What Makes Something Fit (and why it's not just going to the gym)

     Usually, when we think of something or someone being "fit" we imagine a something like that one beefy guy who works out all the time and wants to show you his abs or that keto mom who does cross fit and only eats vegan (she promises). In a biological concept, though, fitness is a lot more broad and harder to pin down. Generally, an organisms fitness can be boiled down to it's reproductive capabilities and contributions to the gene pool within it's population. Measuring that fitness can be a lot more of a hassle to think about. 

    In most scientific literature you will generally hear the measurement of fitness broken down in two ways: absolute and relative. Absolute fitness is essentially how many surviving offspring the individual produced, more specifically measuring gene copies passed down. Relative fitness is a standardized measure of that: where you reference the population's reproductive rate to the individual's. Seems simple enough, right? If the individual can make lots of babies and lots of them survive and have the genes to make subsequent generations better, then you could describe that individual as "fit." There's not necessarily a problem, in my opinion, with the current way to measure and organisms fitness. In this context you can also conclude that an animal's fitness is directly responsible for natural selection:

    Without fitness coming into play, natural selection could not happen and therefore there would be no evolution.

     However, if you put it in the context of evolution and natural selection it becomes a bit more sticky. 

    Let's use, for example, Darwin's finches. Pre-drought the finches were recorded to have relatively small beak sizes. During the drought, beak size raised drastically and, because of what we know today about birds and finches, we can make the reasonable assumption that these beak sizes were a genetic trait being passed on from parent to offspring. After the drought, beak size began to decrease again, though not to the point that it was pre-drought. 

Let's break it down.

    Before the drought the birds who were reproducing with the most surviving offspring were the finches who had smaller beaks. Those birds could be considered "fit" within the population. This does not mean there were no birds with larger beaks, just that it was more advantageous to have a smaller beak.

    During the drought, those birds were no longer considered "fit" in their given environment. They were still capable of reproducing and creating viable offspring, but now, the most "fit" were the birds who had larger beaks. 

    Once the rains returned this was no longer the case. Slowly but surely smaller beaks once again began to be the norm, with those who had smaller beaks creating more offspring and passing on more genes into the gene pool. Once again, those birds could be considered more "fit."

    At no time was there any sort of permanent change in the gene pool or (major*) evolutionary process that took place *(the argument can be made that since the beaks were larger than average post- compared to pre- drought there was some evolution). At no point did the smaller beaked or larger beaked birds find it impossible to reproduce or create viable off spring or just die off completely. The dominant phenotype appeared to ebb and flow as much as the weather. 

What the heck am I trying to say?

    Fitness and the idea of it, especially when placed on a long-term, larger scale, should be seen with the bigger picture in mind. It is not a "yes" or "no" concept, rather something that is a product of the environment the individual is in. What if the finches were in a lab setting? What if we moved them somewhere else? Would the outcome have changed? Unless you are looking at something as simple as "Horse plus donkey make mule and mule don't make baby the end," then just keep an open mind and look at all the possibilities. 

Links:

https://www.biologyonline.com/dictionary/fitness

https://www.biologyonline.com/dictionary/relative-fitness

https://www.biologyonline.com/dictionary/absolute-fitness