Bipolar distributions in vascular plants: A review

Area of Science:

• Biogeography
• Evolutionary Biology
• Plant Science

Background:

• Bipolar disjunct distributions, where plant populations span high northern and southern latitudes, are observed in approximately 30 vascular plant species.
• Existing hypotheses for their origin include vicariance and convergent evolution, but these are largely unsupported by current evidence.

Purpose of the Study:

• To propose a novel framework for defining bipolar disjunctions and establish guiding principles for studying bipolar species.
• To critically evaluate hypotheses explaining the origin of bipolar plant distributions and identify the most probable mechanisms.

Main Methods:

• Review and synthesis of existing literature on bipolar plant species.
• Analysis of distribution patterns, dispersal mechanisms, and evolutionary history.
• Re-evaluation of vicariance and convergent evolution hypotheses against empirical data.

Main Results:

• Vicariance and convergent evolution are rejected as primary explanations for bipolar disjunctions, with the exception of Carex microglochin.
• Human introduction and dispersal, including direct and indirect methods facilitated by various vectors, are identified as the most plausible origins.
• Dispersal predominantly occurs in a north-to-south direction, with diversification times aligning with Pliocene and Pleistocene cooling events.

Conclusions:

• Bipolar plant disjunctions are largely attributed to dispersal events, with intrinsic factors like self-compatibility and extrinsic interactions influencing establishment.
• The genus Carex (Cyperaceae) hosts the most bipolar species, with birds identified as the most probable long-distance dispersal vectors.
• Bipolar Carex species likely dispersed via stochastic events rather than specialized traits, supported by diversification timing linked to climate change.