Fisherian runaway describes how exaggerated female selection of sexually dimorphous traits in males can lead to male health problems and sometimes destruction of the entire species.
For example, in peacocks, the males evolved to have extremely large plumage, due to persistent female preference for for large plumage. However, this preference the female peacocks have for large sex differences has caused the male peacock's plumage to become absurdly large, hindering it's mobility to evade predators. Also, requiring a lot of energy to grow and maintain said plumage.
The irish elk is known to have gone extinct from female selection giving way to fisherian runaway. The female irish elk preferred male irish elk with large horns. Eventually this preference created a feedback loop that resulted in irish elk eventually evolving to carry 3.6 meter wide horns, one meter longer than its height. Energy requirements for irish elk antler growth were 75% as large as the energy requrirements for fat and protein deposition. The enormous amount of calcium required to grow the antlers resulted the calcium being depleted from most of the body causing a state like similar to osteoporosis. The adaptive trait in irish elk became maladaptive and the species died out.
Many men in the MRA hypothesize that the increasingly dimorphic beauty standards that men are expected to have just to get a date in a sexually liberated online dating environment may be the beginning of a fisherian runaway. Only a very small percentage of men are being chosen, and it's the men with the most sexually dimorphic traits: cartoonishly large muscles, cartoonishly large frame, with no selective attention paid to things like loyalty, intelligence, etc. This also results in an even higerh competitive environment among men without phsyically sexually dimporhic traits, and men with dark triad traits getting even more to win female attention, exaggerating a trait which became maladaptive after civilization and the industrial revolution. Warren Farrell warned of such a thing in 1993, in his book, The Myth of Male Power, warning women that their preference for and encouragement of, "hunter-killer", "star quarterback", type men could cause the extinction of the species with the arrival of nuclear technology. He also said that since civilization and the industrial revolution dark triad traits have become maladaptive, as the traits which foster a healthy society have switched from might-makes-right individual brutality, to cooperation, intelligence, empathy etc...