Final answer:
Turnips were selectively bred from field mustard by humans who chose plants with larger roots for breeding, leading to the significant change of the root's size and shape over many generations.
Step-by-step explanation:
Turnips have been selectively bred from field mustard, a wild plant. This selective breeding process involves choosing individual plants with desirable traits and using them to breed the next generation. Over time, this artificial selection results in significant changes to the physical characteristics of the plant. For turnips, humans selected for and cultivated plants that developed larger and more substantial roots, a feature valuable for food storage and consumption.
Just as wheat and brassicas like cabbage, broccoli, and kale are drastically morphologically different as a result of selective breeding, turnips—root vegetables with tap roots modified for food storage—are also the product of selective breeding for specific desirable traits, such as a big, round root unlike the ancestral field mustard.
The history of almost all crop species involves some form of genetic modification through careful selection and cultivation by humans over thousands of years. By continually selecting for larger roots, early agriculturists could manipulate the genetic makeup of field mustard over generations, resulting in the modern turnip we are familiar with today.