Contemporary ecotypic divergence during a recent range expansion was facilitated by adaptive introgression

Area of Science:

• Evolutionary Biology
• Ecology
• Genetics

Background:

• Rapid phenotypic evolution during range expansion is common, but underlying mechanisms are less understood.
• Three-spine sticklebacks (Gasterosteus aculeatus) in Lake Geneva, introduced ~140 years ago, show rapid ecotype formation in lake and stream habitats.
• Parapatric ecotypes occupying contrasting environments present a model for studying adaptive divergence.

Purpose of the Study:

• To reconstruct the evolutionary history of phenotypic divergence in Lake Geneva sticklebacks.
• To investigate the genetic basis and timeline of adaptive differentiation between lake and stream ecotypes.
• To understand the role of introgression in promoting rapid phenotypic evolution.

Main Methods:

• Comparative analysis of museum collections and contemporary population samples.
• Genetic data analysis to infer invasion history and introgression.
• Phenotypic analysis focusing on antipredator defense traits.

Main Results:

• Parapatric phenotypic divergence between lake and stream sticklebacks was evident by the early 20th century.
• The magnitude of phenotypic differentiation increased over time, especially in antipredator traits.
• Recent increases in differentiation are linked to habitat-dependent selection on introgressed alleles.

Conclusions:

• Divergent selection between lake and stream habitats has been a stable force driving phenotypic divergence.
• Hybridization with an external lineage has accelerated the evolution of adaptive traits, particularly antipredator defenses.
• This study highlights the role of introgression in rapid phenotypic evolution and ecotype formation.