Which of these is not a step in natural selection?

Natural selection is a process that drives evolution and consists of several key steps: variation, struggle for existence, and survival of the fittest. Variation refers to the differences among individuals within a population, which can result from genetic differences and mutations. This variation is crucial because it provides the raw material upon which natural selection acts.

The struggle for existence refers to the competition among individuals for limited resources, such as food, mates, and shelter. This struggle can lead to a situation where not all individuals survive to reproduce, creating a selective pressure that favors certain traits.

Survival of the fittest is a phrase often used to describe the outcome of natural selection, where individuals with advantageous traits have a better chance of surviving and reproducing compared to others. This concept emphasizes that fitness is measured by reproductive success, where "fittest" refers to those best adapted to their environment.

Underpopulation, however, is not a recognized step in the process of natural selection. Instead, natural selection operates within populations that are typically subject to competition, predation, and resource limitations. As such, underpopulation does not fit into the framework of natural selection as it implies a lack of competition and struggle amongst individuals.