Are the phenomena of variability applicable to humans?

Yes, applicable. For example, the ability of a human skeleton to change under the influence of an external factor. Depending on the type of activity, the skeleton as a whole and the bones in particular change their structure and shape. Also, the shape of the eyes or skin color of the peoples of Africa (adapted to bright light and the scorching sun) and other examples.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.