SGE- A Detailed Overview
The 3 Ways to Achieve Selfishness
A selfish gene just does not play by the rules. Like all other genes, a selfish one wants to be passed on to the next generation. However, unlike most genes, a selfish gene will actively strive to achieve that goal. In order to be considered selfish, a gene must simply increase it's chances of being passed on relative to normal genes. For normal genes described in this website, there are two copies of the gene in every cell of the organism. Normal genes move into the next generation every time a baby is born, regardless of whether the baby is a boy, girl, puppy, mushroom, or tree. The baby will inherit one copy of every normal gene from each parent.
Selfish genes break this rule of one copy of a gene per baby. Selfish genes cheat by either increasing the number of copies in a baby, increasing the chance that they are the copy that gets into the baby, or moving in the right place at the right time. Sneaky devils. For the most part, there are three main ways of achieving selfishness: Interference, Over-Replication, and Gonotaxis.
Interference
Interference is simply aggressive competition. In other words, interference is when one gene (or allele) harms, obstructs, or even destroys the transmission of another gene (or allele). This gives the selfish gene a very clear and powerful advantage. By actively disrupting the spread of all of it's competition, the selfish gene will be able to get passed on more frequently. This will then lead to a disproportionate representation of the selfish gene in the following generation.
A genetic element known as a sex-linked killer demonstrates interference quite well. What sex-linked killers do is target the opposite sex chromosome and destroy/harm it. For example, a killer-X chromosome will seek out and destroy sperm with a Y chromosome. This results in the birth of a daughter with the killer-X chromosome.
Over-Replication
In order to understand over-replication, one just needs to understand chances. Lets say you have two different colored marbles, one is red and the other is blue. If you randomly selected one of them, there is a 50% chance of you getting a blue one and a 50% chance of you getting a red one. Now lets say you are randomly selecting a marble from a pile that contains 3 red ones and only 1 blue one. Unlike before, you are more than likely going to pull out a red one. This is the basis of over-replication. It gives a gene a better chance of getting passed on to the next generation simply because there are more of them. Over-replication can occur when one gene is basically able to reproduce faster than it's alternative forms.
Transposable elements are a great example of over-replication. Transposable elements are essentially segments of DNA that can change its location within the genome. It can either move up or down its chromosome or jump to an entirely new chromosome. Transposable elements are able to move so often because they can copy and paste themselves. Because of this, they appear in higher frequencies than normal gene within an individual's genome, which means that more copies will be passed onto the eventual offspring.
Gonotaxis
To grasp the idea of selfishness through gonotaxis, one needs to know the difference between somatic cells and germline cells. Somatic cells are cells that make up your body (ex: hair, skin, liver, blood, etc). By comparison, germline cells create gametes or the cells that allow an individual to produce an offspring, such as an egg and sperm. In the cases of most genes, they are present in equivalent numbers in both the somatic and germline cells. However, genes that utilize gonotaxis actively avoid somatic cells. In other words, genes employing gonotaxis bias their movements to the germline so that are passed onto the eventual offspring.
"B" chromosomes serve as an excellent example of a genetic element that employs gonotaxis. "B" chromosomes are chromosomes that can be found in one's nucleus, but unlike normal "A" chromosomes, "B" chromosomes rarely code for anything. In order to ensure representation in future generations, "B" chromosomes will avoid undergoing proper cell division and seek to enter the germline in as large of numbers as possible.