Phenotypic divergence among threespine stickleback that differ in nuptial coloration.

Area of Science:

• Evolutionary biology
• Speciation research
• Population genetics

Background:

• The threespine stickleback (Gasterosteus aculeatus) exhibits remarkable adaptive radiation, with marine populations repeatedly colonizing freshwater habitats.
• Independent colonization events have led to parallel morphological divergence between freshwater and marine stickleback, alongside significant within- and among-population phenotypic diversity.
• Understanding the evolutionary forces driving and maintaining such diversity is crucial for discerning mechanisms of reproductive isolation and speciation.

Purpose of the Study:

• To investigate phenotypic variation and evolutionary processes in a lesser-known threespine stickleback species pair in southwest Washington.
• To examine the role of sensory drive in the evolutionary shift from red to black nuptial coloration in male stickleback.
• To characterize a newly described 'mixed' morph and explore its potential hybrid origin.

Main Methods:

• Phenotypic variation was measured across multiple sites with distinct red and black morph distributions, and at a site of historical co-occurrence.
• Comparative analysis of non-color traits (shape, lateral plating) between red and black morphs.
• Morphological assessment of the 'mixed' morph in Connor Creek to evaluate hybridization hypotheses.

Main Results:

• Significant phenotypic divergence was observed between red and black stickleback morphs in non-color traits like body shape and lateral plate number.
• Evidence supports sensory drive as a potential mechanism driving the evolutionary color switch from red to black.
• The 'mixed' morph exhibits distinct head shape and size, consistent with hybridization between anadromous and freshwater stickleback populations.

Conclusions:

• Phenotypic divergence between red and black stickleback morphs is substantial, extending beyond coloration.
• Sensory drive offers a plausible explanation for the evolutionary shift in male nuptial coloration.
• The 'mixed' morph suggests a role for hybridization in generating novel phenotypes and potentially influencing divergence within stickleback populations.