In partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Quantitative Biosciences

in the

School of Biological Sciences

Christopher Zhang

Will defend his thesis

“THE COSTS AND CONSTRAINTS OF BIOLOGICAL COMPLEXITY”

April 22nd, 2026

2:00 PM

DALNEY 180

https://gatech.zoom.us/j/98792482360?pwd=DLP1U5pLjGt6ssC3iJWXqxL24ylm5f.1

Meeting ID: 987 9248 2360

Passcode: Crisis

Thesis Advisors:

Dr. Brian K. Hammer

School of Biological Sciences

Georgia Institute of Technology

Dr. William C. Ratcliff

School of Biological Sciences

Georgia Institute of Technology

Committee Members:

Dr. Samuel P. Brown

School of Biological Sciences

Georgia Institute of Technology

 Dr. Peter J. Yunker

School of Physics

Georgia Institute of Technology

 Dr. Lauren Speare

School of Biological Sciences

Georgia Institute of Technology

Abstract:

The costs and constraints of biological complexity are central to understanding how sophisticated traits and communities evolve and persist. In this dissertation defense, I will present two stories involving the Type Six Secretion System (T6SS), a complex bacterial apparatus that allows a bacterium to kill an adjacent competitor with remarkable efficiency. T6SSs have been shown to strongly impact microbial population dynamics, and there is an ongoing effort to understand exactly how this occurs. 

First, I will describe my efforts to precisely measure the metabolic cost of utilizing the T6SS, showing that this cost is significantly lower than prior expectations. Then, I will present a combined effort to scan the entire bacterial phylogenetic tree to identify which and how many bacteria actually possess the T6SS, revising a widely cited estimate that has stood for nearly two decades. Together, these two studies refine our understanding of the T6SS as a whole and reshape how we think about the role of bacterial weaponry in shaping microbial dynamics over evolutionary timescales.