Quantitative aspects of evolution

Evolution stands out as a unifying and powerful force that shapes living creatures and human societies alike. The way populations of cells adapt to their environments, the way the languages we speak are structured, and the way culture changes in human societies, are captured by evolutionary principles.;Here, I use the quantitative tools of Systems Biology to investigate the way evolution acts on systems as diverse as microbial populations, spoken languages and human societies.;First, I focus on microbial evolution in the context of antibiotic resistance. In response to the increasing abundance of resistant bacteria, physicians routinely use multiple antibiotics in combination. But the effect of such combination treatments on the evolution of drug resistance is not known. I develop an experimental and theoretical framework for quantifying the potential of bacterial pathogens such as Staphylococcus aureus to evolve resistance to combinations of antibiotics. These results suggest that combinations currently preferred in clinical settings for their enhanced killing efficacy might also increase the potential for evolution of resistance.;Second, I turn to the evolution of structure in spoken language. Languages are made of rules that change in time, but the dynamics of these evolutions are known only anecdotally. Here I consider the rule for past tense conjugation in English, which adds '-ed' to verbs (e.g. walk/walked). Only a few irregular verbs make exception in Modern English (drive/drove), but many verbs that were once irregular now conform to the rule. I quantify the evolutionary dynamics of this grammatical rule by tracking 277 verbs through Old, Middle and Modern English, revealing a mathematical pattern for the evolution of grammar.;Last, I ask about the evolution of culture in human civilizations. Cultural evolution has so far not been amenable to quantitative investigation. Here, I describe a corpus of 5 million digitized books, which represents about 4% of all books ever published. These books, written in English, French, Spanish, German, Russian, Hebrew and Chinese, span the last 200 years with a high temporal resolution. I quantify the evolutionary dynamics of culture through the trajectories of the words in this corpus and the ideas they express.