[Paper Review] Is nestedness in mutualistic networks an evolutionary spandrel?

Sergi Valverde, José M. Montoya+2 more|arXiv (Cornell University)|Dec 6, 2016

Plant and animal studies3 citations

TL;DR

This paper proposes that nestedness in mutualistic ecological networks is not an adaptive trait shaped by natural selection, but rather an evolutionary spandrel—emergent from simple speciation and divergence rules. Using a minimal model of network growth through duplication and divergence, the authors show that observed nested structures and co-extinction dynamics in real mutualistic webs can be reproduced without assuming adaptive pressures, suggesting that nestedness may be a byproduct of network construction rules rather than a functionally selected feature.

ABSTRACT

Mutualistic networks have been shown to involve complex patterns of interactions among animal and plant species. The architecture of these webs seems to pervade some of their robust and fragile behaviour. Recent work indicates that there is a strong correlation between the patterning of animal-plant interactions and their phylogenetic organisation. Here we show that such pattern and other reported regularities from mutualistic webs can be properly explained by means of a very simple model of speciation and divergence. This model also predicts a co-extinction dynamics under species loss consistent with the presence of an evolutionary signal. The agreement between observed and model networks suggests that some patterns displayed by real mutualistic webs might actually represent evolutionary spandrels.

Motivation & Objective

• To investigate whether nestedness in mutualistic networks is an adaptation or a byproduct of network construction.
• To test if a simple model of speciation and divergence can reproduce observed network patterns, including nestedness and co-extinction dynamics.
• To evaluate whether the structural regularities in mutualistic networks reflect evolutionary processes rather than functional adaptation.
• To assess the role of network growth rules in generating complex, non-random features such as nestedness.
• To explore whether evolutionary spandrels—non-adaptive byproducts of construction rules—can explain invariant network features in ecological systems.

Proposed method

• A minimal model of network growth based on duplication and divergence of species in two interacting layers (animals and plants).
• Speciation events are modeled as duplications followed by divergence, simulating evolutionary branching.
• The model generates bipartite networks with no explicit selection or fitness functions, relying solely on growth rules.
• Network structure is analyzed for nestedness, degree distribution, and modularity using standard metrics (e.g., NODF for nestedness).
• Co-extinction cascades are simulated under species loss to compare with empirical data.
• Model outputs are compared with real mutualistic networks to assess structural similarity and evolutionary plausibility.

Experimental results

Research questions

• RQ1 Is nestedness in mutualistic networks a result of adaptive evolution or a byproduct of network construction rules?
• RQ2 Can a simple model of speciation and divergence reproduce key structural features of real mutualistic networks?
• RQ3 Do the observed co-extinction dynamics in mutualistic systems align with predictions from a non-adaptive growth model?
• RQ4 To what extent do network growth rules alone explain invariant network patterns such as nestedness and heterogeneity?
• RQ5 Is nestedness better understood as an evolutionary spandrel than as a functional adaptation in ecological networks?

Key findings

• The duplication-divergence model successfully reproduces the nested structure observed in empirical mutualistic networks without incorporating selection or functional optimization.
• The model generates co-extinction dynamics under species loss that closely match empirical observations, supporting the presence of an evolutionary signal.
• Nestedness and other structural features emerge naturally from the growth process, suggesting they are not necessarily adaptive but may be evolutionary spandrels.
• The agreement between model-generated and real networks indicates that complex network patterns can arise from simple, non-functional rules of construction.
• The results challenge the adaptationist view of nestedness, suggesting that its presence may be a consequence of network architecture rather than a driver of biodiversity persistence.
• The study supports the idea that universal network growth mechanisms, such as duplication and divergence, can explain invariant features in ecological networks across different systems.