“What purpose in the field of evolutionary biology does the loner serve?”
They have adopted a viable survival strategy and manage to get laid. That’s pretty much all there is to evolution.
The Basics of Evolution
First let’s talk about how evolution works.
From Wikipedia: “Evolution is change in the heritable traits of biological populations over successive generations”
Traits are passed down from generation to generation via genes. Over time organisms that have helpful traits survive and reproduce and organisms with really bad genes die before they can reproduce. All that it takes for a gene to get passed on is for an organism to survive long enough to reproduce. Not everything that gets passed down is adaptive. If something is neutral or just not bad enough to stop you from reproducing then it can continue to get passed down.
One thing that makes evolution weird is sexual selection. Take the peacock’s beautiful giant tail. The tail honestly just makes it easier for predators to hunt them down. They are brightly colored and unwieldy. But they get laid. Because of that peacocks with bigger more ridiculous tails get to have baby peacocks with even more elaborate tails.
Evolution of Cooperation
Make no mistake. Cooperation is costly. It involves giving up some of your resources for others. Most animals only cooperate with other individuals in their family groups because that is a good way to ensure your genes are passed down. The more closely related two individuals are, the more genes they share, the more they get out of cooperation.
Humans are one of the rare animals that cooperate with unrelated individuals. This behavior is explained and modeled using an application of game theory. Game theory is basically a system of mathematical models that explain optimum strategies for decision making. It weighs costs and benefits to individuals who choose a certain strategy and comes up with optimum scenarios and equilibrium states for various scenarios. Read more about it; it is fascinating.
For the evolution of cooperation game theory originally named two groups of individuals: cooperators and defectors.
Cooperators pool and share their resources. Doing so created more resources for everyone but it does involve some cost.
Defectors do not share their resources but they take advantage of the resources created by the cooperators.
In order for the cooperative strategy to be stable there cannot be more defectors than cooperators. Defectors are also usually punished by cooperators to encourage cooperation.
A real life example is vampire bats. Vampire bats drink blood from large mammals such as cows. The way they feed means that each individual does not have a successful hunt every night. If the bats don’t eat at least every few days then they die. In order to keep going vampire bats who have a successful hunt share their blood by regurgitating into the mouths of bats that did not have a successful hunt that night. The system works in a “Today you, tomorrow me” way. In this system there are some defectors, individuals who refuse to share their hunt. The next time those individuals don’t have a successful hunt no one will share with them. In this way they are punished for defecting and they are fed the next night if they don’t have a successful hunt again.
The loner is a more recent addition to the models used to study the evolution of cooperation. The loner chooses not to play the game at all. The loner neither cooperates with others nor do they reap the benefits of cooperation. Basically they go get their own resources. The loner strategy is a safe strategy in that they don’t depend on others but the potential payoffs are less because they don’t have access to the pooled resources.
A Warning about Evolutionary Psychology
Beware evolutionary psychology. At the end of the day human beings are individual organisms with their own goals who have developed survival strategies that work to their benefit. At a more basic level humans are just collections of genes that have managed to get passed down either because they are helpful or at least not so harmful that they prevent people from reproducing. Be wary of untestable hypotheses. Evolutionary Psychology is full of them.