Experimentally Testing the Hypothesis That Hybrid Fitness Can Increase Evolutionarily: An Example in Spadefoot Toads

Area of Science:

• Evolutionary Biology
• Genetics
• Ecology

Background:

• Hybridization between species can result in offspring with reduced fitness due to genetic incompatibilities.
• Selection may act against alleles contributing to these incompatibilities within hybridizing populations.
• This process can potentially increase hybrid fitness over evolutionary timescales.

Purpose of the Study:

• To test the hypothesis that hybrid fitness increases in populations with longer histories of hybridization.
• To investigate the role of selection in overcoming genetic incompatibilities in hybridizing species.
• To examine how evolutionary time influences hybrid fitness in natural populations.

Main Methods:

• Utilized a "space for time substitution" experiment to compare hybrid fitness across regions with varying durations of sympatry.
• Studied two species of spadefoot toads, *Spea bombifrons* and *Spea multiplicata*, which have overlapping ranges.
• Assessed hybrid fitness by measuring development time and body size.

Main Results:

• Hybrids originating from regions of "Old sympatry" exhibited larger body size and faster development compared to those from "New sympatry."
• These findings support the prediction that longer periods of hybridization are associated with increased hybrid fitness.
• The results suggest that evolutionary processes can mitigate genetic incompatibilities.

Conclusions:

• Selection appears to reduce genetic incompatibilities, thereby enhancing hybrid fitness over evolutionary time.
• The study provides evidence for the adaptive evolution of hybrid lineages.
• Understanding these dynamics is crucial for comprehending speciation, adaptive evolution, and species range expansion.