Predicting the timing of critical periods in development

Critical periods are specific periods in the development of a living organism during which there is an increased sensitivity to external perturbations. Such perturbations result in a developmental trajectory signifi- cantly different from what is considered the norm. This paper is concerned with the ques- tion of whether the presence and timing of a critical period can be predicted from the developmental profile without perturbation. Taking rate of change as a measure of this pro- file, we put forward the hypothesis that criti- cal periods will occur when the rate of change is greatest. Using a simplified model of differ- ential gene expression, cellular mechanics and evolution, we evolve organisms with different developmental profiles, test the presence of critical periods by systematically exposing the developing system to an external perturbation at different times between runs, and correlate the timing of these critical periods with the proposed developmental measure. We discuss the implications of our findings.