Date Approved

8-28-2023

Graduate Degree Type

Thesis

Degree Name

Biology (M.S.)

Degree Program

Biology

First Advisor

Jennifer A. Moore

Second Advisor

David L. Clark

Third Advisor

Carlos A. Valle

Academic Year

2022/2023

Abstract

Rapid divergence in body size is a well-documented phenomenon among island species, associated with macroevolutionary processes of diversification and adaptive radiation. The effect of insularity in determining larger body size as an optimal phenotype is primarily attributed to lower predation and greater intraspecific competition on islands. Our study examined interpopulation variation in morphology, social behavior and predator avoidance in Galápagos lava lizards endemic to San Cristóbal island (Microlophus bivittatus), and the nearby islet Isla Lobos. Islet populations have fewer competitors than those on the island, where native and introduced predators are abundant. We simulated interactions with predators and conspecifics by conducting stereotyped approaches and presentations of robotic models resembling males of equal and larger size. Arithmetic and geometric analyses of morphology describe islet males as larger than those in the island, with no significant variation in female size. Territorial displays were stronger towards larger robot models by islet males, and towards same-size robot models by island males. Female displays favored larger models in both locations. Predator avoidance was less pronounced on islet lizards, evidenced by shorter escape and flight distances than those of island lizards despite similar distance to refugia. Our results suggest that predation release, supported by strong male intrasexual selection, is a key component behind the evolution of divergent body sizes in this species.

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