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Evolution in Action

Is it better to be a sheep or a maverick?

In everyday life, it is often easiest to follow the crowd and just ‘fit in’; but bucking the trend can offer its own benefits. In evolution, similar rules apply. When the fitness, or evolutionary value, of a given trait or characteristic depends on how common it is in a population, it is called frequency-dependent selection. If being common is best, it is called positive frequency-dependent selection. When it is better to be rare, it is called negative frequency-dependent selection. Interestingly, these types of selection can both eliminate or maintain diversity in a population; and examples of both can be found in everyday life and nature.

// Positive frequency-dependent selection – going with the flow

motorway+at+night+cropped

Going with the flow: traffic all driving on the left in the UK at night.

Sometimes going with the flow is best. In the UK, driving on the left is the law. If you insist on driving on the wrong side of the road – that is to say the right side – you are unlikely to continue driving for long (either a crash or a ban for dangerous driving will see to that). This is not because driving on the left is, in itself, better than driving on the right, but because here it is safer to stick to the established convention.

In nature: striking patterns of red, yellow, black, and white are classic warning signals. These colours are used over and over: by wasps and butterflies, to snakes and frogs. These conspicuous colours often warn that the organism is protected by a foul taste or toxin. By all copying the same signals, these creatures benefit from being avoided by potential predators that have already learnt the meaning of these adverts.

warning colours

Clear and present danger: striking black and yellow warning patterns of wasps, butterflies, snakes and frogs – as well as in our own warning signs. (Images from varied sources via Wikimedia Commons)

// Negative frequency-dependent selection – daring to be different

So, sometimes just being the same as everyone else can offer its own benefits. On the other hand (quite literally), occasionally it is better to dare to be different. Left-handedness is more rare than right-handedness; only about one in ten adults (including yours truly) are left-handed. Take it from me, being a ‘cack-handed’ lefty is awkward at times, but being less common, it could have some advantages. Consider two people in hand-to-hand combat. All other things being equal, theory suggests that a lefty has an advantage when facing a righty in a fight – just from being rare. Since most people are right-handed, most opponents a person will face will also be right-handed. This is true for everyone, and means that the lefty is more likely to have fought a righty before, than vice versa. As such, lefties are unknown quantities and could surprise opponents with unexpected moves – helping them to win. Although perhaps not the complete picture, a study from 2005 showed that the prevalence of left-handedness varies around the world (from 3 – 27%), and was most common in the most violent of societies. Of course, correlation doesn’t equal causation, but these findings were at least consistent with lefties getting a fitness boost in populations where fighting is commonplace.

Fighting robots

Hand-to-hand combat: in fights, and sports such as tennis or boxing, being a lefty could give you an advantage simply because being left-handed is less common than being right-handed.

Choosing lottery numbers is another situation where avoiding what others are doing will stack the odds in your favour. Of course there is no system for predicting the winning numbers (and I’d definitely tell you if there was!). However, by choosing unpopular numbers – such as those greater than 31 (because of the so-called birthday bias) – it means that, IF you do win, you stand a better chance of not sharing the jackpot. However, if more and more people switch to choosing unpopular numbers (making them popular instead) this benefit will be lost.

In nature, some little fish with crooked smiles provide the textbook example of when doing something unexpected is favoured. As the name of the scale-eating cichlid fish (Perissodus microlepis), from Lake Tanganyika in Africa, might suggest – it feeds on the scales from other ‘victim’ fish. A study back in 1993 revealed that these cichlid fish have asymmetric mouths that turn to either the left or the right. This allows them to snatch the scales from the sides of their victims. The ‘left’ mouth type prefers to feed on the scales of the right side, whilst the ‘right’ mouth type bites the scales of the left side. When there are lots of ‘left-mouthed’ cichlids, the other fish become wary of attack on their right sides, making it easier for the ‘right-mouthed’ type to sneak up from the left and take a bite. However, as the right-mouthed types increase in the population, they become a victim of their own success. The other fish soon learn to protect their left sides instead; and things swing back in favour of the left-mouthed cichlids. As such the population seesaws back and forth, and both left- and right-mouthed cichlids are maintained in the population at a ratio of ~50:50.

 The crooked smiles of scale-eating cichlids: views of mouth bending to the right (on the left) and to the left (on the right) #confused? (Image by Lee HJ, Kusche H, Meyer A (2012), via Wikimedia Commons)


The crooked smiles of scale-eating cichlids: views of mouth bending to the right (on the left) and to the left (on the right) #confused? (Image by Lee HJ, Kusche H, Meyer A (2012), via Wikimedia Commons)

So IS it better to be a maverick or a sheep? As with most things, the answer depends on the specifics of each situation. Clearly, living things do not evolve in isolation – and frequency dependent selection reminds us that the best strategy is sometimes only predictable based on what everyone else is doing at a given time. Also as things change, the pay-outs of each strategy can also shift with knock-on consequences. Once more this fascinating process in evolution reminds us that Nature is anything but simple.

References

Endler, J. A., and J. J. D. Greenwood. “Frequency-dependent predation, crypsis and aposematic coloration [and discussion].Philosophical Transactions of the Royal Society of London. B, Biological Sciences 319.1196 (1988): 505-523.

Hori, Michio. “Frequency-dependent natural selection in the handedness of scale-eating cichlid fish.SCIENCE-NEW YORK THEN WASHINGTON- 260 (1993): 216-216.

Faurie, Charlotte, and Michel Raymond. “Handedness, homicide and negative frequency-dependent selection.Proceedings of the Royal Society B: Biological Sciences 272.1558 (2005): 25-28.

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About StuartKing

Hi, I'm Stuart, an Assistant Features Editor at eLife and recent life sciences PhD graduate. I blog about evolution and its weird and wonderful creations.

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