The process of evolution by natural selection is driven by differences in fitness, or how successful an organism is at getting its genes (or characters) into the next generation. In short, fitness is all about how well an organism reproduces. Characters (or genes) that increase an individual's fitness are more likely to be passed to the next generation than genes that don't. This process is how the frequency of genes changes through time.
In the evolutionary process, fitness has nothing to do with how buff you are. It's purely a measure of the differential reproductive success among different individuals, which is a fancy way of saying that it refers to how successful an individual is at producing offspring. If one individual produces twice as many offspring as the next individual, all other things being equal, it's twice as fit.
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