The Mysterious Path of Island Evolution
The Anagenetic Speciation of Endemic Plants on Ulleungdo Island
In a secluded island off the coast of Korea, a maple tree quietly spent millions of years of evolutionary time, eventually becoming a unique existence.
Off the eastern coast of South Korea, 137 kilometers from the peninsula, lies a small volcanic island - Ulleungdo. This mere 73 square kilometers of land, though small, nurtures astonishing biodiversity.
In this isolated "evolutionary laboratory," some plants follow an unusual evolutionary path, quietly transforming into unique life forms found nowhere else on Earth.
Quick Facts
73 km²
Island Area
180万年
Island Age
37
Endemic Angiosperms
Island: A Unique Evolutionary Laboratory
Oceanic islands have long been recognized as fascinating natural laboratories for the study of evolution. 1
Ulleungdo formed about 1.8 million years ago from volcanic activity 1 , with its highest peak reaching 984 meters above sea level. Since it was never connected to the mainland, this island became an independent evolutionary laboratory.
All organisms had to cross the ocean to settle here, and this strict geographical screening shaped the island's unique ecosystem.
Ulleungdo hosts about 700 species of vascular plants, of which 500 are native species, and an impressive 37 angiosperm species are endemic to this island 1 . These numbers are remarkable for such a small island.
Island ecosystems differ fundamentally from continents - geographical isolation prompts species to evolve in unique ways. On Ulleungdo, a pattern called "anagenetic speciation" has become predominant.
Two Evolutionary Paths
In island evolutionary biology, there are two main modes of speciation: cladogenesis and anagenesis.
Cladogenesis is similar to a large tree growing many branches, where an ancestral population diverges into multiple different species on an island. This phenomenon is particularly evident in Hawaiian silverswords 1 and Canary Islands Echium plants 1 .
These plants adapt to different ecological niches, forming diverse species in a relatively short time.
In contrast to cladogenesis, anagenesis refers to an ancestral population gradually evolving into a new species in an island environment without splitting into multiple species.
Stuessy et al.'s 2006 study estimated that approximately 25% of endemic plants on global oceanic islands are formed through anagenesis 1 . On Ulleungdo, this proportion reaches a remarkable 88% 1 , making it an ideal location for studying anagenetic speciation.
Comparison of Evolutionary Patterns
Diagram showing the difference between anagenesis (linear evolution) and cladogenesis (branching evolution)
Maple: The Key to Revealing Evolutionary Mysteries
On Ulleungdo, two endemic maple species - Acer takesimense (Takeshima maple) and Acer okamotoanum (Okamoto maple) - have become ideal models for scientists studying anagenetic speciation 1 .
Acer takesimense
The Takeshima maple likely originated from the mainland A. pseudosieboldianum (Korean maple) 1 .
Acer okamotoanum
The Okamoto maple originated from the mainland A. mono (painted maple) 4 .
Through morphological and molecular phylogenetic studies, scientists discovered that these two maple species are not close relatives but have independent mainland ancestors 1 .
This means they formed new species on Ulleungdo independently through anagenetic speciation at different times.
Crawford's 2010 review listed these two species pairs as good examples of progenitor-derivative species formation 1 , providing an appropriate system for studying plant speciation.
Key Experiment Reveals the Secrets
To deeply understand the genetic consequences of anagenetic speciation, scientists conducted a detailed population genetics study 1 .
Research Methods
Sample Collection
Researchers collected 130 A. takesimense individuals from 7 populations on Ulleungdo and 133 A. pseudosieboldianum individuals from 7 populations on the Korean Peninsula as a comparison 1 .
Genetic Analysis
They genotyped these individuals using 9 nuclear microsatellite markers 1 . Microsatellites are short sequence repeats in DNA with high polymorphism, making them ideal for population-level genetic studies.
Data Interpretation
By analyzing these genetic markers, scientists could assess genetic differences between and within species, understanding genetic changes during anagenetic speciation.
Data Analysis and Results
The research results revealed several key genetic characteristics of anagenetic speciation:
Clear Genetic Differentiation
Microsatellite data showed clear genetic distinction between A. takesimense and A. pseudosieboldianum 1 .
Slightly Lower Genetic Diversity
Compared to mainland A. pseudosieboldianum, A. takesimense has slightly lower genetic diversity 1 .
Weak Geographic Genetic Structure
Unlike species formed through cladogenesis, A. takesimense shows no obvious geographic genetic structure within the island 1 .
| Genetic Parameter | A. takesimense | A. pseudosieboldianum |
|---|---|---|
| Allele Richness | Slightly Lower | Higher |
| Private Allele Count | Less than half | More |
| Rare Allele Count | Less than half | More |
| Genetic Heterogeneity | Slightly Lower | Higher |
| Characteristic | Anagenesis | Cladogenesis |
|---|---|---|
| Speciation Mode | Single Linear Evolution | Multi-branch Divergence |
| Genetic Diversity | Maintained or Slightly Lower | Partitioned and Reduced |
| Population Differentiation | Weak | Strong |
| Adaptation Pattern | Mutation Accumulation, General Adaptation | Strong Adaptive Radiation |
| Niche | Relatively Uniform | Diversified |
The Parallel Story of Island Artemisia
The story of Ulleungdo's maple trees is not unique. A 2023 study revealed similar evolutionary patterns in Artemisia plants on Macaronesian islands (including Madeira, Canary, and Cape Verde islands) 5 .
A. argentea
Endemic to Madeira Island
A. thuscula
Endemic to Canary Islands
A. gorgonum
Endemic to Cape Verde
Three closely related Artemisia species - A. argentea (Madeira Island), A. thuscula (Canary Islands), and A. gorgonum (Cape Verde) - each species is endemic to a single archipelago 5 .
By analyzing plastid and nuclear DNA sequences, researchers found that these three lineages diverged during a coincidental short period in the Pleistocene 5 .
Compared to the mainland relative A. arborescens, these three species have similarly smaller genome sizes 5 , indicating they underwent similar evolutionary processes.
| Island System | Study Species | Common Genetic Characteristics |
|---|---|---|
| Ulleungdo | A. takesimense | Weak geographic structure, slightly lower genetic diversity |
| Ulleungdo | A. okamotoanum | Weak geographic structure, slightly lower genetic diversity |
| Madeira Islands | A. argentea | Similar genetic diversity, smaller genome |
| Canary Islands | A. thuscula | Similar genetic diversity, smaller genome |
| Cape Verde Islands | A. gorgonum | Similar genetic diversity, smaller genome |
This study confirms the universality of anagenetic speciation across different islands and plant groups, providing more evidence for understanding this evolutionary pattern.
Significance and Implications of Anagenetic Speciation
The study of anagenetic speciation not only has theoretical significance but also important implications for conservation biology.
Species formed through anagenetic speciation, although possibly with slightly lower genetic diversity than mainland ancestors, accumulate unique genetic and morphological characteristics, forming independent species over long periods.
These species are unique components of island ecosystems with irreplaceable evolutionary value.
Due to smaller population sizes and limited distribution ranges, these species are often more vulnerable to environmental changes and human activities.
Understanding the process of anagenetic speciation helps us develop more effective conservation strategies to preserve these unique evolutionary products.
Ulleungdo's maple trees evolved quietly in an isolated environment. Unlike Hawaii's honeycreepers or Galapagos finches that gained fame through adaptive radiation, they write the poetry of evolution in a gentler way.
While mainland ancestral species have risen and fallen with environmental changes, these island lives still maintain their unique genetic identities, becoming irreplaceable living fossils in the long river of evolution.