A population is the smallest unit of life that can evolve. This is because evolution is a process that takes place over extended periods of time through adaptations that are passed through the reproduction of individuals who contribute to the population.
Scientists do not believe that individuals evolve in their single lifetime, making it so that an individual or any of the parts that comprise it, such as a tissues or cells, are not capable of evolution. Individuals contribute to evolution through their transmission of DNA that may carry adaptations to their offspring.
As the majority of individuals in a population change in a synchronized way, evolution begins to occur. Populations then contribute to the evolution of their larger communities, followed by ecosystems and finally, the biosphere.