Plant speciation in the face of recurrent climate changes in the Alps

Christian Parisod

doi:10.1007/s00035-021-00259-6

Abstract

The main, continuous mountain range of the European Alpine System (i.e., the Alps) hosts a diversified pool of species whose evolution has long been investigated. The legacy of past climate changes on the distribution of high-elevation plants as well as taxa differentially adapted to the mosaic of edaphic conditions (i.e., surmised ecotypes on calcareous, siliceous, serpentine bedrocks) and the origin of new species are here discussed based on available evidence from endemic taxa across the Alps. The integration of main spatial and ecological patterns within and among species supports speciation driven by spatial isolation in main glacial refugia where plant populations survived during cold phases and hindered by intense gene flow along main expansion pathways during warm phases. Despite patterns of genetic differentiation matching environmental heterogeneity, processes underlying the dynamics of distribution ranges likely promoted recurrent homogenization of incipient divergence and generally hindered the completion of speciation (except for cases of hybrid speciation). Even intense selective pressures on toxic bedrocks such as serpentine seemingly fail to support the completion of speciation. Accordingly, typical scenarios of ecological speciation whereby local adaptation to environmental heterogeneity initiates and supports long-term reduction of gene flow may rarely be at the origin of stable species in the Alps. Although consistent with neutral processes whereby spatial isolation driven by past climate changes promoted reproductive isolation and yielded limited diversification, mechanisms at the origin of new species across heterogeneous landscapes of the Alps remain insufficiently known. Necessary advances to reliably understand the evolution of biodiversity in the Alps and identify possible museums or cradles of variation in face of climate changes are discussed.

Highlights

The Alps represents a coherent biogeographic unit, harboring a regional pool of high-elevation plant species that have evolved in situ and, through exchanges with other mountain ranges of the Mediterranean area, Europe and Eurasia (Ozenda 2009)
I here interpret the distribution of these endemic taxa (i.e., 12.6% of the native flora of the Alps) in the light of patterns left by climate-driven cycles of range shifts to discuss main processes underlying speciation across ecologically heterogeneous landscapes in the Alps
Based on current knowledge of main refugia having supported the survival and isolation of plant populations during the last glacial maximum (LGM) as well as main post-glacial expansion pathways having promoted secondary contact and gene flow among previously isolated populations (Fig. 1), I discuss the legacy of past climate changes on the distribution of variation in high-elevation endemic plants as well as taxa differentially adapted to the mosaic of edaphic conditions

Summary

Introduction

The Alps (i.e., the main range of the European Alpine System) represents a coherent biogeographic unit, harboring a regional pool of high-elevation plant species that have evolved in situ and, through exchanges with other mountain ranges of the Mediterranean area, Europe and Eurasia (Ozenda 2009). Repeated cycles of isolation in glacial refugia and secondary contact following centripetal expansion likely promoted a transverse distribution of endemic species, whereby divergent taxa had the opportunity to accumulate at the extremities of the mountain range in refugia away from the main ice domes and main recolonization pathways; whereas, widespread expanding species underwent genetic homogenization in the Central Alps (Wallis et al 2016) Such a scenario likely explains the origin and persistence of the 28.1% and 40.3% of the strict endemic taxa that grow in the alpine belt of the South-western and the Eastern Alps, respectively (Fig. 1). Gene flow among divergent taxa does not necessarily lead to homogenization and, in cases where

Southern Alps

Edaphically specialized species in the Alps

Findings

Museums and cradles of biodiversity across the Alps