# Optic tectum size correlates positively with eye size across, but not within, populations of Asiatic toads along altitudinal gradients

## TL;DR

This study finds that in Asiatic toads, optic tectum and eye sizes decrease with altitude, but their correlation is only seen between populations, not within them.

## Contribution

The study reveals that the correlation between optic tectum and eye size is population-level, not individual-level, challenging assumptions about functional integration.

## Key findings

- Optic tectum and eye sizes decrease with increasing altitude in Asiatic toads.
- Optic tectum size correlates with eye size across populations but not within them.
- The correlation does not support functional integration or selection on their relationship.

## Abstract

Co-variation with functionally integrated sensory structures contributes to the size evolution of brain regions. In this study, we investigated co-variation between sizes of the optic tectum and eye of Asiatic toads (Bufo gargarizans) along altitudinal gradients. The relative sizes of both the optic tectum and eye decrease significantly along with increases in altitude. Furthermore, optic tectum size is positively correlated with eye size among populations (p < 0.05, R2 = 0.290), but not among individuals within each population, where natural selection occurs. The eye and optic tectum are likely subjected to parallel and independent local adaptation along altitudinal gradients that produce the correlative pattern; however, the pattern does not support functional integration and selection on their correlation. Eye size change may not have a significant impact on optic tectum evolution. Our study highlights the importance of cross-scale comparative analysis for inferring evolutionary processes.