📄 evolutionary biology

Abiotic Factors and Competitive Exclusion Drive Assembly Patterns in Two Insular Gecko Adaptive Radiations Displaying Ecomorphological Convergence

This study demonstrates that in two convergent Australasian gecko radiations, abiotic factors primarily drive phenotypic divergence in allopatry, while competitive exclusion subsequently shapes species overlap in sympatry.

Original authors: Skipwith, P. L., Castillo-Rodriguez, N., Zenil-Ferguson, R.

Published 2026-03-04

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CC BY 4.0

Original authors: Skipwith, P. L., Castillo-Rodriguez, N., Zenil-Ferguson, R.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine two islands, New Caledonia and New Zealand, sitting in the Pacific Ocean. They are like two separate, isolated "laboratories" where nature has been running an experiment for millions of years. In these labs, a specific family of lizards called geckos has exploded in diversity, evolving into dozens of different species.

This paper is a detective story about how these two groups of geckos evolved. The authors wanted to know: Did these lizards evolve because they were competing with each other, or because they were adapting to the weather and landscape?

Here is the breakdown of their findings, using some simple analogies:

1. The "Twin" Mystery (Convergent Evolution)

Even though the geckos on New Caledonia and the geckos on New Zealand are not close relatives (they are like distant cousins who arrived from different places), they look and act surprisingly similar.

The Analogy: Imagine two different chefs, one in Paris and one in Tokyo, who have never met. They both decide to make a "spicy noodle dish." Even though they use different ingredients and started with different recipes, the final dishes taste almost the same because they are both trying to solve the same problem: "How do I make a delicious, spicy meal?"
The Science: The geckos on both islands independently evolved similar shapes to fit similar jobs. Some became giant tree-climbers, some became small rock-dwellers, and some became ground-runners. They "converged" on the same solutions because the islands offered similar "jobs" (niches) to fill.

2. The "Race" vs. The "Weather" (Competition vs. Environment)

For a long time, scientists thought that when animals live together on an island, they evolve to be different from each other to avoid fighting over food (like two neighbors building different fences to keep their property lines clear). This is called competition.

However, this study found that competition wasn't the main boss.

The Analogy: Think of the geckos as hikers.
- The Competition Theory: Hikers avoid each other by taking different trails so they don't bump into one another.
- The Weather Theory (What the study found): The hikers actually choose their trails based on the weather. If it's raining, they all go to the sheltered caves. If it's sunny, they all go to the open rocks. They aren't avoiding each other; they are just all reacting to the same storm.
The Finding: The study found that abiotic factors (non-living things like climate, temperature, and elevation) were the main drivers. The geckos evolved their shapes to survive the specific weather of their island, not to avoid their neighbors.

3. The "Allopatry" Twist (Meeting Later)

The authors propose a fascinating timeline of how these communities formed:

Step 1 (The Solo Act): Geckos evolved in isolation (on different parts of the island or different islands) to fit the local climate. They became "specialists" for that specific weather.
Step 2 (The Reunion): Later, these different species moved into the same area (sympatry).
Step 3 (The Clash): Because they were already adapted to the same climate, they ended up looking very similar and trying to do the same things.
The Result: Instead of fighting to become different (which is what classic theory suggests), they ended up looking similar because the environment forced them into the same shape. The environment acts like a mold, squeezing different geckos into the same form.

4. The "Key Innovations" That Didn't Matter

The researchers wondered if special traits—like having sticky toe pads or giving birth to live babies instead of laying eggs—were the "secret sauce" that made these geckos so successful.

The Analogy: It's like asking, "Did the invention of the wheel make cars faster?"
The Finding: Surprisingly, the answer was mostly no. While these traits are cool, they weren't the main reason the geckos diversified. The main reason was simply where they lived and what the climate was like. The "wheel" (sticky toes) helped them get around, but the "road" (the climate) determined where they could go and how many of them could exist.

The Big Takeaway

This paper tells us that on these islands, nature is the architect, not the neighbors.

The geckos didn't evolve into a complex community because they were constantly fighting for space. Instead, the climate and the landscape acted like a giant filter, shaping the geckos into specific forms. When different species ended up living together, they were already shaped by the same environmental forces, leading to a community of look-alikes rather than a community of rivals.

In short: The weather wrote the script, and the geckos just acted out their parts, often ending up with the same lines even though they started from different places.

1. Problem Statement and Research Questions

The study investigates the mechanisms driving adaptive radiation (AR) and community assembly in two distinct, insular diplodactyloid gecko lineages: one in New Caledonia (NC) and one in New Zealand (NZ). Both lineages are relatively young but species-rich and exhibit remarkable ecomorphological convergence (e.g., gigantism, diurnality, viviparity, and adhesive tail scansors) despite evolving from different ancestral Australian lineages.

The authors address three primary questions:

Convergence: Is there evidence of convergent evolution between the NC and NZ lineages as they independently transitioned into similar ecological guilds (arboreal, scansorial, scansorial/rupicolous)?
Key Innovations: Do traits absent in mainland relatives (e.g., distal caudal scansors, diurnality, viviparity) act as "key innovations" driving speciation dynamics?
Drivers of Assembly: What is the relative importance of biotic factors (interspecific competition/character displacement) versus abiotic factors (environmental filtering/climate) in shaping phenotypic divergence and sympatry in these communities?

2. Methodology

The study employs a comprehensive phylogenomic and morphometric framework:

Phylogenetics: Utilized a time-calibrated phylogenomic tree of 180 diplodactyloid species based on ~4,200 Ultraconserved Elements (UCEs) from a previous study (Skipwith et al., 2019). The analysis focused on 36 NC and 18 NZ species (95% and 90% of species, respectively).
Morphometrics: Analyzed 298 voucher specimens using 15 meristic traits (body size, cranial dimensions, limb/digit proportions).
- Data were corrected for allometry using Generalized Least Squares (GLS) and Phylogenetic Least Squares (PGLS) regression against body size (SVL) or specific regional lengths.
- Dimensionality reduction was performed using both Conventional PCA (cPCA) and Phylogenetic PCA (pPCA) to generate multivariate datasets for cranial and limb traits.
Disparity and Convergence Analysis:
- Disparity Through Time (DTT): Calculated the Morphological Disparity Index (MDI) to test for "early burst" patterns of diversification.
- Adaptive Shifts: Used PhylogeneticEM (a likelihood-based method) to detect shifts in adaptive regimes (stabilizing selection) without a priori grouping.
- Convergence Testing: Employed RRphylo (phylogenetic ridge regression) to test for phenotypic similarity between guilds using the search.conv function. Convergence strength was quantified using the Wheatsheaf index ( $w$ ) via the Windex package.
Community Assembly Drivers:
- Character Displacement: Applied the TRIBE model (in the Julia package TAPESTREE) to quantify the effects of sympatry on trait evolution ( $w_x$ ), colonization ( $w_1$ ), and extirpation ( $w_0$ ).
- Causal Pathways: Used Phylogenetic Path Analysis (PPA) via the PHYLOPATH package to model causal relationships between speciation rates, trait evolution rates, climate niche evolution, and habitat.

3. Key Results

A. Disparity and Speciation Rates

No Early Burst: Neither lineage exhibited the "early burst" pattern typical of classic adaptive radiations. Disparity in cranial and limb traits was not exceptional relative to lineage age.
Body Size Exception: The New Caledonian clade showed a significant early burst in body-size disparity (MDI = -0.26, $p < 0.01$ ), but not in other morphological traits.
Speciation Rates: Speciation rates were unexceptional compared to other diplodactyloids, contradicting the expectation that high species richness correlates with rapid speciation.

B. Convergent Evolution

Strong Convergence: Despite different ancestral ecologies (NC ancestors likely scansorial; NZ ancestors likely arboreal/scansorial), both lineages independently evolved similar guilds.
Statistical Support:
- RRphylo: State-informed analyses confirmed significant convergence ( $p < 0.01$ ) between NC and NZ guilds for arboreal and scansorial forms across cranial and limb datasets.
- Wheatsheaf Index: Strong support for convergence ( $p < 0.05$ ) was found for most guilds, particularly in cranial traits and scansorial/rupicolous forms.
Adaptive Shifts: PhylogeneticEM identified multiple adaptive shifts in morphospace that corresponded with transitions in ecological guilds, particularly in the NC Correlophus-Mniarogekko-Rhacodactylus clade and the base of the NZ clade.

C. Drivers of Community Assembly (Biotic vs. Abiotic)

Lack of Character Displacement: The TRIBE model found no evidence of character displacement ( $w_x < 0$ ) in either lineage. Instead, there was evidence of phenotypic convergence in sympatry ( $w_x > 0$ ), particularly in NZ limb traits.
Competitive Exclusion & Filtering:
- Phenotypically similar taxa had lower colonization rates ( $w_1 < 0$ ), suggesting competitive exclusion prevents similar species from entering occupied niches.
- Extirpation rates ( $w_0$ ) were not significantly affected by sympatry, implying that once established, similar species can coexist or that environmental filtering is the primary barrier to entry.
Abiotic Dominance: Phylogenetic Path Analysis (PPA) revealed that climate and elevation niche evolution are the primary drivers of speciation rates.
- The best-supported model for both clades was: Climate Niche Evolution $\rightarrow$ Limb Dimension Evolution $\rightarrow$ Speciation Rate.
- This indicates that adaptation to abiotic factors (climate/topography) drives morphological change, which subsequently influences diversification, rather than interspecific competition driving divergence.

D. Key Innovations

Traits such as diurnality and viviparity (absent in mainland relatives) appear to facilitate niche expansion but were not found to be the primary drivers of speciation rates in the broader analysis.
Adhesive structures (toepads, tail scansors) did not correlate with colonization success or speciation rates, challenging the view that they are the sole key innovations for island colonization.

4. Significance and Conclusions

This study provides a robust, modern assessment of convergence in diplodactyloid geckos, offering several critical insights into macroevolutionary theory:

Refutation of Strict Competition-Driven AR: The findings challenge the "Court Jester" vs. "Red Queen" dichotomy by showing that while biotic interactions (competition) play a role in community assembly (via exclusion), they do not drive the initial phenotypic divergence. Instead, abiotic factors (climate) drive the diversification.
Mechanism of Sympatry: The study suggests a model where species diverge in allopatry driven by environmental adaptation. When they come into secondary sympatry, they may exhibit phenotypic convergence due to shared environmental constraints. Coexistence is maintained not by character displacement, but by competitive exclusion preventing phenotypically similar species from colonizing the same niche, or by niche filtering allowing only specific phenotypes to persist.
Convergence as a Deterministic Process: The repeated, independent evolution of similar ecomorphs (giant arboreal, small scansorial) in NC and NZ underscores the deterministic nature of selection in response to available niches, even when lineages have different deep-time histories.
Methodological Contribution: The study highlights the importance of combining multiple analytical approaches (TRIBE, PhylogeneticEM, PPA) to disentangle the complex interplay between abiotic filtering and biotic interactions in adaptive radiations.

In summary, the assembly of these insular gecko communities is driven primarily by abiotic environmental filtering and competitive exclusion, rather than by interspecific competition driving character displacement. The lineages converged on similar phenotypes because they faced similar selective pressures from their environments, not because they were forced apart by competitors.