How Important Is Variation in Extrinsic Reproductive Isolation to the Process of Speciation?

Area of Science:

• Evolutionary Biology
• Speciation Research
• Population Genetics

Background:

• Reproductive isolation (RI) strength varies with ecological conditions.
• Most speciation research examines RI across broad environments, neglecting finer-scale spatial or temporal heterogeneity.
• Variation in RI may inhibit speciation (leaky barriers) or promote it (reinforcement).

Purpose of the Study:

• To investigate the impact of spatial and temporal variation in RI on speciation.
• To examine how variation in hybrid fitness across time and space affects gene flow.
• To explore the role of variable RI in the challenges of completing speciation.

Main Methods:

• Literature review of field studies on RI variation.
• Individual-based simulations to model hybrid fitness variation.
• Analysis of gene flow dynamics under static vs. variable hybrid fitness.

Main Results:

• Variation in hybrid fitness across space and time generally increases gene flow compared to static conditions.
• This increase in gene flow is linked to a convex relationship between gene flow and selection strength on hybrids.
• The effect of variable RI on gene flow is most significant when average RI is low.

Conclusions:

• Spatial and temporal variation in RI is a prevalent, under-explored factor influencing speciation.
• Variable RI, especially when weak, can increase gene flow and challenge the completion of speciation.
• Further empirical research is needed to document and understand the prevalence of extrinsic RI variation.