Fisher’s Fundamental Theorem of Natural Selection has done a lot to convince many professional academics of the truth of Darwinian evolution.
The theorem states that the rate of increase in fitness of any organism at any time is equal to its variance in fitness at that time. In other words, fitness must go up! Because it is a mathematical equation, many are convinced that selection must be effective.
However, it turns out that Fisher’s theorem actually says very little about fitness, or even biology. While this has been pointed out before, a recent paper shows (a) the limitations of the theorem to tell us anything helpful about selection, (b) the limitations of the theorem to tell us anything about evolution, and even (c) the limitations of the theorem to tell us anything about biology.
Fisher’s theorem has been lauded as “biology’s central theorem” because it describes why biologists expect fitness to increase. However, the meaning of “fitness” in this sense is so weak as to not be helpful in any way to describe biological expectations.