Evolution in Sun and Shade
Rapid divergence in growth and development rates of CT wood frog populations

March 2008 - Current

This independent ecology project is my senior thesis research for the Intensive B.S. in Ecology and Evolutionary Biology at Yale. I'm working in the Skelly Lab under Professor David Skelly of the Yale School of Forestry and Environmental Studies. I'm broadly interested in rapid evolution of populations in response to environmental variables, and this specific study looks at the growth and development rates of Rana sylvatica (commonly known as wood frogs), from ponds of varying forest canopy cover in Yale Myers Forest.

I selected six ponds representing a broad range of canopy cover. The amount of canopy cover over a pond greatly affects the pond's temperature, and since amphibians are ectotherms (animals whose body heat is controlled by the environment) temperature should have a strong effect on their growth and development rates. My experiment is designed to look at whether these populations have evolved differences in their growth and development due to the canopy and temperature conditions in their ponds.



To isolate for genetic effects I'm performing a "common garden experiment" where I raise individuals from the different environments together under controlled lab conditions. In March, I collected eggs from each of my ponds (within 24 hours of being laid) and hatched them in the lab. I then placed a number of individuals from each population in incubators of different temperature... and proceeded to dedicate the next 4 months of my life (and counting) feeding the needy little buggers until they reach metamorphosis and become little tiny froggies. I also monitor their growth and development by regularly weighing them and noting their stage along the way to metamorphosis. As of writing this (August 3) I'm down from 384 individually housed tadpoles to only 10!

In addition to raising tadpoles in the lab, I resample the ponds every two weeks to estimate the growth and development rates of the wild populations. That way, I can see both how they grow under the natural environmental conditions and in a controlled environment, where most of the environmental variables have been removed. My hypothesis was that the populations from colder ponds (those with more canopy cover) would grow faster in the lab than those from warmer ponds (with less canopy cover), but that in the field the tadpoles from cold ponds would grow slower since subjected to the cold temperature conditions. This would be a counter-gradient trend suggesting that populations in colder environments evolve to allocate more energy toward growth and development.

At this point I'm beginning to analyze the data, and some very interesting trends are becoming apparent (though not necessarily those that I predicted)! I've got a ways to go on the analysis before I can draw any sound conclusions... but for now I can tell you that it's intriguing stuff! I'm planning work in with Dr. Gisella Caccone this fall to perform microsatellite analysis of genetic samples from my populations, which will show their degree of genetic isolation from each other. This will shed light on the time scale over which the observed evolution has occurred, as well as the amount of migration between the ponds and its effects on divergence and the genetic stability of the populations over time.

I'd also like to look at certain known functional genes that may contribute to growth and development rates of R. sylvatica tadpoles, and perform histological analysis of tissue samples from lab and field individuals to look for trade-offs associated with faster growth and development. The results are already raising more questions, and there's so much more to learn... but just not enough time to do it all!

Questions, comments or suggestions? Email me.