Why biological randomness matters

Biological events are often uncertain at the individual level. Birth, death, infection, recovery, mutation and molecular reactions do not normally occur at exactly predetermined times.

Deterministic and stochastic descriptions

A deterministic model gives one trajectory from a fixed initial state and parameter set. A stochastic model assigns probabilities to possible events and therefore produces a distribution of possible trajectories.

Why the distinction matters

Randomness can be especially important in small populations, at the beginning of an epidemic, near extinction, or when rare events have large consequences. Two populations with identical initial conditions can then have different outcomes.