For the past 10 to 25 years, farmers have planted crop seeds that have been genetically modified to withstand treatment with a common weed killer. This allows the farmers to spray their fields to get rid of the weeds without harming their crops. Recently, more and more farmers have discovered that their fields have weed killer- resistant Pigweed (a weed) growing along with their crops. Use what you’ve learned to explain how this came about.