Banthas are a large, elephant-like creature with ivory tusks. The tusks can come in different shapes: unbent, spiral (multiple circles), or simple (one) circle. In a second generation of banthas, investigators counted the number of banthas with different tusk shapes and found that 19 had spiral tusks, 11 had simple circular tusks, and 2 had unbent tusks. What is the most likely explanation for those numbers