In ecology, we usually assume that population sizes or interspecies relationships will eventually settle into a steady state. This appears as the stabilization of traits in evolutionary biology, meaning the population or the trait reaches a stable or equilibrium condition. Allele frequencies likewise converge on a relatively stable balance in genetics. Across biology and ecology, such stability is collectively termed an “equilibrium state”, which in physics corresponds to “equilibrium” or “conservation”.

When discussing “equilibrium”, “stability” or “conservation”, we are tacitly assuming symmetry: the interacting parties are mutually symmetrical. That is, swap their positions in space and the system looks exactly the same. Likewise, time is fully reversible and the system would remain unchanged. Only under such conditions of complete symmetry can stability, equilibrium, or conservation emerge. In physics, this is known as Noether’s Theorem.

Just as truly perfect symmetry rarely exists in physical systems, absolute symmetry is almost impossible to find in real-world living systems. Take ecosystems as an example: the spatial distribution of species is often uneven, making their interactions intrinsically asymmetric. Furthermore, individuals within a population exhibit asymmetry due to structural characteristics such as age structure, hierarchical differences, or sex ratios. Any spatial exchange—whether between populations within an ecosystem or among individuals within a population—inevitably alters the system's properties and may even cause collapse. Social insects provide the clearest illustration: if worker ants replace the queen's position, the system will inevitably collapse and require reconstruction.

This review uses interspecific mutualistic systems as a case study to demonstrate the types of asymmetry exhibited in interspecific cooperative systems and how this asymmetry generates uncertainty in interactions within the system. Asymmetry in reciprocal cooperation systems primarily stems from the following aspects:

1. Asymmetric Payoffs

Asymmetry in mutualism first manifests as asymmetric payoffs, where the benefits gained by each partner are unequal. In many host–symbiont relationships, the resources or services exchanged between the host and symbiont are not equivalent. In plant-insect pollination, legume-rhizobia, and ant-plant mutualistic symbioses, the host provides resources or habitat while the symbiont delivers services (pollination or nitrogen fixation). Significant disparities also exist in resource ownership, with hosts monopolizing more resources.

Hosts often possess the power to control resource allocation, limiting symbionts’ access to resources, while symbionts typically depend on specific resources provided by the host. This leads to asymmetry in resource acquisition and benefit distribution. Such asymmetric resource allocation results in unequal benefits within mutualistic relationships, thereby causing asymmetry.

2. Asymmetric Evolutionary Rates

Asymmetry also arises from differences in evolutionary rates between hosts and symbionts. In general, symbionts have shorter lifecycles and more frequent generational turnover, potentially accumulating greater genetic variation per generation. Hosts, with longer lifecycles, may evolve at a slower pace, lagging relatively in the evolutionary process. This disparity in evolutionary pace enables symbionts to adapt more rapidly to environmental or host changes. By adjusting their behavior or traits—or even developing deceptive or non-cooperative behaviors—they can alter their role within the mutualistic relationship, further deepening asymmetry within the cooperative system.

3. Asymmetric Information

Information asymmetry in mutualistic cooperation refers to incomplete knowledge between hosts and symbionts regarding each other’s behaviors and intentions. Hosts may struggle to accurately identify which symbionts are cooperators and which are cheats; conversely, some less tightly cooperating symbionts may also fail to clearly discern when the host will implement punishment or reward. In fig tree–fig wasp mutualisms, the host may be unable to distinguish which symbionts provide pollination services and which merely “free-ride” without pollinating. Consequently, the host’s punishment mechanisms may fail to function effectively. This allows non-cooperative symbionts to gain higher net benefits without providing sufficient returns to the host, thereby exacerbating asymmetry within the cooperative relationship and undermining the system’s stability and cooperative patterns.

The emergence of asymmetry in mutualistic systems primarily stems from disparities between hosts and symbionts in resource exchange, evolutionary rates, and information acquisition. These asymmetries introduce complexity and uncertainty into interactions, undermining cooperative stability and long-term evolution.

About the author

Ruiwu Wang, “Qiushi” Distinguished Professor and PhD supervisor at the College of Life Sciences, Zhejiang University, China. Recipient of the National Science Fund for Distinguished Young Scholars in 2013 and the Chinese Academy of Sciences Outstanding Young Life Scientist Special Fund in 2011. Former Associate Editor of Proceedings of the Royal Society B and Editorial Board Member of Zoological Research: Diversity and Conservation. His research focuses on the evolution of reciprocal cooperation and ecosystem ecology, and he proposed the theory of asymmetric and uncertain selection in cooperative systems.