CHALLENGE SUMMARY
The smallest group has just two members. These two members may be genetically the same, as would be the case if the group is made up of two attached yeast cells that failed to separate, or they may be genetically different as is the case of two parents collaborating to rear their young. Though a group of two is small, the behavior, physiology, and environment of the two social individuals can differ dramatically from the single, isolated individual. Furthermore, the group of two individuals can take advantage of new opportunities or experience different consequences. Two can be double the size of one, perhaps making them much less likely to be predated. Two can guard a resource 100% of the time. Two can specialize, perhaps one guarding more while the other forages more. Two can also compete over resources.
If we want to understand the fundamental principles and mechanisms underlying collective action, an excellent place to start is with groups of just two, for several reasons. (1) Such groups are likely to be simple, yet the effects of doubling the number of members will be great, much more than adding an individual to a group of 20, for example. (2) Groups of two are found across the tree of life, from microbes to insects to vertebrates, making it interesting to explore similarities and differences among different kinds of pairs. (3) Sexual reproduction combined with parental care creates a group of two parents, and the dynamics between these two individuals and their offspring can have great ramifications for fitness. (4) Groups of two are common at various stages of social bee and wasp colonies, presenting an
ideal experimental framework to understand how interactions between two individuals can scale to a larger social groups. (5) Groups of two can be fraternal, made up of genetically related individuals, or egalitarian, made up of genetically unrelated individuals, allowing a comprehensive understanding of the possible factors facilitating social behavior. (6) Processes important in evolving groups of two are likely to be important in larger groups and much more apparent.
Key Questions
What are the costs and benefits of groups of two compared to singletons?
What are the pressures on groups of two making them drop to one, or increase to three or more?
Do groups necessarily grow from one to two, or can they grow from one to many, and what are the consequences?
How is conflict controlled in groups of two?
What is the impact of group stability and duration on the nature of the group?
How are groups of two distributed phylogenetically? What can we learn from these patterns?
How do fraternal and egalitarian groups of two differ in costs and benefits?
What can we learn from microbial groups of two of relevance to animal parents?
Reading
Brand N and Chapuisat M. Impact of helpers on colony productivity in a primitively eusocial bee. Behavioral Ecology and Sociobiology 2014;68:291-298.
Cockburn A. Prevalence of different modes of parental care in birds. Proceedings of the Royal Society B: Biological Sciences 2006;273:1375-1383.
Field J, Shreeves G, Sumner S, and Casiraghi M. Insurance-based advantage to helpers in a tropical hover wasp. Nature 2000;404:869-871.
Hogendoorn K and Velthuis HH. The sociality of Xylocopa pubescens: Does a helper really help? Behavioral Ecology and Sociobiology 1993;32:247-257.
McLanahan S, Tach L, and Schneider D. The causal effects of father absence. Annual Review of Sociology 2013;39:399-427.
Ratcliff WC, Denison RF, Borrello M, and Travisano M. Experimental evolution of multicellularity. Proceedings of the National Academy of Sciences of the United States of America 2012;109:1595-1600.
Stevens MI, Hogendoorn K, and Schwarz MP. Evolution of sociality by natural selection on variances in reproductive fitness: Evidence from a social bee. BMC Evolutionary Biology 2007;7:153.
Wong JW, Meunier J, and Koelliker M. 2013. The evolution of parental care in insects: The roles of ecology, life history and the social environment. Ecological Entomology 2013;38(2):123-137.
IDR TEAM MEMBERS
Anamaria Berea, University of Maryland
Itai Cohen, Cornell University
Maria R. D’Orsogna, CSUN
Kingshuk Ghosh, University of Denver
Nigel Goldenfeld, University of Illinois Urbana-Champaign
Charles J. Goodnight, University of Vermont
Glenn Hampson, National Science Communication Institute (nSCI)
Andrew M. Hein, Princeton University
Lisa A. McGraw, North Carolina State University
Anand D. Sarwate, Rutgers, The State University of New Jersey
James Urton, University of California, Santa Cruz
IDR TEAM SUMMARY—GROUP 6
James Urton, NAKFI Science Writing Scholar University of California, Santa Cruz
IDR Team 6 was asked to probe the fundamental principles of the simplest group, that of two individuals.
The Challenge of Group Formation
Groups pass through phases of formation, growth, decline, and dissolution. They also change through the type and number of interactions among members. Rejuvenated or worn down by these alterations, the group can press on or drift into obscurity based on the health, number, and type of interactions that flow among group members.
One approach to understand group formation is to distill groups to their constituent components to draw out the basic principles of their interactions. This is an attractive approach given that the complexity of
groups, in size and the nature of interactions, can strain resources to study them. Models of group behavior struggle to explain how groups form and change, or are too vague to have real-world applicability. Field observations lack the robust data measurements needed to capture the complete picture of group evolution. One reason for these difficulties is the lack of guiding principles of group formation, maintenance, and dissolution, especially the most fundamental steps that seed the group and root its purpose.
The simplest group consists of just two individuals, a dyad. Its formation is both fundamental and minimalist. There are only two members to form pairwise interactions and respond to the external environment. If common principles underlie the formation of the group of two, then the dyad forms the foundation of all group structures and could be a common framework to understand both group behavior and history.
With the importance of this undertaking in mind, IDR Team 6 confronted this challenge through two fundamental questions:
Are there fundamental principles in the formation, maintenance, and dissolution of a group of two?
What role does the group of two play in the formation, maintenance, or dissolution of larger groups?
One Plus One
The group of two permeates nature, from hydrogen gas to binary stars. The simplest definition of the group is two individuals joined or bonded by an interaction. But, in their deliberations, IDR Team 6 members quickly concluded that there is little else that universally links all groups of two. The examples from nature, history, sociology, and psychology are too varied in composition and interactions to draw more comprehensive conclusions or divine general principles for the formation of groups of two.
But, the examples the IDR team explored did illustrate how consideration of groups of two could be useful for understanding many different aspects of group formation, maintenance, growth, evolution, and dissolution. Team members believe their discussions will be most useful for future cross-disciplinary collaborations and investigations into group activity or collective behavior.
The Problem of Scale
The largest hurdle IDR Team 6 encountered in their deliberations centered on the fundamental principles of the group of two, or dyad. Armed with examples of dyads from across the natural world, from protein dimers to merging corporations, they readily identified qualities of different dyads and instances where some dyads shared characteristics, such as the chemical bond interactions between atoms or molecules, or social interactions among animals. But, no other commonality permeated all groups of two.
This difficulty centers on the issue of scale. Identity as a group or individual can vary depending on the observer’s scale. Two genes can form pairwise interactions as a simple group. These genetic interactions can combine with other genes and environmental factors to form a more complex genetic network. This complex group, with a myriad of members and interactions, in turn defines a new level of individual, the living cell. Living cells can form new interactions and complex groups, forming a person. People interact to form societies, and so on. Each level is defined by a collection of individuals, each of whom is made of the complex group from the previous level. The only unifying principle for the dyads at each level is the involvement of two individuals, be they genes, cells, people, or societies.
Mergers
A group of two may revert to an individual identity rather than a cooperative dyad. Corporate mergers are an obvious example at a societal level. But, even the fragmentation of large corporations can eventually lead to the reformation of the original corporation. AT&T, for example, was recently reconstituted into a single corporate identity from the merger, acquisition, and absorption of smaller companies that had previously broken off from the telecoms giant. In Protestant Christianity, some denominations fragmented and later merged again in response to internal and external factors. Sexual reproduction is also a type of merger in which egg and sperm unite to form a single zygote, a new and novel individual.
Mergers are an important issue to understand in medicine. In organ transplantation, the donor tissue must be successfully integrated into the recipient’s body without rejection. In autoimmune disease, the immune system no longer recognizes a specific cell type or tissue as a part of the whole body. An ideal treatment would merge the two parts into an individual identity.
The Dyad
In their discussions, IDR Team 6 members emphasized a distinction between collective behavior and cooperation when it applies to the group of two. The collective behavior of groups centers on the acquisition of a new function or role that is absent among the individuals. Cooperative activity, in contrast, is the sharing of tasks or roles among members of a group.
Biology is replete with dyads that demonstrate cooperative properties. Many aspects of parental behavior in animals involve cooperative behavior, where one parent might guard a nest while the other searches for food. But, cooperation can even exist on smaller scales, such as gene duplication. One homolog might assume a greater share of the ancestral gene function, while the other might take on a different function through mutation or regulatory changes.
IDR Team 6 members were less clear on whether the dyads always demonstrate cooperative behavior, and whether they take on any truly collective behaviors. Collective behaviors could include coordinated endeavors such as synchronization or a more generalized output such as mass hysteria. Either way, pairwise interactions can form the basis of later collective behaviors in larger groups, but it is unclear whether the dyad itself can exhibit these behaviors alone.
The dyad can also include unique evolutionary processes in which both members are changed irrevocably by selection pressures. In a “red queen” evolutionary arms race between a pathogen and a host, strong selection can ensure that both members of the group change dramatically through mutation accumulation as each tries to outmaneuver the offensive and defensive strategies of the other. In a whole-genome duplication event, evolutionary forces can act differentially on the homologous copies, yielding genetic and epigenetic divergence.
Dyads can also take on roles that are greater than the sum of interactions between the two individuals. These synergistic groupings include positive epistasis between two genes, drafting in migrating birds, and receptor-ligand binding in cell-cell interactions.
The Role of Two in Larger Group Formation
IDR Team 6 then took on the role that dyads play in the formation of larger groups and more complex networks. Many large groups, especially in cellular biology, are constructed from dyads. For example, the cytoskeleton,
which plays indispensible roles in cell structure, transportation, and division, consists of polymers of protein dimers. Theoretical models of group formation indicate that the group of two can serve as a fundamental building block for constructing more complex and larger groups, but with a less structured network topology than trios.
Larger groups exhibit unique patterns that may reflect a common framework for formation and growth. For example, many large groups show a skewed distribution of sizes. Cities tend to be very large or small, with fewer mid-sized communities. Many other group phenomena, from chromosome sizes to journal citation patterns, also exhibit this “power-law” distribution pattern. Some theoretical models of group formation and game theories posit that pairwise interactions may underlie this skewed distribution of group size. But in general, data to support these models are lacking.
Technological and Experimental Gaps
IDR Team 6 members think there are now sufficient tools to track and manipulate group formation and response to ask whether the dyad is a common building block for larger complex groups. Tracking software can now measure position, speed, and coordination among group members, often on an automated basis. These tools have already been used to measure complex groups such as schools of fish and the myriad of interactions among dancers. Optogenetic techniques can manipulate nerve activity in model organisms such as fruit flies, allowing experimenters to isolate the neural and muscular responses necessary for collective behaviors. In vitro viral capsid and cytoskeletal assembly assays can measure assembly and disassembly of complex biochemical groups from their building blocks. Repeated and robust experiments with these new tools should reveal the fundamental building blocks of more complex groups at a variety of scales, potentially boosting the importance of the dyad in group formation and maintenance.
Implications for Group Formation
Groups of two are ubiquitous and enigmatic. They are also the gateway to understand the basic interactions among individuals in groups of all sizes, and potentially the building block of large and complex networks. The dyad in all of its iterations may aid in understanding how deleterious groups, from cancer to terrorist cells, form and thrive, adding urgency to this quest.
Recent advances in technological and experimental tools are the key
to advancing group studies beyond abstract models and theories. The collective behavior field will soon be in a position to collect data that could address many of the IDR team’s outstanding questions and concerns. The group of two is not as simple as 1+1=2, but its complexity is also not too far out of our reach.
