Theory of random sexual selection of mates. Used to explain passing on of useless or 'expensive' genes which may create liabilities. It's called 'runaway' selection because the gene traits carried on may become liabilities if carried to extremes. The genes may in fact exaggerate physical characteristics which makes the animal less able to survive, rather than more so, in normal evolutionary theory. The animals with characteristics of runaway selection are more subject to predation, less mobile, etc. This theory is used to deal with apparently bizarre, useless characteristics of species and to trace their origin more effectively than the logical natural selection approach.