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The context: the other day I was reading an article about a peculiar species of ant that when threatened by a predator would “explode” in order to help protect the rest of the colony, although it kills the individual ant. I was wondering how this kind of behavior could evolve via natural selection if it only picks out the individual organisms best suited for the environment - because it seems like a self-destruct type of behavior wouldn’t be helpful to the ants that are trying to survive and reproduce. Though it would make sense if natural selection also applies to entire populations as it does to individual organisms - picking out entire populations that are best suited for the environment. It would make sense because populations that have individuals willing to explode to help protect the other individuals would have an advantage over the ones that don’t.

The question: does natural selection apply to entire populations as it does to individual organisms or have I made a mistake somewhere in my logic - and if so, where?

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  • $\begingroup$ I’ll leave a proper answer to someone more qualified than myself, but in the meantime you could lookup kin and group selection. $\endgroup$
    – canadianer
    Commented May 29, 2018 at 1:59
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    $\begingroup$ In an evolutionary sense, you can't really think of worker ants as individuals, since they can't reproduce. The "individual" is really the entire hill. See "Aunt Hillary" in Hofstadter's " Godel, Escher, Bach". $\endgroup$
    – jamesqf
    Commented May 29, 2018 at 5:09
  • $\begingroup$ Don't feel bad for feeling confused about this subject. The problem of the "unit of selection" has long been a philosophical and empirical discussion among biologists. For example, Richard Dawkins has strongly argued that even the 'organism' as the unit of selection can be misleading. He argues that the unit is even deeper, the "replicator" (gene). He argues this in his books The Selfish Gene and The Extended Phenotype. $\endgroup$
    – Eff
    Commented May 31, 2018 at 10:35

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Unit of selection

Your question is about the concept of unit of selection. Selection acts on any populations of things that have heritable features which explain at least part of the fitness variance in this population. The "things" can be genes, cells, organisms, groups of organisms, species, lineages, etc...

The standard is to view selection acting on population of individuals. But of course, selection can also act on populations of groups of individuals (or populations of populations if you prefer) or populations of species. When talking about selection on a population of groups of individuals, we talk about group selection (a concept very related to kin selection) When talk about selection on a population of species, we talk about species selection (or lineage selection).

Social evolution and altruism

An altruistic phenotype is defined as being a phenotype that is causing a decrease in the fitness of the carrier of the phenotype coupled with an increase in the fitness of one or more recipient individuals. Altruism can evolve at a level, only by selection applying at levels above it.

Imagine for example a population of individuals. If one individual mutates and as a result display a behaviour of donating food to the others. The cost for this individual is -0.5 and the benefit for the recipient is say 0.7. In absence of reciprocity, this mutation will not be passed on on future generations because this individual carrying it have lower fitness. Therefore altruism should not evolve in this population. Imagine however if there are many populations and that some populations are altruistic and other aren't. Then, the altruistic population will likely outcompete the non-altruistic populations and therefore over time only the altruistic populations will remain.

It is by the way the existence of an altruistic behaviour that brought to wonder about the level at which selection can act.

The field that studies the evolution of such social phenotypes is called social evolution

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