Frequency Dependence, Which Happens When Uncommon Traits Are Extra Profitable Than Frequent Ones, Helps Clarify Sure Types of Genetic Variation
Our story begins in 1954, on the shores of Lake Tanganyika in what was then Belgian Congo, with somewhat paper about fish diets titled “A Curious Ecological ‘Area of interest’ among the many Fishes of Lake Tanganyika.”
The authors, biologists Georges Marlier and Narcisse Leleup, describe a little-studied species of cichlid fish. In line with their findings, adults subsist primarily on the scales of different fish, which they tear off their dwelling prey with fearsome tooth. Marlier and Leleup be aware that the people they held in an aquarium wouldn’t eat “earthworms, fish powder, or bugs” or the rest they introduced aside from the scales of dwell fish.
A couple of a long time later, the eminent Harvard fish biologist Karel Liem and his coauthor Donald Stewart investigated the mechanics of how these fish advanced to feed on scales, with a particular deal with their jaws and tooth. They described a brand new species with notably excessive laterality (a very robust twisting of the pinnacle towards the left aspect or the precise) and proposed that asymmetry within the form of those fish’s skulls was related to their macabre diets, probably offering a bonus in feeding. Additionally they famous that there have been comparable numbers of right- and left-twisted people.
The laboratory work supplied necessary insights, however it was a long-term area research, showing in 1993 in Science by Michio Hori, then a biologist at Wakayama Medical School in Japan, that started to clarify the persistence of each left and proper mouth laterality in Tanganyika’s scale eaters. For many of a decade, I introduced the work described on this paper each time I taught a course in evolution to undergraduate biology college students. It’s fairly actually a textbook research.
Hori discovered that his research animal, Perissodus microlepis, sometimes assaults by hanging the prey fish on the aspect of the physique, approaching from the rear to be much less seen. When he towed a prey fish behind a ship and captured wild scale eaters after they struck it, people attacking from the precise at all times had a mouth with a left twist, whereas people attacking from the left at all times had a mouth that twisted proper. He was additionally in a position to determine scales, from Perissodus stomachs, as having come from the left or proper aspect of the prey based mostly on their pore patterns. A lot as with strike observations, fish with right-twisted mouths had eaten scales from the left aspect of their prey and left-twisted scale eaters had eaten scales from the precise.
However why ought to the dimensions eater inhabitants present such distinctive variation, with the mouths of some people twisting visibly to the precise and others to the left?
Hori’s reply—frequency dependence—and the proof he collected to help it, are the explanation the paper attracted enduring consideration. Frequency dependence happens when uncommon traits or methods are extra profitable than widespread ones. It’s a precept properly illustrated by means of sport.
They’d not eat something aside from the scales of dwell fish.
When there are direct contests between people, a combatant-competitor can gain an advantage by having a technique that’s unfamiliar to opponents. For instance, left-handed fencers, who’re usually much less widespread, shall be accustomed to the assaults and defenses of right-handed opponents, as right-handers are ceaselessly encountered. However right-handers could have much less expertise of lefties—giving the uncommon left-handers a bonus. Consequently, lefties are at a better frequency amongst aggressive fencers than within the basic inhabitants. The identical goes for cricket, baseball, desk tennis, and martial arts. In these extremely interactive sports activities (and others), having the much less widespread laterality—being a leftie—is advantageous. Success is frequency dependent, negatively so to be exact: As one’s frequency goes up, success goes down and as frequency goes down, success will increase.
Hori hypothesized that scale eaters are a bit like fencers. If scale eaters at all times struck from one aspect, say, the precise, their prey would solely have to protect towards predictable assaults from a single route and will in all probability achieve this successfully. Any uncommon scale eater biting from the left ought to do notably effectively on this scenario, towards unprepared prey. In fact, as lefties grow to be extra widespread, the prey ought to shift their consideration accordingly. Thus the scenario ought to result in frequency dependence and the persistent presence of each left-and right-attacking scale eaters within the inhabitants, at one thing near 50-50.
Hori was in a position to take a look at for frequency dependence by making the most of pure ups and downs within the relative abundance of fish with right- or left-twisted mouths. About each 2.5 years, the inhabitants shifted from primarily lefty fish to primarily fish with right-twisted mouths. By scars on the edges of prey fish, Hori was in a position to estimate the relative success of the morph that was much less widespread. He discovered that, as anticipated, whichever twist kind was much less considerable left extra scars and achieved extra profitable bites, with success charges flipping when the uncommon morph’s frequency elevated and it grew to become the considerable selection.
Understanding frequency dependence is important to greedy how essentially the most speedy permutations in any vertebrate have occurred. These have usually advanced by means of coevolutionary processes, very similar to the evolutionary tango of the dimensions eaters and their prey. 
This text is tailored from Jeffrey McKinnon’s Our Ancient Lakes: A Natural History.
Lead picture courtesy of Hori, M., et al. Dynamics of laterality in Lake Tanganyike scale-eaters pushed by cross-predation. Symmetry (2019).
Get the Nautilus publication
Reducing-edge science, unraveled by the very brightest dwelling thinkers.