There are deep, yet largely unexplored, connections between computer science and biology. Both disciplines examine how information proliferates in time and space. Central results in computer science describe the complexity of algorithms that solve certain classes of problems. An algorithm is deemed efficient if it can solve a problem in polynomial time, which means the running time of the algorithm is a polynomial function of the length of the input. There are classes of harder problems for which the fastest possible algorithm requires exponential time. Another criterion is the space requirement of the algorithm. There is a crucial distinction between algorithms that can find a solution, verify a solution, or list several distinct solutions in given time and space. The complexity hierarchy that is generated in this way is the foundation of theoretical computer science. Precise complexity results can be notoriously difficult. The famous question whether polynomial time equals nondeterministic polynomial time (i.e., P = NP) is one of the hardest open problems in computer science and all of mathematics. Here, we consider simple processes of ecological and evolutionary spatial dynamics. The basic question is: What is the probability that a new invader (or a new mutant) will take over a resident population? We derive precise complexity results for a variety of scenarios. We therefore show that some fundamental questions in this area cannot be answered by simple equations (assuming that P is not equal to NP).evolutionary games | fixation probability | complexity classes E volution occurs in populations of reproducing individuals. Mutation generates distinct types. Selection favors some types over others. The mathematical formalism of evolution describes how populations change in their genetic (or phenotypic) composition over time. Deterministic models of evolution are based on differential equations. They assume infinitely large population size and ignore demographic and other stochasticity. The more precise descriptions of evolutionary dynamics, however, use stochastic processes, which take into account the intrinsic randomness of when and where individuals reproduce and how many of their offspring survive. They also describe populations of finite size.A well-known stochastic process of evolution was formulated by Moran in 1958 (1). In any one-time step, a random individual is chosen proportional to fitness for reproduction and a random individual is chosen for death. The offspring of the first individual is added to the population. The total population size remains constant and is given by N. The original process was formulated for constant fitness, which means the fitness value of individuals does not depend on the relative abundance of various types in the population; it is a fixed number. The crucial question is: What is the probability that a newly introduced mutant will generate a lineage that takes over the entire population? This quantity is called the fixation probability. For the original Moran process, the...