Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, UNITED STATES OF AMERICA.
Behavior determines how organisms interact with their environment, and has long been posited as a pacemaker for evolution. The classical view is that novel behaviors expose organisms to new selective pressures, in turn “driving” evolution. Behavior can also restrain evolutionary change. Some behaviors, such as thermoregulation, help organisms maintain a constant selective environment, thus “inhibiting” evolution. This thesis seeks to understand the role of behavior in influencing the evolutionary process.
In the first part, I test the hypothesis that the same behaviour can simultaneously impede and impel evolution in different traits. I focus on the lizard, Anolis cybotes, from the Caribbean island of Hispaniola. Through a replicated field experiment I show that behavioral flexibility allows these lizards to maintain a constant body temperature in markedly different thermal habitats. I determine that this similarity in body temperatures is associated with physiological stasis, as the preferred temperature and heat tolerance are nearly identical among populations. I demonstrate that the behavioral change allowing lizards to maintain a constant body temperature involves a perch switch. Finally, I demonstrate that this shift in structural habitat use from trees at low elevation to rocks at high elevation in turn impels morphological evolution in traits associated with rock use, and that these traits are likely genetically based. Thus, a perch switch to rocks at high elevation is simultaneously impeding physiological evolution, whilst impelling morphological evolution.
In the second part of my study, I asked whether rates of evolution differ among physiological traits, and how thermoregulation influences these rates. I found that cold tolerance evolves significantly faster than heat tolerance in the cybotoid anoles, a clade of anoles that contains A. cybotes and its relatives. I demonstrate that thermal variation is considerably greater during the day than at night and, at high elevation, nighttime temperatures are so cold that they would incapacitate most lizards. In the absence of thermal refuges and behavioral buffering, lizards at high elevation have no choice but to adapt their physiology. Thus, the ability to thermoregulate during, but not at night, likely influences differences in rates of evolution between heat and cold tolerance.