?

The selection pressure for Howea palms in this context relates to environmental conditions that affect their growth and survival. The pH levels of soil can significantly influence nutrient availability, root health, and overall plant health. If the soil's pH levels vary, it could select for specific physiological traits in the palms that allow them to thrive in those conditions, such as the ability to tolerate more acidic or alkaline soils. This would lead to differential survival and reproduction based on how well individual palms can adapt, ultimately shaping the population's genetic makeup over time.

While different soil temperatures, varying levels of rainfall, and differences in sunlight exposure can all be important environmental factors influencing plant growth, the question specifically identifies soil pH as a critical selection pressure in this scenario.