Aligned selection in cold climates breaks the rules of trade-offs in adaptation

Identifying the underlying causes of species’ distributions improves our understanding of biodiversity and evolution. Many species’ distributions appear to be limited by trade-offs between their ability to compete with closely related species and their ability to tolerate environmental challenges like climate. However, we do not know how widespread these trade-offs are across taxa and environments. Here, we tested for widespread signatures of trade-offs, aligned selection, or no interaction between competitive ability and climatic tolerance across 362 pairs of closely related (same-genus) avian species. If adapting to climatic challenge compromises competitive ability, subordinate competitors should occupy climates which are more diverged from their genus’ average climate (H1). Conversely, if selection from climatic challenge and competition align on the same favoured traits, then dominant competitors should occupy more divergent climates (H2). To test these hypotheses, we used linear mixed models to assess whether relative competitive ability (dominant vs. subordinate rank) varied consistently with relative thermal divergence within pairs. We accounted for potential impacts of the direction of thermal divergence (coldward vs warmward), migratory behaviour and ranges, the age and body size of the pair, and the temperature at which species experience divergent temperatures. Overall, we found that trade-offs are widespread under some conditions, including among pairs experiencing climatic challenge in milder climates and among older, more anciently diverged pairs. However, we also found that aligned selection is widespread, especially among (1) pairs tolerating extreme cold via larger size and (2) younger, more recently diverged pairs. In these cases, widespread aligned selection appears to ‘break the rules’ of adaptation by allowing species to escape from trade-offs. Consistent patterns of widespread aligned selection should lead to reduced diversity, greater thermal tolerance, and larger geographic ranges, and thus could contribute to broad patterns of biodiversity.

Department dc:contributor.department

Biology

Year dc:date.issued

2026

Author dc:creator

• Sheppard, Brendan

Advisor dc:contributor.supervisor

• Martin, Paul

• ecology
• evolution
• trade-offs
• biodiversity
• climate
• competition

Statement dc:rights

• Attribution-NonCommercial-NoDerivatives 4.0 International

Licence dc:rights.uri

• http://creativecommons.org/licenses/by-nc-nd/4.0/

Language dc:language.iso

eng

Handle dc:identifier.uri

https://hdl.handle.net/1974/36282

OAI identifier oai:identifier

oai:queensu.scholaris.ca:1974/36282