SURVIVAL OF THE FITTEST.
Are you picturing a desperate struggle for life in the depths of the darkest jungle? Two tigers are chasing an antelope, all three of them fighting to survive and reproduce. But only one tiger gets the prize, leaving the other hungry.
Natural selection is the most prominent force of evolutionary change. It works because some members of a species are more successful than others. Take our two tigers, for example. One of them caught the antelope because it was a better hunter than the other. That tiger can reproduce more because it is better at acquiring the resources needed to support babies than the loser tiger.
And because all those adorable baby tigers inherited ferocious sharp teeth and fast legs from their parent, they will be more successful too and have more offspring. The descendents of the winning tiger will be much more numerous than those of the loser tiger, and so as a whole the species will be more similar to the winning tiger than it used to be. This is evolution by natural selection.
So in a group of tigers there’s a struggle over who is the best tiger and who has the most tiger babies. What makes a tiger the best are it’s physical traits – sharp teeth and claws, warm fur, etc. A parent passes these traits to their children through genes – the DNA blueprints for all the millions of little components that work together to make a tiger. Together, all of the genes an organism possesses are called the genome. And even though the different parts of the genome work together to make up a whole tiger, they don’t necessarily always play nice with each other. Natural selection can occur within the tiger, specifically between all the genes the tiger carries. Just like there are many different tigers living together in the jungle, there are many different individual genes together in a genome.
Natural selection within the genome means that the genes that give the organism a better chance of surviving and reproducing will become the most common. But some selfish genes have the ability to mimic the effect of natural selection by sabotaging another part of the genome in order to increase their own transmission to the next generation. This genetic warfare is incredibly common, comes in many different varieties, and can have serious consequences for a species. One of the best examples of genetic conflict are transposable elements, aka “jumping genes”.
Transposable elements were first discovered by Barbara McClintock in the 1940s, back before anyone even knew genes were made of DNA. She was figuring out basic genetic principles using color in corn kernels when she noticed pigment patterns that could only be due to spontaneous failure of certain color genes in just a few cells. She determined the cause was genes that could “jump” around the genome.
Instead of staying in their set places in a row on a DNA strand like normal genes, these jumping genes were inserting themselves into different places in different developing kernels. If one landed in the middle of a gene required for pigment production, it would prevent the gene from working properly. McClintock was awarded a Nobel Prize in 1983 for her discovery.
Transposable elements don’t just move around in the genome, they make copies of themselves and then insert the new copy in a different place. Through this copy-paste behavior, the number of duplicates of a particular transposable element in the genome increases. It’s similar to how a tiger that is very good at hunting will have a lot of babies, but in the case of a transposable element, ”hunting” is “making copies of yourself and sticking them random places in the genome”. Transposable elements that are the best at this become the most numerous – the transposable element doesn’t actually do anything other than hang out in an organism’s genome. In some organisms, particularly plants, transposable elements have been incredibly successful. The corn genome, for example, is about 90% transposable elements!
But transposable elements aren’t just quietly sitting in the genome minding their own business. They came to the attention of Barbara McClintock because they were disrupting the function of the genes that controlled kernel color. A corn plant can survive if the kernel color is a little messed up, but if a transposable element jumps into the middle of a gene that is required for something more important it’s a different story.
This is what makes jumping genes selfish and puts them into conflict with the rest of the genome. The organism with the genome that is best able to prevent transposable elements from jumping around and screwing up vital processes will be more successful, so there is natural selection for the genome to stop transposable elements. It’s exactly the same principle as the tiger chasing the antelope, except this time the battle is taking place inside the tiger’s genome.
Transposable elements have had a huge impact on evolution, affecting both the content of the genome and how it functions to make an organism. Selfish transposable elements might even provide raw materials for the evolution of new, beneficial genes. And transposable elements are only one kind of selfish gene – there are many, many more. If there’s a theoretical way for one part of the genome to take advantage of another part, it probably already exists in nature. “Survival of the fittest” is the law of the land, and that includes everything from tigers and transposable elements.
About the Author
|Katie Pieper is a PhD student in the Department of Genetics at the UGA. She studies the molecular evolution of sex chromosomes in fruit flies. In her free time, she enjoys baking delicious desserts and winning at trivia contests. She is also the head tweeter for the Athens Science Café official Twitter account @AthSciCafe. Get in touch with Katie at @kpeeps111or firstname.lastname@example.org. More from Katie Pieper.