# Evolutionary responses to conditionality in species interactions across environmental gradients

## TL;DR

The study shows that teosinte plants and soil biota co-adapt better in stressful cold climates, supporting a new hypothesis about mutualistic interactions under stress.

## Contribution

The paper introduces the Stress-Gradient-Adaptation hypothesis, extending the SGH by predicting co-adaptation under stress.

## Key findings

- Local adaptation between teosinte and soil biota occurs in cold (stressful) climates but not in warm (benign) ones.
- Sympatric plant-biota combinations from cold sites improve plant fitness and adaptation.
- Warmer-sourced biota provide greater benefits than colder-sourced ones, even for cold plant populations.

## Abstract

The outcomes of many species interactions are conditional on the environments in which they occur. A common pattern is that outcomes grade from being more positive under stressful conditions to more antagonistic or neutral under benign conditions. The evolutionary implications of conditionality in interactions has received much less attention than the documentation of conditionality itself, with a few notable exceptions. Here, we predict patterns of adaptation and co-adaptation between partners along abiotic gradients, positing that when interactions become more positive in stressful environments, fitness benefits of interactors become more aligned and selection should favor greater mutualistic adaptation and co-adaptation between interacting species. As a corollary, in benign environments, if interactions are strongly antagonistic, we predict antagonistic co-adaptation resulting in Red Queen or arms-race dynamics, or reduction of antagonism through character displacement and niche partitioning. We predict no adaptation if interactions are more neutral. We call this the CoCoA hypothesis: (Co)-adaptation and Conditionality across Abiotic gradients. Here, we describe experimental designs and statistical models allowing us to test predictions of CoCoA, with a focus on positive interactions. While only one study has included all the elements to test CoCoA, we briefly review the literature and summarize study findings relevant to CoCoA, and highlight opportunities to test CoCoA further.