What principle states that more heavily pigmented individuals are generally found closer to the equator?

The principle that states more heavily pigmented individuals are generally found closer to the equator is Gloger's Rule. This rule explains the relationship between pigmentation in human populations and environmental factors, particularly climate. As you move towards the equator, there is increased exposure to ultraviolet (UV) radiation, which encourages darker skin pigmentation as an adaptive response. Melanin in the skin provides protection against the harmful effects of UV rays, such as skin cancer and folate degradation, which are particularly relevant in regions with high sunlight exposure.

The other principles, while important in understanding human adaptations and biological diversity, focus on different aspects of biological variation. Allen's Rule relates to the body shapes of animals and humans in different climates, indicating that animals in colder climates tend to have shorter limbs to conserve heat. Bergmann's Rule pertains to body size and shape, suggesting that larger bodies are found in colder environments as a means to retain heat. Lyell's Law, associated with geology and uniformitarianism, does not relate to human pigmentation or adaptation in relation to climate at all. Therefore, Gloger's Rule is the principle that directly answers the question regarding pigmentation and proximity to the equator.