Up until now, we have been talking about evolution by natural selection. But humans are taking evolution into our own hands through selective breeding and genetically modified organisms (GMOs).
Selective breeding is the way people develop new or improved varieties of crops. This works by exploiting natural variation that already exists. Just like tall parents often have tall children, large pumpkins often have large seeds. And large seeds grow into large pumpkins. Those 1700-pound pumpkins that take home blue ribbons at pumpkin weigh-offs didn't just spontaneously come into being. They were bred to be big.
A really good example of artificial selection is the various dog breeds humans have come up with. All dogs are the same species, Canus lupus. But through years and years of breeding little fluffy dogs with other little fluffy dogs, and big hunting dogs with other big hunting dogs, we ended up with the whole spectrum of dog shapes and sizes:
(Public domain image, by Heinrich Sperling from Nordisk familjebok, available at http://commons.wikimedia.org/wiki/File:Hundraser_3.png)
In the same way that humans have been breeding dogs, and cows, and prize-winning pumpkins, we have also bred many different plants so that we have the fruits, vegetables and grains that so many of us eat on a daily basis. Humans have been doing this for thousands of years—This is also called artificial selection, since humans are choosing the traits instead of natural selection.
Genetically modified organisms build on the same principles of artificial selection but take it a step farther. Instead of picking a prize-winning plant and harvesting its offspring, scientists modify the specific DNA sequence that will produce a desired trait.
Instead of artificial selection happening on the farm, it can now happen in a lab. What once took generations of plant growth can now happen very quickly. Some of the most exciting prospects for GMOs is using the technology to improve crops to be more nutritious, particularly for areas of the world that don't have access to a wide variety of foods.
One example of a promising, yet controversial GMO is golden rice. Golden rice is a variety of rice that contains beta-carotene, the same pigment that makes carrots orange. Beta-carotene is the precursor to vitamin A in the human body. In areas where rice is a main food source, people are often deficient in vitamin A because it does not occur naturally in rice. Proponents of golden rice hope that their product can reduce malnutrition in developing areas where vitamin A deficiencies cause blindness and mortality.
Many people don't like the idea of GMOs, because the genes that are imported into plant genomes would not get there naturally. Who knows what might happen to these unnaturally mutant plants in the wrong circumstances; the DNA that was transferred to the plant might be transferred to other organisms. Certain sources of DNA might give people allergic reactions, too. If you are allergic to strawberries, then you might also be allergic to a fruit that had strawberry DNA added. On the other side is the argument that if it doesn't hurt us and has the potential to solve problems, why not? Of course the real issue is that we don't know what the impact of eating food with modified genomes is, and not too many people are willing to volunteer to find out.
One could argue that modifying DNA in a lab is only an extension of the artificial selection we have been doing since the agricultural revolution 10,000 years ago, but sped up in laboratories and using more technology. Usually though, people do not like to think about eating something that has been developed in a lab instead of a farm. Another argument against GMOs is the fear that biotechnology companies are taking over the food industry. We'll leave it up to you to decide how you feel about GMOs, but keep in mind that there are many players involved with differing goals, opinions and backgrounds. Understanding the science behind genetic modifications is an important first step to being an informed decision-maker.