Environmental predictability drives different routes to adaptation

Climate change is altering thermal environments, yet we know little about
how environmental predictability shapes species' adaptive responses.
Different species may rely on plasticity or evolution to survive
environmental change, but how these strategies depend on environmental
predictability remains unclear. Experimental evidence that distinguishes
between plastic and evolutionary responses to different patterns of
environmental variability has been lacking. Here we present the first
experimental demonstration that compares adaptive responses to predictable
versus unpredictable thermal variation disentangling plastic from
evolutionary changes. Using Drosophila melanogaster populations evolved
for 11 generations under constant, predictably fluctuating, and randomly
fluctuating thermal regimes, we assessed survival and fecundity: a
plasticity assay testing flies directly from their evolutionary
environments to capture total phenotypic responses, and a common garden
assay after two generations of standardized rearing to isolate genetic
changes. Strikingly, environmental predictability shaped divergent
life-history strategies that were only revealed by comparing our two
assays. Populations from predictably fluctuating environments evolved
enhanced survival over generations, but this benefit was only visible in
the common garden assay, not when tested directly from their evolutionary
environment. Conversely, populations from randomly fluctuating
environments showed reduced survival in the plasticity assay and
consistently lower fecundity in the common garden assay, though this
reproductive cost was completely masked in the plasticity assay. These
contrasting responses demonstrate that environmental predictability
fundamentally determines life-history evolution: predictable variation
favors investment in stress-resistant longevity, while unpredictable
variation imposes both immediate survival costs and constitutive
reproductive constraints. Our findings challenge the traditional view that
environmental variation uniformly selects for increased plasticity,
instead revealing that the predictability of environmental change
determines both the target and mechanism of adaptation. As climate change
increases environmental variability and reduces environmental
predictability, these insights provide crucial guidance for predicting
species persistence and developing effective conservation strategies.