February 21st, 2015
New study explains suicide by natural selection and ‘survival of the fittest’.
Single-celled organisms regularly kill themselves in reaction to environmental stresses. From an evolutionary point of view, this behavior is mysterious. What is the fitness advantage to the dying individual? Why would natural selection permit suicide?
A new study provides some answers. The researchers found that in single-celled algae, suicide is altruistic and helps the organism’s relatives. We ask Dr. Pierre Durand from the University of Witwatersrand in South Africa questions about his findings and the implications for human suicide.
You found that suicide benefits the organism’s relatives. Could you explain how this mechanism works?
It appears that cells that die release nutrients that can be used by relatives but which suppresses the growth of some non-relatives. This may not be the case in all unicellular systems but we have some evidence to show that this is the mechanism in unicellular green algae.
In the research you used a type of alga as a model organism. How do you detect suicide (programmed death) in alga?
There are very specific features of programmed and non-programmed death. Programmed death requires energy and regulated gene expression; we detect it by the ordered fragmentation of DNA and the exposure of phosphatidylserine (a marker of programmed death) on the cell surface. This doesn’t occur in non-programmed death which is a haphazard, passive death process whereby the cell breaks up and its contents are liberated into the micro-environment. These contents are harmful to all other cells. It is only when programmed cell death occurs and the cellular contents are processed such that they can be used by others as nutrients and no longer harmful that the benefits are observed.
Programmed death/suicide seems to be triggered by is a stressful environment. Does this imply that suicide rates will increase due to, for example, climate change?
Stress induces programmed death in our model organism and in many other unicellular organisms. I have no doubt that stress increases suicide in humans as well, whatever the source of the stress.
What else may the findings tell us about suicide among humans?
It would be unreasonable to try and draw any comparisons with humans, although there must be evolutionary explanations for the epidemiological patterns of suicide in humans. Our data indicate that suicide in single celled organisms can be explained by natural selection and “survival of the fittest”. With regard to suicide in humans, sure, evolutionary biology is the only system we have for explaining the ‘why’ of any of life’s phenomena. The details of human suicide, I am not an expert on, but I have no doubt that evolution, whether by adaptive or non-adaptive mechanisms, will be the only coherent explanation for human death in whatever form it takes.
Sociologist Émile Durkheim argued that the causes of human suicide were to be found in social factors. He, for example, explained suicide by decreasing social integration and increasing individualization. Could you respond to this theory based on your own findings?
Durkheim’s arguments are excellent, but they address the causes of suicide from a mechanistic standpoint. He explains how it comes about, the mechanisms leading up to a suicide event. Our study addressed the selective pressures associated with programmed suicide; why a phenomenon like suicide exists at all. We argue that it may exist, in unicellular organisms anyway, because of the fitness benefits it provides to relatives. These benefits mean that the phenomenon can be maintained because it provides selective advantages to a group rather than an individual.
Photo: Flickr, wolfgraebel
Durand, P., Choudhury, R., Rashidi, A., & Michod, R. (2014). Programmed death in a unicellular organism has species-specific fitness effects Biology Letters, 10 (2), 20131088-20131088 DOI: 10.1098/rsbl.2013.1088
suicide, evolution, biology, single-celled organisms, alga, programmed death, climate change