Adaptation to complex environments is associated with higher fitness costs under benign conditions

Populations evolved under pairwise combined stressors across 14 species show lower fitness increases and, in benign conditions, higher fitness reductions than populations evolved under single stressors.

• Why it matters: Combined environmental stressors are common, but it is unclear how this selective complexity changes evolutionary pace and the tradeoffs that emerge when stress is removed.
• What they did: In a coordinated multi-species experimental evolution, 14 species were exposed to single stressors versus pairwise combinations over multiple generations with comparable overall stress level, then fitness was measured during evolution and again in control benign conditions.
• The result: Lines from combined-stressor evolution had lower fitness gains and a shallower link between initial maladaptation and fitness gain, and in benign conditions they exhibited higher fitness reductions with higher cross-species consistency than the direct evolutionary response.

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