Evolution is how species experience heritable (passed from one generation to the next) changes in their traits over time. In order for evolutionary changes to occur, many generations over thousands to millions of years are often required – meaning, these adaptations don’t happen overnight! The ability of humans to digest lactose as adults is a perfect example of this. As babies, a majority of humans have the ability to digest lactose. As we develop into adults, some people lose the ability to digest lactose, and others continue to have no problem digesting milks, cheeses, and other food products containing lactose. A hereditary mutation responsible for the trait allowing humans to digest lactose well into adulthood is kept “on”, resulting in lactose tolerance. This trait is a result of a mutation from thousands of years ago. The mutation causing the trait was beneficial and heritable, so it spread throughout the human population and many of us today have this trait!
You may have heard about antibiotic resistance. Bacterial infections in humans are commonly treated with antibiotics. However, because of their overuse and misuse, some pathogenic bacteria have become resistant to some of these treatments. This is dangerous for humans, because treatments that used to cure potentially fatal diseases are now less effective in some cases, or not effective at all. How have antibiotics become less effective?
When you have an infection, the multiplying pathaogenic bacteria inside of you are not all identical. Most of them are the same, but every now and again one of of them will be slightly different genetically. If they were all genetically identical, they would react to their environment the same way and all be harmed by the same things. One harmful thing could wipe out the entire species. But when there are slight genetic differences, harmful conditions, like the presence of an antibiotic, might kill most of the pathogenic bacteria but some may be better able to survive and then thrive. These characteristics are favored in this environment so these bacteria can continue to thrive, and make you ill. Different environments favor different traits and so natural selection has taken place!
There are other types of selection, in addition to natural selection, that are out there in the world. Think about some decisions you make about the types of pets you want or what kind of foods you prefer to eat. Artificial selection, also called “selective breeding”, is where humans select for desirable traits in agricultural products or animals, rather than leaving the species to evolve and change gradually without human interference, like in natural selection.
Can beneficial traits arise in more than one area by accident? Yes! Let’s go back to our lactose tolerance example at the beginning of the article. When multiple environments favor the existence of a trait, these beneficial traits can pop up through mutation and spread throughout their individual populations completely independently. Evolutionary biologists call this convergent evolution. In the lactose tolerance example, this is exactly what happened. A population in Europe evolved the ability to digest lactose as an adult independently from an African population. Both populations had begun farming dairy, and both traits arose around the same time. What’s cool is that, when the lactose tolerance trait arose, these populations were far enough away that they were not able to reproduce with each other making the development of the trait a neat coincidence.