Evolution by natural section describes a process by which replicators (things that make copies of themselves) tend to increase in numbers, but compete for limited resources, so that replicators with altered (by random mutation) features have (context dependent) variable rates of replication. The result is that those alterations that foster increased replication in the prevailing local context become more prevalent in that population. This view requires identification of the replicator, and genes (or some smaller fragments of DNA) are the natural candidate for life on Earth as we have ever seen it. The idea of group selection, of which earlier crude versions had been reasonably dismissed, has made a rebound. The new versions (sometimes more accurately referred to as multilevel selection) have been called upon to explain culture, morality, altruism, cooperation, and similar phenomena in a seemingly plausible way. But most proposed new versions of group selection are also flawed, and some other explanation is needed for these social phenomena.

First, to understand what is wrong with these recent forms of group selection, you can just read this article by Steven Pinker. Though I don't think everything there is quite right, I am not going to go into that now. I am going to focus on a compatible analytical concept that I call multiscale selection. The idea is linked to multilevel emergence (an open problem in complex systems). The drivers of evolution (the replicators) only exist at one level, because the casual forces of any system can only reasonable exist at one level (though you can pick whatever level you want to consider). Phenomena at other levels of organization are just different descriptions of phenomena occurring at the causally efficacious level. Some phenomena are easier to recognize (by humans) at certain levels, and we build a conceptual hierarchy out of these levels (although it's not really a hierarchy). As a result selection pressure seems to occur at multiple levels, but that is not a necessary or supported model/theory of evolution.

I use "multiscaled selection" to mean that the process is recognizable at many scales, and "multilevel selection" in the standard way that selection operates at many levels. Scales and levels have a similar meaning here; so cell -> organ -> body can be seen as both increasing level and increasing scale (the difference is that levels are determined by the appearance of coherent phenomena, whereas scales can be defined arbitrarily). The salient difference here is that "selection" describes a behavior, and a necessarily causally efficacious one. Selection can only happen at one level because it is causally efficacious and causes can only operate within a level (i.e., not across levels). Although certainly selection at one level may produce recognizable changes in the proliferation of features found/defined at different scales. This is what the group selection and multilevel selection advocates (typically) want to deny...that selection can only happen at one level. We can explain all the social phenomena under consideration, and a great deal more, without the multilevel section pressures; therefore Occam's razor should lead us to abandon it. Here's how to generate multilevel and multiscale selection and compare them.

To keep things simple, let's assume that a level-1 group consists of two agents, and a level-2 group consists of two level-1 groups, and a level-2 groups contains two level-1 groups. Selection requires some property that affects replication. At the agent level, let's give all agents the property + or . Level-1 groups with ++ inside have property A, groups with + or + inside have property B, and groups have property C. Level-2 groups can only contain level-1 groups of AA, AB, AC, BB, BC, or CC (assuming order doesn't matter), and these combinations are assigned F1, F2, F3, F4, F5, F6 respectively.

Each higher level property is determined by its lower-level constituents, but these are not lossless bridge laws. Note that you can reduce any level-2 group all the way down to possible sets of agents: an F3 groups contains AC groups, which contains ++. But given the unordered set ++, it could also be ++, so you don't know if that's AC or BB, hence F3 or F4. Decreasing scale (reduction) is unambiguous, but increasing scale (emergence) is ambiguous even in a simple formal model like this.

In a good complexity model, all the behavior would be programmed at the agent level, and higher-level phenomena generated from that. So there would be rules for what +s and s do by themselves and when they interact. Perhaps they interact with multiple other agents at the same time...or (let's say) just one at a time. At any given time, whatever pairs exist will have the property A, B, or C. But what would identify the level-2 properties if there are no pairs of pairs? If you want another level you have to add something to the model.

One thing you could do is allow agents with partners to join with other agents with partners; that allows pairs of pairs, but you might also get chains. So you limit this bonding of pairs to other non-bonded pairs. That's an ad-hoc rule to get what we want for sure, but it may actually be appropriate for some kinds of agents. Okay, that generates level-2 groups of pairs of pairs of agents and each one will have exactly one property F1-F6. You can then specify other agent rules such as preferential attachment and detachment and/or birth and death processes and produce some selection dynamics. For example, 1) assign a rule that all agents will connect to a + if one is available (otherwise a ) 2) in every iteration each agent makes X copies of itself, where X = #of+s connected to it, and 3) each agent lives 3 iterations. In most cases the population will very quickly explode with + agents, A groups, and F1 groups...multiscale selection without needing to specify rules beyond the agent level (I did something like this in my dissertation for the Biased Lane Choice game, but only two scales).

Another thing you could do is identify groups of agents, and specify rules for duplication at all three levels: for example, 1) half of the agents in F6 groups die, 2) F1 groups duplicate themselves, 3) A groups join with other A groups 75% of the times, etc. Those are all phenomena that the bottom-up agent-based model described above would produce, but without requiring the higher-level selection rules. This second, higher-level type of rules are what group selection (sometimes) is claiming. Something like "when two level-2 groups are competing for +s, the one that better fosters +s and + acquisition will flourish and reproduce at the expense of the other group, and having more +s in the group is that property that makes it better for +s." This is a description at the higher level of what is happening at the lower level, and the agent rules can explain this, but if you define this rule at the higher level, then you've lost all explanatory power.

Of course, if you don't care about explanatory power, and you just want to create different (possibly competing) dynamics at multiple levels without figuring out how to get the macrolevel or mesolevel phenomena from the microlevel, then this is a way to explore the effects of multiscale selection. That model will be conceptual nonsense, and I would argue it will likely also be useless for any purposes because it fails to preserve conservation laws in causal efficacy. But there may be some specific questions and narrowly focused problems that such a model can address.

My point is that the thing that most people are actually interested in is the multiscale selection generated by microbehaviors, and there is a major difference between identifying phenomena at higher level (because they are interesting, measurable, and comparable to data), and defining rules for those higher level phenomena as causally efficacious. Downward causation is worse than wrong, its confused and unnecessary. So is causation at multiple levels/scales in the same model. If you want to explain the phenomena you generate in a model via the mechanisms that may have actually generated those phenomena in the world, then selection (and causation in general) can only be at a single level...choose wisely.