Farmers have been selectively breeding plants for thousands of years. Let’s look at the process of breeding and growing some of the plants we eat regularly (e.g. corn, bananas, potatoes) from a biological perspective.
With all the uproar about genetically modified organisms, people tend to forget an important fact: humans have been modifying plant genomes for thousands of years. Just like dogs have been bred to look and behave differently from their wolf cousins, most of the plants we eat are very different from their wild relatives. Wild cucumbers are round and spiky, wild rice is black and gray, and the wild form of corn, teosinte, isn’t even edible. So how did we get to the point where we have big juicy fruits and vegetables, and nutritious grains that can be harvested and turned into wheat, corn and rice products?
About 10,000 years ago, our ancestors started taming wild plants and created the first farming societies. Since then, various human societies have been refining wild plants to make them more delicious, more nutritious, or easier to harvest and process. Plant breeders have done this by exploiting natural variation in plants. Suppose you had some corn, but you wanted bigger corncobs. You might take the seeds from the plant that produced the biggest ear of corn and plant those seeds exclusively for the next year’s crop. The resulting corncobs should be bigger, on average, than the first crop. Then next year you would plant the seeds from the plants that produced the biggest corncobs, and so on, selecting the plants that produce the biggest corncobs each year. For this to succeed the variation has to be heritable, which means it is passed on through genes; luckily for us and the Neolithic farmers who domesticated corn this way, the variation is heritable.
Humans have fiddled with lots of plants this way, though not necessarily always for size of corncobs. Some plants have come into being by mating two different species or strands. Bananas are a prime example of this, as are many orchid varieties. The bananas we eat actually have a triploid genome (3N), meaning they have three sets of each chromosome. They got that way through a cross between a diploid (2N) and a tetraploid (4N) banana species. The triploid genome makes bananas sterile, so they can only be propagated through cuttings.
Speaking of bananas, remember how bananas are good at producing ethylene and helping other fruits ripen? In large agricultural productions, farmers and fruit distributors control ethylene so that they have ripe fruit when they want it. Some fruits, like apples, are stored in containers that have higher than normal carbon dioxide. The reason for this is that carbon dioxide inhibits ethylene production, keeping the fruits immature indefinitely. When it is time to take the fruits to market, the fruits can be moved to containers with increased ethylene to speed up the ripening process. Unfortunately, too much ethylene can overripen fruit. Don’t forget the importance of other hormones either: gibberellins make grapes big and juicy, and can induce flowering of long day plants.