Population bottleneck triggering millennial-scale morphospace shifts in endemic thermal-spring melanopsids

Area of Science:

• Evolutionary biology
• Paleontology
• Malacology

Background:

• Lake Pețea, Romania, is known for its endemic fauna and flora, particularly the gastropod Microcolpia parreyssii.
• Subfossil records show significant morphological diversity in M. parreyssii during the early Holocene, leading to numerous taxonomic classifications.
• Currently, only a single phenotype of M. parreyssii exists in the lake.

Purpose of the Study:

• To critically review the taxonomic interpretations of M. parreyssii's morphological variability.
• To investigate the evolutionary mechanisms, including speciation, behind the observed changes in M. parreyssii.
• To analyze new morphometric and isotopic data in conjunction with existing information.

Main Methods:

• Morphometric analysis of shell outlines from subfossil and recent M. parreyssii specimens.
• Oxygen and carbon isotope analysis of shell material.
• Synthesis of new data with existing paleontological and taxonomic information.

Main Results:

• A significant shift in M. parreyssii morphology occurred during a period of rapid peaty deposit accumulation.
• This environmental change created a small, eutrophic swamp, leading to a population bottleneck and reduced population size.
• The data suggest genetic drift, not natural selection, was the primary driver of morphological change following the bottleneck.

Conclusions:

• The apparent speciation event in M. parreyssii was likely driven by genetic drift after an environmental bottleneck.
• Environmental changes, specifically the formation of a eutrophic swamp, played a crucial role in reducing population size and subsequent genetic diversity.
• Careful objective evaluation of morphological variability is essential for accurate interpretations of evolutionary mechanisms.