On the assumption that sexual preferences are heritable, RA Fisher (1930) argued that selection for exaggerated secondary male ornamentation is coupled with selection for exaggerated female sexual preference for the ornament. This produces positive reciprocal feedback such that more exaggerated males and more choosy females will be produced in successive generations. There is runaway selection for still further exaggeration of both the ornament and the preference, until the cost of the ornament and (or) the limitations of choice outweighs there reproductive benefit. The cost of ornamentation in males is assumed to be the typical limit, as it is males that display the extremes of ornamentation in sexually di-morphic species. The classic example is the tail display of the male Indian Peafowl (Pavo cristatus) [below], which has a high 'cost' in terms of developmental, flight energy, and exposure to visual predators. Seeking an explanation for it by his theory of Natural Selection, Charles Darwin in 1860 wrote to Asa Gray, "The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick." However, Fisher presented an explanation:
"[If] we consider that the tastes of organisms … be regarded as the products of evolutionary change, governed by the relative advantage which such tastes may confer. Whenever appreciable differences exist in a species … there will be a tendency to select also those individuals of the opposite sex which most clearly discriminate the difference to be observed, and which most decidedly prefer the more advantageous type.
[I]n a species in which the preferences of … the female, have a great influence on the number of offspring left by individual males. ... [ornamental] development will proceed, so long as the disadvantage is more than counterbalanced by the advantage in sexual selection … there will also be a net advantage in favour of giving to it a more decided preference.
The two characteristics affected by such a process, namely ornamental development in the male, and sexual preference for such development in the female, must thus advance together, and … will advance with ever increasing speed. [I]t is easy to see that the speed of development will be proportional to the development already attained, which will therefore increase with time exponentially, or in a geometric progression.... Such a process must soon run against some check. Two such are obvious. If carried far enough … counter-selection in favour of less ornamented males will be encountered to balance the advantage of sexual preference; … elaboration and … female preference will be brought to a standstill, and a condition of relative stability will be attained. It will be more effective still if the disadvantage to the males of their sexual ornaments so diminishes their numbers surviving, relative to the females, as to cut at the root of the process, by demising the reproductive advantage to be conferred by female preference."
Fisher RA (1930). The Genetical Theory of Natural Selection. Oxford Univ Press; reprinted by Dover.
Peacock in Flight [By Servophbabu - Previously published: https://www.facebook.com/photo.php?fbid=549409641739213&set=a.523320851014759.131395.100000105472880&type=3&theater, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=22719490].
Peacock display & text ©2021 by Steven M Carr
