Final answer:
William Sheldon proposed the theory of somatotypes, classifying individuals into three body types: Endomorphs, Ectomorphs, and Mesomorphs, supposedly correlating with personality traits.
Step-by-step explanation:
The individualistic theories related to modern biological theories include those of William Sheldon, who proposed the concept of somatotypes. Sheldon's theory suggests that individuals can be classified into three body types: 1) Endomorphs, characterized by a rounder and softer body, 2) Ectomorphs, characterized by a leaner and more delicate build, and 3) Mesomorphs, known for their muscular and well-built physique. These somatotypes are thought to correlate with personality traits, although modern science challenges the extent of this correlation.