[Paper Review] Diversity patterns and speciation processes in a two-island system with continuous migration

Débora Princepe, Simone Czarnobai+5 more|arXiv (Cornell University)|Feb 23, 2022

Genetic diversity and population structure68 references15 citations

TL;DR

This study investigates how continuous migration influences speciation and diversity in a two-island model with finite genomes, revealing that intermediate migration rates maximize species richness by enabling both founder-driven speciation and sympatric speciation. Key results show that small genomes favor speciation at low migration, while large genomes sustain diversity under high migration, with optimal migration intensity balancing speciation modes and species richness.

ABSTRACT

Geographic isolation is a central mechanism of speciation, but perfect isolation of populations is rare. Although speciation can be hindered if gene flow is large, intermediate levels of migration can enhance speciation by introducing genetic novelty in the semi-isolated populations or founding small communities of migrants. Here we consider a two island neutral model of speciation with continuous migration and study diversity patterns as a function of the migration probability, population size, and number of genes involved in reproductive isolation (dubbed as genome size). For small genomes, low levels of migration induce speciation on the islands that otherwise would not occur. Diversity, however, drops sharply to a single species inhabiting both islands as the migration probability increases. For large genomes, sympatric speciation occurs even when the islands are strictly isolated. Then species richness per island increases with the probability of migration, but the total number of species decreases as they become cosmopolitan. For each genome size, there is an optimal migration intensity for each population size that maximizes the number of species. We discuss the observed modes of speciation induced by migration and how they increase species richness in the insular system while promoting asymmetry between the islands and hindering endemism.

Motivation & Objective

• To understand how continuous migration affects speciation and species richness in a two-island system with finite genome sizes.
• To investigate the interplay between population size, genome length, and migration intensity in shaping diversity patterns.
• To identify the mechanisms—specifically founder events and sympatric speciation—by which migration promotes diversification.
• To analyze the emergence of asymmetry in species distribution and the decline of endemism under varying migration regimes.
• To explore the conditions under which ring-like species and cosmopolitan species form due to gene flow.

Proposed method

• Uses a stochastic, individual-based, neutral model with non-overlapping generations and continuous migration between two identical islands.
• Each individual is represented by a binary string of B biallelic loci (genome size), with reproduction based on genetic similarity and local mating.
• Employs a fixed population size M per island, with migration occurring at probability ϵ per generation, allowing for fluctuating population sizes during migration.
• Simulates evolutionary dynamics over time, tracking species formation through genetic divergence and reproductive isolation.
• Analyzes species richness, endemism, cosmopolitanism, and asymmetry in species distribution across islands under varying migration rates and genome sizes.
• Conducts simulations across multiple parameter combinations (migration probability, population size, genome size) and validates results with statistical analysis and supplementary figures.

Experimental results

Research questions

• RQ1 How does migration intensity affect the number of species formed in a two-island system with finite genomes?
• RQ2 What role does genome size play in enabling sympatric speciation and maintaining diversity under gene flow?
• RQ3 How do founder events and sympatric speciation contribute differently to speciation dynamics under varying migration rates?
• RQ4 Why does species richness peak at intermediate migration levels, and what mechanisms underlie this optimization?
• RQ5 How does migration influence asymmetry in species distribution between islands and the loss of endemism?

Key findings

• For small genomes, low migration induces speciation on isolated islands, but increasing migration reduces diversity to a single cosmopolitan species across both islands.
• For large genomes, sympatric speciation occurs even under strict isolation, and species richness per island increases with migration, though total species count decreases due to cosmopolitanism.
• An optimal migration intensity exists for each population size and genome size that maximizes total species richness, indicating a trade-off between speciation modes and gene flow.
• Founder-driven speciation dominates at low migration, leading to asymmetric species distributions and increased endemism, while sympatric speciation under moderate migration increases shared species and total richness.
• Ring-like species formations emerge primarily under high migration and large genomes, indicating active gene flow connecting populations, though these are transient and less durable.
• Species richness is sustained under high migration only with large genomes, as small genomes lack sufficient genetic variability to maintain diversity when gene flow is high.