The principle behind it is that, in a population where certain conditions are met (see below), the frequency of the alleles in the gene pool will be constant. To read more about the Hardy-Weinberg Theory and Equation, check out this page from Nature.
Although meningiomas are referred to as brain tumors, they do not grow from brain tissue. They arise from the meninges, which are three thin layers of tissue covering the brain and spinal cord. These tumors are most often found near the top and the outer curve of the brain. Tumors may also form at the base of the skull. Meningiomas usually grow inward, causing pressure on the brain or spinal cord. They also can grow outward toward the skull, causing it to thicken. Most meningiomas are noncancerous, slow-growing tumors. Some contain sacs of fluid (cysts), mineral deposits (calcifications), or tightly packed bunches of blood vessels.
As a pituitary tumor grows, the normal hormone-releasing cells of the pituitary may be damaged. This results in the pituitary gland not producing enough of its hormones. This condition is called hypopituitarism. The causes of pituitary tumors are unknown.