What Is Fitness?

   Technically speaking, fitness is the quantitative representation of natural selection. There are many variables that have an affect on fitness. Some of these variables include environmental factors such as resources, competition, etc. at the given time. Fitness cannot properly be measured for an entire population because it is too abstract. If you go out in the wild and try to measure a population, there would be no way to accurately measure and track all of a population. Fitness can be measured by fitness proxies instead. A fitness proxy can be measured by the quantity as well as the quality of offspring. 

    As an example, if I were to measure the fitness of a zebra in a natural environment, I would tag and track 20 moms and babies that originate from different environments in Africa. I would track the offspring as they grow up and see how many offspring they produce as one way to measure fitness. I would also compare how well they survive and if they thrive in their environment. I would also be sure to take other factors into consideration such as resources, competition, weather conditions, etc. The zebras will be deemed most fit based on the highest number of offspring produced from individuals. I think this would be fairly easy to test the fitness of a mom, but figuring out the fitness of a dad could be more difficult in my opinion. I think males would be more difficult to test fitness on because it would be hard to tell which offspring was theirs if you were not there at the time of mating.