When an individual within a population reproduces, he/she is contributing his/her genes to the gene pool. In the process of natural selection, sometimes a trait is preferred to the other choices available, something that helps that particular organism better survive within the community. I was reviewing students for a section of genetics last year when I came across an example that talked about a population of mice that had white fur color and brown fur color. It was noticed a year later the white mice had virtually disappeared, while the brown mice were in abundance. Obviously, the brown fur predisposed the brown mice for better survival because they were able to blend in with their surroundings and avoid their predators. The white mice, however, stood out like sore thumbs, and were obvious targets at lunchtime. Such an advantage effects a change within the community.
Whenever an individual is deemed to be "fit" and survives within its environment, it also will reproduce. Whatever adaptations it had to help it to survive may possibly be passed on to the next generation. Genetically, it may have a favorable combination of traits that allow it to survive. It may even have a mutation that helped it to deal with an environment that is not static, but ever changing. An example of this is the peppered moth. During the Industrial Revolution in England, almost every moth was light colored with a very small number of dark individuals. As factories released pollutants into the air, some soot ended up coating the trees in the forests. Darker moths that blended into the background avoided predation, survived and reproduced, while their lighter counterparts were eaten. Over a short period of time, the dark moths outnumbered the light. Whenever someone reproduces and contributes its genes to the gene pool, it may help to change the frequency of certain genes in the future.