BODY SIZE, NATURAL SELECTION, AND SPECIATION IN STICKLEBACKS.

Area of Science:

• Evolutionary Biology
• Speciation Research
• Behavioral Ecology

Background:

• Divergent natural selection is often theorized but rarely proven as a primary driver of speciation.
• Reproductive isolation is a critical factor in the formation of new species.
• Sympatric species provide a unique opportunity to study the mechanisms of speciation in nature.

Purpose of the Study:

• To investigate the role of body size differences in premating isolation between two sympatric stickleback species.
• To determine if adaptations to alternative foraging habitats, reflected in body size, contribute to reproductive isolation.
• To understand the behavioral mechanisms underlying mate choice and reproductive isolation in sticklebacks.

Main Methods:

• Laboratory-based mating trials were conducted with two sympatric stickleback species differing in body size.
• Assortative mating patterns were quantified, and hybridization events were noted.
• The correlation between body size differences and the probability of interspecific mating was analyzed.
• Male aggression and courtship behaviors were assessed in relation to size disparities between paired individuals.

Main Results:

• Strong assortative mating was observed, but limited hybridization occurred.
• Interspecific mating probability was significantly correlated with body size differences, with mating occurring only between individuals at the extremes of the size range.
• Mating rates between similar-sized individuals of different species were comparable to within-species rates.
• Increased size differences between males and females led to heightened male aggression and reduced courtship, disrupting mating.

Conclusions:

• Body size is a dominant factor in interspecific mate preference for these sympatric sticklebacks.
• Natural selection, acting on body size adaptations to different environments, plays a crucial role in the speciation process.
• Divergent selection on traits like body size can directly lead to reproductive isolation and the formation of distinct species.