Every new form of biological organization becomes a new unit of natural selection
Group selection has been a controversial topic in evolutionary studies. Indeed, the whole matter of where selection operates has been a confusing mess. Richard Dawkins battled his way to fame by arguing that genes were the target of selection, and that we as animal bodies were merely automata driven to unwittingly propagate them by various unconscious means. When considering the unit of selection, one could go even to the individual nucleotide, which is ultimately what is extinguished or propagated by the action of mutation and selection, plying its tiny oar towards the survival of its gene, its genome, its cell, its organism, its society, ... its blessed plot, this earth, this realm, this England!
Traditionally, the individual organism has been viewed as the main unit of selection. But can groups, when they form societies like bee hives or human tribes, be objects of selection as well? A paper reviewing the mathematics of evolution and selection makes the crucial distinction between the mechanism underlying heritability of traits, which might be a gene or nucleotide, and the unit of selection, which is the level of biological organization that exhibits traits upon which natural selection acts. The color of our eyes may be a cellular and organ-level trait, based on genes and nucleotides, but the unit of selection remains the individual, since that is where selection- via mate choice, disease, and whatever other ramifications eye color may have- acts directly to promote or inhibit reproduction. Likewise, social traits such as altruism, cooperation, detection and policing of cheaters, etc. may be in large degree be relevant and selected at the individual level, but at least some of their power and selectivity comes in the competition between groups, i.e. group selection.
It should be clear that selection happens at all sorts of levels, indeed at every level where a new form of biological organization emerges. The "unit of selection" is not singular, but manifold, and is defined, not absolutely, but by the level and properties of the trait being considered. We who inhabit multicellular bodies have pretty definitively ended competition / natural selection among the cells that compose us- those cells are not individual units of selection, since they do not persist after we are gone (even in the case of cancer where their replication has gone haywire). The closest might be competition among male sperm cells, which evidently do compete in their final voyage, though not to the extent of taking up arms against each other. Thus generally, our genes are only indirectly targets of selection, in that they generate traits that manifest on the cellular, individual, and indeed group level, with consequent selection at those levels and differential reproduction that change gene frequencies in the future.
This is called multi-level selection. The socio-biologists got into hot water back in the 1970's by asserting that group traits are at least in part biologically based, as are individual psychological traits, and thus that groups must act as units of selection. This did not sit well with the politically correct of the day, who wanted as a matter of principle to believe that humans (and especially subgroups such as ethnicities and races) are all created equal, and that any talk of heritability of traits such as intelligence, aggressiveness, altruism, etc. was, if not wrong, at least socially devisive and certainly damaging to a proper communist / constructivist view of the malleability of the human condition. While constructivist views of our social psychology, relations and conflicts certainly have significant truth, they can be taken too far, such as the arch-feminist idea that male-ness is purely a social construction, and that some counter-programming is all it would take to make a utopian, de-gendered world.
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But that is all in the past, and not only are social and group traits increasingly recognized as biological and to some degree heritable, but our evolutionary history is unthinkable without a lot of specific socially relevant traits being encoded, evolved, and put to the test in group-group competition, whether via direct competition or just relative success of independent groups without direct interaction. A set of papers made a review of this field and developed a general mathematical treatment of multi-level selection (MLS), postulating that any biological entity or level of organization can be a unit of selection- when traits can be defined pertaining to that level. This is especially relevant to emergent traits that can not be defined at lower levels of organization.
Alcoholism, for instance, is hard to define at the cellular or single gene level, but can be easily defined at the organismal level. So it is selected at the level, where individuals suffer and die due to its effects and impair the lives of others along the way. While it necessarily has genetic components and heritability, and those genes can be thought of as being selected for or against, they often drag along many other genes, and have complex relations with other genes in the trait's expression, leaving the definition of the trait and its interaction with natural selection at the individual level. The unit of selection is a separate concept from the genetic and developmental processes that generate the trait. In alcoholism, the adult is the unit of selection, consituting a collection of characteristics that develop out of genes and other sources, whose frequencies may change based on that selection.
"The genetical theory of MLS ... describes the action of group selection in terms of change in a genetical character. As discussed in the previous section, a genetical score may be assigned to any biological entity that contains genes – such as an entire population – and change in this genetical score can be computed, irrespective of how that population is subdivided into groups and individuals, or the biological level of organization at which the corresponding phenotype actually manifests. ... the theory of natural selection is ‘genetical’: this adjective pertains to the medium by which characters are inherited, rather than to the unit of selection itself."
It may be that all this is just a matter of convenience and book-keeping, as traits are defined (by us) on a macro basis. A gene's-eye view of the situation would focus on its own gains and losses in the rough and tumble of life. But in that case, we could not speak of alcoholism as a trait, but would have to speak of the gene's eye view of all the pressures it finds itself under, which would range widely over molecular, cellular territories and beyond, and violate our basic conceptions of a trait that is under natural selection. That is why a trait is defined at a particular level of organization where that characteristic becomes manifest, rather than at at gene level. There is no gene for alcoholism, though the trait is composed of / developed out of many heritable elements.
Imagine, in contrast, that alcoholism had no genetic component at all, but was purely random in genetic terms, not even affected by, say, genetic susceptibility to advertising blandishments. Such a trait would be subject to natural selection (i.e. death and other forms debility). But all that selection on the trait would have no effect on the next and future generations, due to its lack of heritability. It would have no genetic implications, by definition. So the unit of selection and trait being selected are separate issues from the genetic elements that might underpin it, particularly the degree or lack thereof of its genetic basis.
While we are discussing this particular trait, it might be worth noting that in group terms, affinity to alcohol might be considered a positive trait, contributing to group bonding through the ages. Thus alcoholism might be a matter of stabilizing selection, trading off between its individual harms and its group benefits, particularly in the prehistoric setting where alcohol concentrations tended to be low, social controls strong, and alcoholism proper quite hard to develop.
This discussion, based on the paper series, is all based on the Price equation, which apparently underlies the field and is an extremely general statement / definition of natural selection. It contains basically two terms, which provide for a separation between the aspects of biological change derived from natural selection, and all the rest of the sources of change- drift, environmental change, etc. The selection portion it expresses as co-variation between traits in two populations (such as in successive generations) and the success of individuals (or other units of selection) carrying that trait. The whole equation rests on four key terms, none of which are explicitly genetic:
- The unit of selection- the biological organization that exhibits the trait, whether an individual, group, etc.
- The arena of selection- the population of units within which selection and evolution take place.
- The character under selection- the trait at issue, at whatever appropriate level of organization.
- The target of selection- the quantity (fitness) by which the character / trait is either good or bad, thus being selected.
As far as the unit of selection and the trait that pertains to that unit, any level will do, as long as it corresponds with a unit, or trait, that is definable to us and selectable in nature.
"Between-group selection is directly analogous to standard, individual-level natural selection, but with the group taking on the role of the unit of selection, the group's phenotype acting as the character under selection and group fitness being the target of selection."
"... by framing selection in its full generality from the outset, Price's equation reveals that kin and group selection are components of natural selection, and we obtain their dynamics by drawing them out of—rather than adding them into—the basic form of Price's equation. Moreover, by showing how the kin selection and group selection viewpoints both emerge from the mathematics of natural selection, Price's equation shows that these are not competing hypotheses for the evolution of social behaviour but simply different ways of conceptualizing the very same evolutionary process—and that a fierce, decades-long debate had been largely over nothing."
"For group selection to overcome selection within groups, less than one successfully reproducing migrant may be exchanged per two populations per population lifetime. ... Indeed, if groups are long lived, successful migrants must be very rare, and within-group inbreeding intense, for group selection to prevail over equally intense within-group selection."
Each level of selection can operate on many different traits, however, some of which may not directly conflict. So leaving aside the direct competition between individual and group interests, there is a rich field of action for group selection. This observation of the great sensistivity of group benefits to the rate of migration, especially for traits that conflict between individual and group benefits, gives us a clue about the origins of tribalism, which makes a practice of accentuating infinitesimal differences (or entirely imaginary ones) and using them to justify xenophobia, war, and genocide. It is a key legacy of evolution, particularly group evolution, and one that we struggle to overcome.
So group selection is perfectly consistent with evolutionary theory, (though some rather testy controversies remain). Does that mean that racism is OK? Do group differences justify tribalism and oppression? Well, our instinct for tribalism is certainly testament to a long evolutionary history of group selection, with its tireless focus on tiny, or even nonexistent, differences. The fact is that among humans, group differences are always swamped by within-group variation. We also do not generally discriminate so harshly against the differently abled and neuro-diverse *within tribes as we do against those we perceive outside them. So the practical and moral basis of discrimination and oppression is very poorly founded. True group selection is also virtually powerless against high migration rates, which we have throughout the modern world in any case. Thus the tribal instinct, which is now so flexibly deployed for nebulous groupings as nation states or sports teams, is totally out of its natural element, were we even inclined to mount some new eugenic project of any nature, whether individual or group.
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