Fitness refers to the fact that organisms have varying abilities to survive and reproduce. Individuals with beneficial traits, called adaptations, are more capable of survival and reproduction in their environment.
These variations occur at the microscopic level of DNA and proteins. Differences in DNA allow organisms to produce different proteins. These proteins can then respond differently to stimuli in their environment.
In the context of this unit, some organisms are better than others at harnessing energy. Perhaps an organism has a better ability to hunt and consume energy. Maybe one organism is capable of going through aerobic respiration, while another is stuck in anaerobic respiration. Any of these differences will create selective pressure that allows some organisms to survive and reproduce more than others.
Survival and Reproduction
If an organism is more capable of survival and reproduction, then they are passing their DNA down more frequently than those with less desirable traits. This makes the next generation resemble the adaptations of the organism that is more capable of survival and reproduction. In these situations, environmental pressures can truly change the genetic makeup of a population over time.