f I were to take a guess, the most memorable phrase you have from a biology class (besides “the mitochondria is the powerhouse of the cell”, which is both grammatically incorrect and really only useful for memes) is probably “survival of the fittest,” and this is well-founded—both the concept of “survival” and the concept of “fitness” are central to the theory of evolution, which is a central topic in biology. The question of focus of this theory is what happens when a diverse population of replicating units must compete for something in order to replicate, and why it happens. What evolutionary biologists have found is that each individual replicating unit in a population can be assigned a level of fitness based on how adapted to the environment it is, and the units with higher levels of fitness will tend to survive to replication more often than the units with lower levels of fitness, thus resulting in the “survival of the fittest” units and death of the least fit units. This process is known as natural selection, and is responsible for the take-over of increasingly complex units produced through mutations.
Now I’m going to take another guess: you were taught that, for life as we know it, the units of natural selection are species—or more specifically, the organisms within those species. Since most multicellular organisms reproduce sexually within their species—a group of organisms with practically identical DNA—you can consider the species to be the unit of selection. Then, each species can be assigned a level of fitness according to its adaptations to the environment, and the higher fitness species will survive while the lower fitness species will die off over time. While most evolutionary biologists would agree that this method of analyzing evolution is logically sound, not all would agree it is the best—as it turns out, there is an equally valid unit of selection for life on Earth: genes.
Dating back to as early as the 1920s and 30s, scientists have discovered evolved genetic elements in DNA that appear to be non-beneficial or even parasitic to the species that contain them—in other words, a gene seems to be able to spread throughout a species even if it decreases the species’ level of fitness. For example, in 1928, a Russian geneticist named Sergey Gershenson discovered a so-called “driving” X chromosome in Drosophila obscura that had the ability to spread throughout the species and potentially drive it extinct due to the female-biased sex ratio it produced. These are known today as “selfish genes,” a term popularized by Richard Dawkins’ influential book “A Selfish Gene.” In fact, Dawkins argues that all genes are “selfish,” even if they work together in groups. So why are genes selfish, and what does this imply?
It is important to note that genes do not have brains; they cannot consciously decide what to do, and therefore cannot determine their own futures—their futures are determined for them by their environments (in fact, I would argue that all evolutionary units should be viewed as such). Thus, since they must be contained within an organism and passed down to new organisms through reproduction, their future is determined solely by the reproductive fate of the organism as well as its reproductive mechanism. If the organism successfully reproduces and the gene by chance appears in the fused zygote (which is 50-50 if the organism contains only one copy of it), the gene has successfully replicated. If the organism dies before being able to reproduce, has a reproductive disability, or does not by chance pass on the gene to any offspring, the gene has failed to replicate, and is thus, in essence, dead. If genes were not able to affect their encompassing organisms, the ones which successfully replicate and “survive” would be completely random, as the organism’s reproduction would be determined by factors completely unrelated to genes. However, as it turns out, genes can greatly influence an organism’s or its reproductive mechanism’s actions and abilities. Therefore, a gene can cause its encompassing organism to be more likely to reproduce or even increase its own chances of appearing in the fused zygote, thus affecting its own ability to replicate. For this reason, genes can be assigned fitness levels just like organisms according to how likely they are to cause their own replication. However, fitness levels associated with organisms arise in
However, I have still not explained the “selfish” nature of genes and why the example above supports a “gene’s-eye view” of evolution. First of all, genes do not propagate together—well, as least they don’t have to. The very definition of propagation is replication within a species, so the only way a gene’s replication would coincide with that of another gene is if the genes worked together and the organism relied on both genes in order to successfully reproduce. However, it is often the case that a gene does not have to work with another one to produce a positive effect, and it is often the case that an organism’s reproduction does not rely on the presence of a given gene. Therefore, genes are selfish in nature, at least in relation to themselves, because their replication is not required to rely on the replication of other genes. Genes are also selfish in relation to the encompassing species as a whole because they don’t propagate based on what’s best for the species in the long run—they propagate based solely on how good they are at replicating. This is a point that so many people get confused, and why the above example is reason to consider genes as the unit of selection. Mutations do not become present in an entire species at the same time, they become present in individual genes in individual organisms. Thus, the obtaining of a different level of fitness actually occurs through the propagation of those mutations, which is determined by the genes’ levels of fitness. Additionally, the effect on the level of fitness of both the gene and species may not even be positive in the long run—only a positive short-term effect on the gene’s level of fitness matters in producing its propagation. This is why it is impossible to determine the reason for the propagation of the X chromosome in the above example based on the selection of organisms/species—it can only be determined by the genes’ fitness function, which need not be correlated with the species’ fitness function. For these reasons, obtaining a gene’s-eye view, with the gene as the unit of selection, is important for truly understanding the process of evolution.
PS: One interesting thing is that genes do not solely determine an organism’s actions and abilities—factors like thoughts in the brain can also affect them. Therefore, we actually have the ability to affect the propagation of our own genes by decision. This is why we can use birth control—the thoughts in a person’s brain can actually affect the genes’ fitness function, and the only way for them to prevent that is to “fight” the brain. Maybe it is possible for a gene to someday develop a mutation that limits the effect of the brain, allowing it to obtain a higher fitness—I don’t know, but it’s fascinating to think about.