At the core of this potential future shift in biological sciences is the recognition that all biological systems are what have come to be known as complex adaptive systems, in which macroscopic patterns reflect the collective dynamics of individual units at lower levels of organization and feed back to affect those more microscopic dynamics. Evolutionary changes operate on multiple levels and multiple scales: from cells, to organisms, to populations, to communities and the biosphere. As my Princeton colleague, Philip Anderson, wrote years ago, “more is different.” Although the details at lower levels govern the behavior at higher levels, understanding those details is not sufficient for understanding how macroscopic patterns emerge or how natural selection operates at lower levels to lead to those patterns. Where those patterns refer to properties of the organism, natural selection operates to modify the details, such as the rules that govern organismal development due to feedbacks from fitness differences among organisms. On the other hand, where those properties refer to those of the biosphere, there is no comparable process of natural selection choosing among competing biospheres. What properties arise are hence largely emergent, reflecting selective events at much lower levels of organization. This is the principal reason that our biosphere is in trouble. It also emphasizes the importance of understanding at what levels selection operates most strongly.The questions that biologists from diverse subdisciplines are asking have commonalities that make clear the continued existence of fundamental challenges that unify biology and that should form the core of much research in the decades to come. Some of these questions are as follows: What features convey robustness to systems? How different should we expect the robustness of different systems to be, depending on whether selection is operating primarily on the whole system or on its parts? How does robustness trade off against adaptability? How does natural selection deal with environmental noise and the consequent uncertainty at diverse scales? When does synchrony emerge, and what are its implications for robustness? When and how does cooperative behavior emerge, and can we derive lessons from evolutionary history to foster cooperation in a global commons?
These are among what we identify as fundamental questions in biology, cutting across subdisciplines and with the potential to reunify the subject. To encourage recognition of these challenges, PLoS Biology is publishing a series of brief discussion papers raising core issues and designed to be provocative (the first in the series is published today [DOI: 10.1371/journal.pbio.0040299]). Contributions to the Challenges Series are encouraged; ideas should be sent to biology_editors@plosbiology.org.