Self selection

Humans breed domestic animals; ants farm aphids. If evolution could create these mechanisms, why couldn’t it create self-selection?

Self-selection can be indirect and direct

Indirect:

An instinct can make an animal behave in ways that favor certain traits. Take mice: a drive to run and explore could help them spread into new territory, but it could also expose them to predators. Slower mice may be more likely to be caught, favoring traits that help them escape. In this way, an evolved behavioral drive could accelerate adaptation.

Direct:

Depression

What is depression? Why the most successful (intelligent) species on Earth have depression and what is its purpose?

How would such a mechanism work? Imagine that when an animal cannot fulfill an important instinctive need, its drive to act is suppressed. It may stop doing the things it needs to survive and reproduce. If this response affects which traits are passed on, it could act as a form of self-selection.

In an experiment with meadow voles, access to a running wheel restored reproductive function in males whose reproductive system had been suppressed by short days. This suggests that the opportunity to perform a behavior can, under some conditions, change whether an animal is reproductively active.

Calhoun’s Universe 25 experiment offers an example worth investigating. The mouse colony had abundant food and water. It grew at first, but eventually reproduction stopped and the colony died out. This does not establish depression as the cause, but it shows that food and water alone do not guarantee that a population will keep reproducing.

I would propose to repeat the experiment with several comparable colonies: some with running wheels and some without. My prediction is that access to running would reduce behavioral deterioration and allow reproduction to continue longer. Birth rates, survival, and behavior could be compared between the groups.

Another animal example comes from an experiment with American mink. Males raised in enriched enclosures mated more often than males raised in barren ones. The enriched enclosures offered more space, objects to interact with, and access to water. The experiment changed several things at once, so it does not tell us which particular behavior mattered or whether the animals experienced anything like depression. But it does show that conditions affecting what an animal can do may also affect its reproduction.

Anthony Stevens and John Price proposed a related evolutionary idea: depression might lead an individual to withdraw from reproduction. Their proposed trigger was a judgment of one’s own prospects relative to others. The benefit: more resources for relatives.

I also made a maze simulation of competing populations. In this toy model, reproduction is limited by the territory a group controls. One group has an additional penalty to its chance of having offspring if it has not reached new territory; the other does not. It illustrates how such a rule can favor expansion with fewer individuals.

Could something similar happen in humans? Severe depression can make a person unable to get out of bed, let alone pursue a goal. Perhaps a similar shutdown mechanism is involved, but triggered by a different instinct. What that instinct might be is a question for another article.