How Lake Size Shapes Evolution
The Story of Postglacial Diversification in Arctic Charr and Beyond
An Evolutionary Mystery in Icy Waters
Imagine a scientist peering into the crystal-clear waters of a Greenland lake, expecting to find a single type of fish, but instead discovering six completely distinct forms—each with unique body shapes, feeding habits, and lifestyles—all descended from a common ancestor in the relatively recent past. This isn't science fiction; it's exactly what researchers found in Lake Tasersuaq, home to the most diverse Arctic charr assemblage known to science 5 .
Postglacial Environments
Following the retreat of continental glaciers roughly 12,000 years ago, vast landscapes became available for colonization by freshwater organisms.
Ecological Opportunity
These newly formed lakes and rivers presented unprecedented ecological opportunities for the few fish species that managed to colonize them.
The study of these natural laboratories has revealed that ecosystem size serves as a powerful predictor of eco-morphological variability—the differences in body shape and structure that enable species to exploit various ecological niches. From the Arctic charr of Greenland to the whitefish of Fennoscandia, the same pattern repeats itself: larger lakes containing greater ecological opportunity consistently foster more extensive diversification than smaller water bodies 5 6 .
Key Concepts: Ecological Opportunity and Adaptive Radiation
Ecological Opportunity
Refers to the availability of underutilized resources and niches that provide organisms with the chance to evolve new adaptations 5 .
Adaptive Radiation
The rapid evolution of multiple ecologically differentiated species from a single common ancestor 6 .
Ecomorphs
Ecologically and morphologically distinct forms within a species adapted to specific niches 6 .
Terminology in Postglacial Diversification
| Term | Definition | Example from Research |
|---|---|---|
| Ecomorph | A distinct form within a species adapted to a specific ecological niche | Littoral benthic, profundal dwarf, pelagic planktivore |
| Gill rakers | Bony projections along the gill arch that aid in feeding | High number for plankton feeders, low number for benthic feeders |
| Ecological opportunity | Availability of underutilized resources and niches | Untapped profundal zone in newly formed lakes |
| Adaptive radiation | Rapid evolution of ecological diversity from a common ancestor | Six Arctic charr species in Lake Tasersuaq |
| Phenotypic plasticity | Ability of one genotype to produce different phenotypes in different environments | Brown trout developing different head shapes based on diet 3 |
Phenotypic plasticity—the ability of a single genotype to produce different phenotypes in response to environmental conditions—plays a crucial role in the initial stages of diversification 3 . This flexibility allows populations to rapidly adjust to new ecological opportunities without requiring genetic changes.
A Natural Experiment: Arctic Charr in Greenland's Lakes
The Eqaluit River drainage in southern Greenland provides an ideal natural laboratory for testing the relationship between ecosystem size and diversification. This remote catchment contains a series of lakes of varying sizes that were colonized by Arctic charr after the retreat of the Greenland ice sheet approximately 10,000 years ago 5 .
Lake Size vs. Charr Diversity
Ecomorph Distribution in Tasersuaq
Lake Size and Charr Diversity in the Eqaluit Drainage
| Lake Name | Surface Area (km²) | Number of Charr Ecomorphs | Number of Genetically Distinct Species |
|---|---|---|---|
| Timerliit Lake 1 | 0.09 | 1 | 1 |
| Small lakes | 0.1-0.5 | 1 | 1 |
| Intermediate lakes | 0.6-2.5 | 2 | 2 |
| Tasersuaq | 8.93 | 5 | 6 |
Arctic Charr Ecomorphs in Lake Tasersuaq
Anadromous charr
Migrate to the sea and return to freshwater to spawn
Littoral benthics
Feeding on invertebrates along the shore
Profundal dwarfs
Specialized for deep-water habitats
Planktivores
Consuming zooplankton in open water
Piscivores
Preying on other fish (with two genetically distinct species)
The correlation between lake size and diversity was not merely numerical. The researchers discovered that larger lakes supported more extreme forms of ecological specialization, particularly toward profundal, pelagic, and piscivorous lifestyles 5 . These niches require the most specific adaptations and appear only when ecosystems reach a certain size and complexity.
The Scientist's Toolkit: Methods for Studying Diversification
Unraveling the mysteries of postglacial diversification requires an interdisciplinary approach and specialized techniques.
Gill Net Sampling
Capturing fish from different depth zones to sample littoral, pelagic, and profundal fish communities
Geometric Morphometrics
Quantifying body and head shape differences to identify ecomorphological adaptations
Microsatellite Markers
Assessing genetic differentiation between populations to determine distinct species
Stable Isotope Analysis
Tracing dietary sources and trophic position to confirm niche partitioning
RNA Sequencing
Measuring gene expression differences to identify molecular mechanisms
Genetic Analysis
Reconstructing phylogenetic relationships and estimating divergence times
Gill Raker Adaptations
Adapted for filtering small prey from water
Adapted for processing larger food items
Parallel Patterns: Evidence From Other Systems
The pattern observed in Greenland's Arctic charr is not unique. Similar relationships between ecosystem size and diversification have been documented across numerous fish taxa and geographic regions, strengthening the case for a general evolutionary principle.
European Whitefish
In northern Fennoscandia, European whitefish have repeatedly diverged into distinct ecomorphs specializing on littoral, pelagic, and profundal niches 6 .
Dark-eyed Junco
Research on the songbird genus Junco revealed speciation within the last 10,000 years following northward expansion 1 .
Brown Trout
A 2024 study found that resource instability undermined predictable plasticity in head and jaw morphology 3 .
Comparative Diversification Across Species
| Species/Group | Region | Diversification Pattern | Timeframe |
|---|---|---|---|
| Arctic charr | Greenland | Up to 6 species in largest lakes | ~10,000 years |
| European whitefish | Fennoscandia | Multiple ecomorphs in larger lakes | ~12,000 years |
| Dark-eyed junco | North America | At least 5 distinct morphotypes | ~10,000 years |
| Three-spined stickleback | Multiple regions | Consistent benthic/limnetic pairs | ~12,000 years |
The remarkable consistency in adaptive trajectories across different systems highlights the predictable nature of evolution when faced with similar ecological opportunities. As one research team noted, "The replicated ecological and morphological post-glacial diversification of fishes into distinct trophic specialists in freshwater lakes is a powerful natural experiment for testing evolutionary predictability" .
Conclusion: Conservation in a Changing World
The study of postglacial diversification offers more than just insights into evolutionary processes—it provides crucial lessons for conservation in an era of rapid environmental change. These naturally replicated evolutionary experiments demonstrate how biodiversity develops and responds to new ecological opportunities.
Conservation Implications
As climate change alters freshwater ecosystems worldwide, understanding the relationship between ecosystem size and diversity becomes increasingly important. Larger, more complex lakes may serve as biodiversity reservoirs and potential sources for recolonization following environmental disturbances.
Research Limitations
A 2024 study on brown trout found that resource instability undermined predictable plasticity in head and jaw morphology 3 . When prey resources alternated even sub-seasonally, the association between diet type and phenotype was overwhelmed.
Future Research Directions
Future research will likely focus on identifying the specific genetic mechanisms underlying eco-morphological adaptations and determining how environmental factors beyond mere size—such as productivity, complexity, and stability—influence diversification trajectories.
What remains clear is that the legacy of the glaciers continues to shape life on our planet. The empty landscapes left behind by retreating ice sheets became laboratories of evolution, where ecosystem size emerged as a powerful predictor of biological diversity. As we confront unprecedented environmental changes, understanding these natural evolutionary experiments may prove crucial for protecting the diversity they have generated.