Colour polymorphism torn apart by opposing positive frequency-dependent selection, yet maintained in space.

Area of Science:

• Ecology
• Evolutionary Biology
• Behavioral Ecology

Background:

• Aposematic species often exhibit warning coloration, but the maintenance of color polymorphisms is poorly understood.
• Predator learning typically favors common morphs via positive frequency-dependent survival selection, posing a challenge to polymorphism.
• Differential mating success is hypothesized to provide negative frequency-dependent selection, favoring rare morphs.

Purpose of the Study:

• To investigate if differential mating success can explain white and yellow color polymorphism in male wood tiger moths (Parasemia plantaginis).
• To determine the role of mating success in maintaining warning signal diversity in aposematic insects.

Main Methods:

• An experiment was conducted in semi-natural conditions to assess mating success of white and yellow male moths.
• Mating success was evaluated across three different morph frequencies.
• A mathematical model was developed and parameterized with experimental data and known spatial variation in survival advantages.

Main Results:

• Mating success was found to be positively frequency-dependent for both white and yellow morphs.
• Contrary to the hypothesis, rare morphs did not exhibit higher mating success.
• The model predicted that polymorphism can be maintained with approximately 20% migration between subpopulations.

Conclusions:

• Differential mating success alone does not explain the observed polymorphism in wood tiger moths.
• A combination of spatial variation in predator-driven survival and migration may maintain polymorphism.
• A selection mosaic, driven by spatial variation in selection pressures, likely prevents the fixation of a single morph.