What Is Dan Janson's Hypothesis?

Why are tropical mountain passes ‘higher’ than temperate ones? Why do some organisms regulate their temperature better than others, and what effect does this have on evolution?

On this episode, we talk with Martha Muñoz, a professor in Yale’s Department of Ecology and Evolutionary Biology. We frame the chat in terms of two big ideas in thermal physiology – Janzen’s hypothesis and the Bogert Effect. Dan Janzen famously predicted that tropical ectotherms, with their relatively narrow thermal performance curves, would have a harder time moving up and over cold mountain passes. For organisms that rely on the environment to regulate their body temperatures, mountain passes appear higher in the tropics. These effectively higher mountains should lead to greater isolation of local populations and potentially greater rates of speciation because dispersal becomes more difficult. Charles Bogert focused on interactions between behavior and thermal evolution in ectotherms. He suggested that species with better capacity for behavioral thermoregulation would evolve more slowly – because thermoregulation shields thermal traits from the brunt of strong selection. We talk with Martha about these two ideas, and about how her work on Anolis lizards illustrates them.

Cover art: Keating Shahmehri