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. Individuals with favorable variations tend to survive, and those with less favorable variations die.
Survivors reproduce others with favorable traits.
A gene pool contains all the traits or genes in any one population that can be passed on through generations. The gene pool may change slightly over long periods of time as traits change for better survival in specific habitats, but the pool is mainly consistent to ensure the success of the population. Plants and animals with biological characteristics better suited for their environment are better represented in future generations. So through successive generations, gradual and continuous changes occur in the population. Eventually these changes result in new adaptations and species.