Investigating evolutionary transitions in individuality, whereby biological entities team up to form greater entities.
As readers are probably aware, they comprise myriads- legions of cells, each teaming with molecules and genes, plus the galaxies of the microbiome. We feel like individuals, but have been assembled over billions of years of evolution out of alot of smaller components, with dramatic steps taking place in what are called evolutionary transitions in individuality, or ETI. A recent paper introduced me to this literature, but it is so poorly written and conceived that we won't mention it any further.
Some of the landmark transitions of this type are from an RNA world of individual replicators to that of cells, or at least blobs that collected replicators into genes, now organized on chromosomes. Next was the eukaryotic cell, which arose as the joint project of at least two quite disparate microbial cells, one the bacterium that formed the contemporary mitochondrion, and the other the controlling nucleus. Multicellularity came next, building animals out of these eukaryotic cells, first simple, like sponges, but eventually, something amazing. Last came the animal societies, of which ours has only partially transitioned to a new stage of individuality, but where the insects such as termites and ants have completed virtually complete transitions to being super-organisms.
It is inherent in contemplating these transitions that they are examples of group-level selection. In order to achieve higher levels of organization there must be selective benefits of operating at the group level, often in competition with benefits occuring at the individual level. For groups inherently need to regulate and sometimes kill off their members, whether that is a gene that is sliced apart and recombined to serve the immune system of its host, the leukocytes we sacrifice to ward off infections, or the soldiers we send off to battle. Group selection is very real, whether one wants to recast every selective event as gene-centric, (where the mutation takes place), as Richard Dawkins has, or recognize the actual ecological / operational level of selection.
One important form of regulation is of reproduction, without which a true individual will never emerge at the higher level. The group (say, an organism with lots of cells) needs to suppress to reproduction of most of its members, eliminating individual competition and selection. Instead, it will need to select some representatives to stand for the whole and carry out reproduction, while not themselves having an independent livestyle that would conduce to selection on any relevant traits- i.e. those that might compete with the important traits of the collective. That is where germ cells come from, as much-reduced versions of actual organisms that so briefly carry out the hapoid phase of most organism's lives.
Humans are obviously nowhere near this kind of reproductive control, outside of science fiction and societies that, if they have ever occurred, are extremely rare in their degree of totalitarian control. So our groups are nowhere near becoming a new level of biological individual. This is unlike the most developed social insects, whose reproduction is totally controlled at the group level. One interesting paper in this field brought up the case of dictyostelium, a soil amoeba whose life cycle, while mostly individual and independent, includes occasional mass aggregation to produce traveling slugs and fruiting bodies, which put out reproductive spores. In their words:
"The single-cell bottleneck and subsequent clonal development is thus a key trait facilitating the evolution of higher-level complexity in fraternal transitions. Two widely studied social organisms, the slime mold Dictylostelium discoideium and bacterium Myxococcus xanthus, appear stuck in the transition to multicellularity, despite ample time to evolve multicellular complexity (>400 Myr ago for the Dictyostelid cellular slime molds and >650 Myr ago for the myxobacteria). While both organisms possess multicellular life histories that include cellular division of labour, neither life cycle includes a single-cell bottleneck, and genetic conflict is rampant."
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| The general life-cycle of Dictyostelium. Many of the aggregated amoebae get to form spores, but not all. |
The dictyostelium aggregate slug forms a base, stalk, case, in addition to the spores, so it is evident that some of the individuals that came together will be left behind, and not have even a chance at reproduction. At the same time, many individuals do get that chance, out of the body they form as a community. So this species, like humans, forms collectives of convenience, while not controlling either its own clonality or the reproduction of its members, and thus is quite far from achieving any higher level of biological individuality.
So, the key requirements of individuality at any level are:
- The entity must have traits subject to darwinian selection
- These traits must be heritable, enabling selection
- The entity must reproduce, to make new entities and enable selection
- The reproduction must not be subject to competition by entity members, (i.e. by their own individual reproduction, with their own selective competition and imperatives)
Clearly, clonal colonies of some sort make it easier to form coherent higher level collectives. But that is not enough- reproduction needs to go through a single germ cell bottleneck, preventing the competition among the vast majority of the collective members, while reflecting the collective's overall genetic complexion.
This paper also presented an experiment of selecting yeast cells for collective behavior / structure by selecting those that precipitated rapidly from their growth medium. This is a selection for multi-cellular aggregates, which is relatively easily done in yeast. Successful isolates tend to have defective cell wall separation machinery, so that cells remain attached after division. These entities reproduce, by breaking off occasional flakes of aggregate. This reproduction is largely clonal, with lineal descendents being attached in local aggregates which break off. And whatever trait the aggregate has is reflected in the descendents genetically. So is this a new level of biological individuality? They claim that yes, it is, though limited to this totally artificial regime of selection.
"Snowflake yeast display a key emergent property: as clusters grow larger, tension among cells increases until it exceeds the tensile strength of a cell–cell connection, resulting in the release of a multicellular propagule5. Once clusters have evolved, they readily become a unit of selection, as whole clusters either settle rapidly enough to survive, or fail to do so and perish. As a result of this shift to cluster-level selection, we observe extensive cluster-level adaptation, including the evolution of larger size, elevated apoptosis and more spherical, hydrodynamic clusters."
These cells would never succeed in the wild, where entirely different and diverse selective pressures exist. Yet the experiment shows that this transition is not intrinsically as hard as evolution makes it seem. Evolution is terribly conservative, with intense selective pressures to innovate only at the margins, given the network of constraints already satisfied by one's ancestors. Coming together to recognize new cooperative opportunities, while giving up one's individuality, is, frankly, anathema.
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