Biological Fitness vs Physical Fitness
Fitness- Seems like an easy enough word to understand. If you want to be fit you eat right and cross train.
Ahh but that is physical fitness, and although physical fitness deals with biological organisms, it is different from, though may affect, biological fitness. IOW you don't have to be the most physically fit to be the most biologically fit, although it may help.
Biological fitness can refer to either a population's or individual's reproductive success. It is basically an after-the-fact assessment. If the population size increases it has a positive fitness. If it decreases it has a negative fitness and if it stays the same it is in equalibrium. And the individuals in the population that have the most offspring are the fittest, even if they get their ass kicked by others who have fewer offspring. They are the lovers, not the fighters.
What's the point?
Look at humans. With some exceptions it appears the "fittest" humans are the biggest losers- and I ain't talkin' 'bout no game show.
People pumping out kids just to stay on welfare- let the State support you for being so fucking fit! :)
IOW it just seems like a useless concept.