Reciprocal selection causes a coevolutionary arms race between crossbills and lodgepole pine

Area of Science:

• Ecology
• Evolutionary Biology
• Zoology

Background:

• Reciprocal selection and adaptation are crucial for coevolutionary systems.
• Few wild studies demonstrate reciprocal selection and adaptation concurrently.
• Previous research suggested reciprocal adaptations between lodgepole pine and crossbills in isolated mountain ranges.

Purpose of the Study:

• To investigate reciprocal selection and adaptation between lodgepole pine (Pinus contorta spp. latifolia) and red crossbills (Loxia curvirostra complex).
• To compare selection patterns in areas with and without red squirrels (Tamiasciurus hudsonicus).
• To determine if selection patterns align with existing evidence of reciprocal adaptations.

Main Methods:

• Comparative analysis of selection patterns on lodgepole pine cones.
• Assessment of selection exerted by red crossbills and red squirrels.
• Examination of geographic patterns of cone variation in relation to species presence.

Main Results:

• Crossbill selection on lodgepole pine differed from red squirrel selection.
• Divergent selection was observed between areas with and without red squirrels, correlating with cone variation.
• Red crossbills experienced stabilizing selection on bill size in areas without red squirrels, with cone structure as the selective agent.

Conclusions:

• Lodgepole pine and red crossbills exhibit reciprocal adaptations driven by reciprocal selection.
• The presence or absence of red squirrels influences selection pressures.
• Cone structure and bird bill size are key traits mediating this coevolutionary relationship.