Evolution by natural selection says that those characteristics that enable organisms to survive and reproduce more than others will tend to end up dominating the population. In that model, organisms seek to propagate their genes as much as possible. Suicide as a biological instinct is clearly not advantageous and should be selected against and disappear over time. So what are we to make of some mice that seem to commit suicide by actually running towards cats and being killed and eaten by them?
Ed Yong explains that it is caused by a parasite that infects many animals but can only complete its life cycle in the guts of a cat. So how does the parasite do that? By invading the brains of mice and causing permanent changes in its neuronal structure, causing them to lose their fear of cats and run towards the source of cat urine. So basically this parasite ends up controlling the brain of a mouse, at least as far as this particular behavior is concerned.
This example because shows the danger of taking a superficial view of behavior and that the organism for which we should apply natural selection may not be the one we see. Traits that seem maladaptive on the surface when looked at in terms of one organism (the mouse in this case) turn out to be the byproduct of a deeper mechanism that is beneficial for a different organism (the parasite).
It struck me that to the extent that we assign consciousness to a mouse, as far as the mouse is concerned it might think that it was acting of its own volition in going towards the cat. It had essentially ‘chosen’ to commit suicide. This has implications for free will in that it reinforces the argument that those actions that we think of as freely chosen are really the result of brain processes over which we have no control.