Which tracts originate from the cell bodies of upper motor neurons, synapse on interneurons that communicate with cranial nerve nuclei at various levels of the brainstem, and do not decussate and enter the spinal cord

Which of the following statements is true about the relationship between genetic variation and natural selection? a. genetic variation must be present in a population before natural selection can act on it. b. genetic variation arises in a population as a result of natural selection. c. natural selection can act on a population whether there is genetic variation within the population or not. d. after natural selection acts on a population, the amount of genetic variation in the population always increases.