A species is a group of plants or animals that are genetically similar and so are able to reproduce with each other. A population is a group of plants or animals of the same species that live in the same area and have the opportunity to breed with each other. For example, there is a population of killer whales in Pacific Northwest waters and a separate population in the Antarctic. Although the animals are the same species, they are geographically isolated and therefore do not commonly breed with each other. Natural selection refers to proecesses that result in the survival and reproduction of individuals best suited for the environment in which they live.
All species tend to produce more offspring than the environment can sustain.
Not all individuals can survive. Some fish lay millions of eggs, only a few of which will live to be adult fish and reproduce. Variety is the basis for natural selection. Individuals within a population are not all exactly the same. Genetically, they vary.