It’s election season here in the USA. This year is particularly cringeworthy, but the general format is the same as it’s been since long before I was born. Two major parties each nominate a candidate. Both candidates win a reasonable share of electoral votes. No one else ever gets any electoral votes. Neither party shows a consistent advantage over the decades. Occasionally third parties and independents have made a good showing, but this two-party system has been remarkably stable over our nation’s history. That amazes me. Let me clarify. I’ve seen a lot written about the dominance of Republicans and Democrats, and how Duverger’s Law impedes the success of new parties. But that’s not what interests me. As a biologist, I’m used to competitive exclusion: no two species can occupy the same niche indefinitely. Within a species, a genetic difference does not persist long in the gene pool except under unusual conditions. A balanced polymorphism requires an explanation. So my question isn’t why we don’t have more political parties. It’s why we don’t have fewer.
To be clear, I’m not advocating for a one-party state. Democracy doesn’t function well when we don’t have choices. But that’s exactly why the two-party stability is perplexing to me as a population geneticist. It’s not written into the Constitution that we need to have two parties to give voters a choice. No one had to legislate it, because the duality emerges naturally. But why? Think of a simple political model. Imagine that voter needs are distributed uniformly along an axis of policy. A perfectly centrist candidate should always outperform a left- or right-leaning party, in terms of being more appealing to more voters. Red and blue candidates should always lose to a perfectly purple candidate. If voter needs show a so-called normal or bell-shaped distribution, with more people in the middle, the centrist candidate will have an even greater advantage. This is what usually happens in evolution, after all. Most traits are under stabilizing selection, meaning the individuals with intermediate features do the best. Likewise, a single purple political party should emerge as the reliable winner election after election. That obviously hasn’t happened.
“But…,” you protest. You can come up with a litany of reasons why my model is oversimplified. It’s because voters tend to shun the status quo. It’s because the political landscape is constantly fluctuating. It’s because people just naturally have either a liberal brain or a conservative brain, so voter needs are actually bimodally distributed. It’s because of campaign financing laws. It’s because parties don’t actually stand for anything besides their own self-propagation. I agree, one or more of these factors may be at play. But which ones? Do we really know? All models are wrong, but some models are useful. In biology, we have simple mathematical formulas that can explain when two gene variants will be maintained by natural selection. It takes a specific set of circumstances. What’s going on in politics? Is there a simple, plausible mathematical model that you can plug into a simulation, which yields a permanent balance between two non-centrist parties? I have yet to see it.
Evolution isn’t all stabilizing selection, of course. Traits do vary and change, otherwise we’d all still be floating in the primordial soup. One explanation for all this change is colorfully dubbed the Red Queen Hypothesis. In Lewis Carroll’s Through the Looking-Glass, the Red Queen tells Alice that she must run as fast as she can just to stay in one place. As a species evolves, other species around it are also evolving, finding new ways to infect, eat, or poison it. To stay alive, a species needs to keep up with the Joneses by constantly evolving as well. The Red Queen even explains why sex exists: you need to keep rearranging the genotypes of your offspring, or you’ll fall behind in the race. Maybe the reason for Earth’s luxuriant biodiversity is that we’re all just trying to stay ahead of each other. But the Red Queen doesn’t explain why differences are maintained within species. Take my favorite example, the sexes. Simple logic implies that an organism who can reproduce with anyone else should have higher fitness than either sex. Yet over and over again, evolution favors separate sexes. Genes that are more favorable in one sex versus the other then pile onto the sex chromosomes. Soon, each sex expresses a distinct suite of information. Eventually, you get two highly differentiated body types, and there is no place in the system for an intermediate form. The dichotomy is enforced because it depends on the choices of individuals, and most individuals prefer extremes. If this is starting to sound like our government, well, you can see where I’m going with this. A model that can explain political polarization, call it the Purple President, may also explain biological polarization.
This is just the beginning of an idea. If we can figure out exactly why the two-party system is so robust, maybe that will tell us something about genetic diversity within species. Conversely, maybe learning more about biological balancing selection will give us political insight. The Red Queen explains why we have sex at all and why species change. Perhaps the Purple President can explain why we have two sexes and why species harbor genetic differences. I’m no political scientist, but I’d happily discuss these ideas with one if there are any takers out there. In the meantime, everybody please vote.