How is sexual selection evolutionarily stable?

How is sexual selection evolutionarily stable?

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It seems like in any population subject to evolutionary pressure, sexual attractiveness will inversely correlate with other kinds of reproductive fitness. If you see a peacock with a small tail feather, this is information that it is likely above average in other areas, because otherwise its genes would already have been wiped out. So intuitively, you would think that it would benefit individuals to find less conventionally attractive mates.

So far, I have encountered two possible explanations for this. One idea was that the things that make an animal attractive correlate with evolutionary fitness. I'm not an expert here, but it feels like the correlation here will diminish over time as soon as there is direct pressure to increase attractiveness.

The other, more convincing, explanation was that it is advantageous to produce attractive offspring. So, there might be a feedback loop here where if members of a population care about sexual attractiveness more, then attractiveness would confer a greater advantage, making attractive offspring more important, increasing the evolutionary pressure for individuals to choose attractive mates.

Can someone give an intuition (or better, the relevant math) for why these two latter effects dominate my original intuition? Or are there other effects at work here?

first off you are talking about runaway sexual selection, run away sexual selection works because attracting mates matters MORE than individual survival. An immortal survivally perfect creature that cannot mate is an evolutionary dead end and might as well not exist where its genes are concerned. A bright red neon sign on your back that draws the attention of potential mates is an advantage as long as it attracts mates more often than it attracts predators. Coincidentally this is why so many sexual displays are things you can turn off, even a peacock can fold its tail fan down. These displays often exploit existing attention factors, like eye shaped objects, bright contrasting colors, loud noises, ect, things organisms are already predisposed to pay attention to. Of course once neon sign displays exist your second idea comes into play, once neon signs are in play a female is best served if her own male offspring and descendants also have neon signs, which makes attraction to the neon signs (and thus the genes making them) an advantage.

For normal sexual selection both your ideas also come into play, likely more often. common attractive features are things like size, symmetry, health, or excess energy, direct signs of fitness.

Most of the time displays reaches a point of stability where the cost is balanced by the attractiveness, this is how most displays work. But sometimes runaway selection can drive it past the point of no return, where the cost of reducing it is so high (lack of mates) the only winning strategy is to invest more and more, on and on until the cost is so high you never get a chance to mate.

You gave the example of a brightly colored male bird… and what about the females? The are well camouflaged and optimally fit to evade predation. So, even if 50% of males are caught and eaten due to theor bright colors, the demography stays the same.

The species as a whole is the same with or without the bright feathers. The sexual display amplifies the pressure for fitness selection, to obtain more food, evade predators better, and invest more energy in sexual lures, horns, feathers, song.

You are right, if a predated population places sexual selection on the females, it is counter productive. Fish, birds, insects, frogs, they nearly always have super robust camouflaged females and males that fight and display their high fitness, except parrots and reef fish.

Birds are so good at evading predators and flying to food, that even the females are colorful in some species i.e. parrots, and thats a less easy example, why do female parrots risk having no camouflage? Perhaps their climate doesnt provide enough selection.

Evolutionarily stable strategies of age-dependent sexual advertisement

In various models of sexual selection mediated by the viability indicator (“good genes”) mechanism, a sexually selected trait will truly reflect male quality if its expression is costly for the male. However, in long-lived species, the expression of a trait often increases with age while the genotype of the male remains unchanged. This fact may obscure the indicator mechanism. Hitherto, game theory models of honesty in sexual advertisement have not taken life-history effects into account, whereas life-history models of reproductive effort have only seldom considered the dependence of mating success on the actions of other individuals. Here, the two approaches are combined, and I examine whether honesty is maintained if males can divide their advertisement effort over their lifetime. The model shows that an increase in the expression of the sexually selected trait over several years is an evolutionarily stable strategy (ESS) under a wide range of situations, so that a correlated preference for old age can emerge through a viability indicator mechanism. Honesty in the strict sense is not preserved: an optimally behaving low-quality male will in some cases advertise more than a high-quality male of equal age, to the extent that the strongest advertisement found in the population can be associated with a low-quality male. Due to life-history trade-offs, however, honesty in an average sense holds true over the lifetime of individuals: “cheater” age classes will remain small enough, that a female will obtain a higher expected mate quality if she trusts in the trait as an indicator of viability.

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10.9 What is the evidence for sexual selection in humans?

Evolution by sexual selection has been invoked to explain a number of human anatomical features, which appear useless or detrimental to human survival. These include hairlessness, male facial hair, rounded breasts, pubic hair, and penis size. In the next few pages we will talk about some of the more interesting examples of traits that some scientists have argued are products of sexual selection: primary sexual traits (e.g. male penis size), female orgasms, large brains, our extensive vocabularies, and many aspects of our cultural behaviors.

Biology is Sexy!

Why do human men have nipples?

Early evolutionary biologists were keen to observe that humans, especially compared to nearly all other mammals, were much less hairy. Many hypothesized that because human females were much less hairy than their male counterparts, the loss of hair was due to selection by pre-historic human males for less hairy female partners. But if this were the case, and men were selecting for less hairy women, than why would hairlessness be present in human males, and not just human females? The answer will show up often as we discuss evolution- by-products of similar biological pathways in male and female animals can give rise to traits that are only selected for in one sex, but will often show up in the other sex. An interesting example of this is the presence of nipples in mammals. In most mammals, including human females, the nipple is important for transporting milk from inside to outside of the body. Given that human women are solely responsible for breast feeding, why do male animals, like human men, have nipples? Maybe this is not a question you have ever thought about after all, it would be quite startling to suddenly see a handful of animals, including humans, without nipples. But evolutionarily, this question is quite interesting and the answer will come up a lot in our discussions of human evolution, so it’s worth spending some time on it.

Have you ever noticed most animals have a similar body plan? For example, most animals have a set of eyes somewhere on their head, the head rests atop a thorax or torso, and legs protrude downward from the thorax. While aberrations from this body plan can happen, often as the result of mutation, most animals have this body plan. The reason is that all animals, in fact, all living things, evolved from a common ancestor. As such, the developmental pathways that give rise to the traits we see in animals are highly conserved. This means that across a wide variety of animal species, pathways sharing a common function, also share a common origin in the biological past. Evolution typically takes the path of least resistance and which organisms may not be completely optimized, we are often just “good enough” to survive and reproduce.

This same logic applies to the case of nipples in humans. The developmental pathway from embryo to fetus is highly conserved, with little variation in the early stages of embryonic development. Human fetuses do not develop sex-specific characteristics, like a penis or vagina, until certain genes are turned-on around week 7 of development. Because nipples are not a trait that is determined by male or female-specific genes, all human fetuses will develop them. While they do not serve the function of nourishing offspring in males, they also do not affect survival and reproduction. Because they are not costly, nipples persist in men as it is not an evolutionary priority to get rid of them, and a re-working of the entire embryonic developmental plan would be extremely difficult.

Fun Fact: Why do humans have two nipples?

A good rule of thumb for mammals is to have twice as many nipples as offspring that you produce at one time. A female cow typically has two offspring at a time, and has four nipples a small dog has eight nipples and a large dog has ten nipples because typical litter size is four to five pups at once. Thus, human females, who typically gestate one fetus at a time, have two nipples.

Large penises and breasts

Human penises are quite unique! Despite common slang terms that imply otherwise (e.g., “boner”), the human penis contains no bones. Unlike most of our closest evolutionary relatives, like chimpanzees and bonobos, human males do not have a penis bone, or baculum. Instead, human males must maintain an erection by pumping blood into the penis. Evolutionary biologists have speculated that the loss of the baculum in humans may be due to sexual selection by human females. Because the human erection relies on a type of hydraulic pumping system, erection failure can be an early warning of certain health conditions. Thus, human females are able to use the male erection as a clear sign of good health in potential partners.

Interestingly, the human penis is also much thicker than the penises of other great apes. Some have suggested that the evolution of the human penis towards a large size, both in length and diameter, has been the result of sperm competition. However, sperm competition typically favors larger testicles, not larger penises. Others have suggested that it is the result of mate competition, because a larger penis will be more efficient in displacing sperm from rival males during sexual intercourse. Support for this idea is limited. In fact, one study found that the amount of semen displacement during sexual intercourse was related not to penis size, but to the depth of pelvic thrusting. However, these researchers also stated that a longer penis would be more capable of leaving semen in less-accessible parts of the vagina, making it more difficult for subsequent males to remove or displace the semen.

Similarly, human females have much larger breasts than other primates. Because the additional fatty tissue in human breasts does not contribute to milk production, many think breast size evolved as a courtship signal. Many scientists think that large, round breasts and larger penises may once have served as signals of health and fertility, but many of these traits are now the product of “runaway selection”- a positive-feedback loop in which strong mate choice leads to the further exaggeration of a sexual trait.

Figure 10.14 Humans have large penises and breasts relative to body size compared with other great apes.

An evolutionary biologist misrepresents sexual selection in The New York Times

Friday’s New York Times contained an article on sexual selection in birds (link and title in the picture below) by Richard O. Prum, the William Robertson Coe Professor of Ornithology, Ecology and Evolutionary Biology at Yale’s Peabody Museum of Natural History. Prum has a new book out, The Evolution of Beauty: How Darwin’s Forgotten Theory of Mate Choice Shapes the Animal World — and Us, which I intend to read. For now, though, he’s given us a take in the Times which is both erroneous and confusing, for it misrepresents sexual selection, natural selection, and modern evolutionary theory.

Here’s the Amazon summary of the book, which explains how Prum is trying to revive Darwin’s theory of sexual selection—a theory, which, by the way, has not been forgotten, but either refined with additional assumptions or discarded outright because Darwin didn’t know genetics or had no evidence to support his views:

In the great halls of science, dogma holds that Darwin’s theory of natural selection explains every branch on the tree of life: which species thrive, which wither away to extinction, and what features each evolves. But can adaptation by natural selection really account for everything we see in nature?

Yale University ornithologist Richard Prum—reviving Darwin’s own views—thinks not. Deep in tropical jungles around the world are birds with a dizzying array of appearances and mating displays: Club-winged Manakins who sing with their wings, Great Argus Pheasants who dazzle prospective mates with a four-foot-wide cone of feathers covered in golden 3D spheres, Red-capped Manakins who moonwalk. In thirty years of fieldwork, Prum has seen numerous display traits that seem disconnected from, if not outright contrary to, selection for individual survival [JAC: We’ve known this for a long time: it’s reproduction, not survival, that is impelling the evolution of these male traits.] To explain this, he dusts off Darwin’s long-neglected theory of sexual selection in which the act of choosing a mate for purely aesthetic reasons—for the mere pleasure of it—is an independent engine of evolutionary change.

It’s true that Darwin was the first person to ponder sexual dimorphism—the extraordinary difference in ornamentation, weapons, and behaviors between the sexes—and to speculate about its causes. He suggested “sexual selection”, and gave two hypotheses about how it worked.

The first, which Darwin called “the law of battle,” was correct: males are larger and have weapons or features that enable them to compete for females, as when elephant seals or elk fight it out for mates. The ultimate cause of this difference, which I’ve described before, is the difference in gamete size between the sexes (sperm vs eggs), which ultimately leads to females being a scarce resource for which males have to compete. I won’t describe it further you can consult a good evolution textbook, such as Futuyma and Kirkpatrick (it shows some examples of sexually selected traits on the cover below), or read the Wikipedia article on sexual selection, which is okay but not great. Lots of experiments and observations confirm that males do fight over females, and the weapons and their size make a difference. (Males also compete for females after fertilization: the so called “gamete competition.” One example is in damselflies, in which a male, before inseminating a female, will use a scoop on his penis to remove the sperm of the previous male. The selective advantages of having such a device are obvious.)

Darwin’s second theory, however, was largely wrong, as it was based on females preferring certain traits of males because of their appeal to the females’ aesthetic sense. Here are the two theories given by Darwin in his 1871 book, The descent of man and selection in relation to sex.

“The sexual struggle is of two kinds: in the one it is between the individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive while in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners.”

The problem here is that it takes the aesthetic sense of females as a given rather than something that can itself be the product of evolution. And of course it implies abilities not present in many species, like flies, who surely don’t have “aesthetic senses”. While Darwin’s “aesthetic” theory can be modified to take into account pre-existing female preferences that are either evolved or the byproduct of some other evolved trait (Ronald Fisher was responsible for this advance), in itself it doesn’t explain much. Darwin was correct, though, that female preferences can cause males to evolve traits that hurt the males’ survival (as in the elaborate tails of peacocks), so long as the males’ loss in “fitness” due to survival costs is more than compensated by their gain in fitness due to females mating more often with males having exaggerated calls, behaviors, or ornaments.

The mechanisms of sexual selection and causes for female preference still remain mysteries, for there are many reasons why females can prefer the traits of males that make them so bizarre—traits like the plumes, ornaments, and behaviors of the New Guinea birds of paradise. And distinguishing among these hypotheses—which include the “runaway hypothesis”, direct benefits models, handicap models, sensory bias models (a refinement of Darwin’s ideas), “good genes” models, and so on—is difficult, especially because they can work in tandem. To see the hypotheses for the evolution of female preference (which Darwin took as a given) and the difficulty of testing them, have a look at this 2009 PNAS paper by Jones and Ratterman. The paper shows this table listing the varieties of ways that female preferences for male traits can evolve:

In his New York Times piece, Prum ignores most of these, claiming that sexual selection is not a form of natural selection, does not lead to adaptation, and leads to “maladaptive decadence.”

Here’s an example. After describing the elaborate display of the male club-winged manakin, which lures females by rubbing together its wing feathers (a trait that has caused the evolution of thick, flight-impeding wing bones, and whose results can be seen in the video at bottom), Prum says this:

This [manakin song] is an evolutionary innovation — a whole new way to sing. But the evolutionary mechanism behind this novelty is not adaptation by natural selection, in which only those who survive pass on their genes, allowing the species to become better adapted to its environment over time. Rather, it is sexual selection by mate choice, in which individuals pass on their genes only if they’re chosen as mates.

It’s hard to make THREE errors in two short sentences, especially when the writer is an evolutionary biologist writing about evolution, but that’s what Prum has done. Here are the errors:

1.) Sexual selection is a subset of natural selection: the consistent differential reproduction of genes based on their advantage in replication. Sexual and natural selection are not two distinct processes. Here’s Futuyma’s definitions from the 3rd edition of his textbook.

“Natural selection” is defined as “The differential survival and/or reproduction of classes of entities that differ in one or more characteristics.”

“Sexual selection” is defined as “differential reproduction as a result of variation in the ability to obtain mates.” These definitions, which are held by nearly all evolutionists, clearly show that sexual selection is a subset of natural selection, the subset affecting traits involved in mate competition. Prum’s claim that the two processes are distinct is confusing and wrong.

2.) Prum conceives of natural selection as only differential survival, whereas it’s differential reproduction that is key. Differences in survival produce selection only if they’re associated with differential reproduction. (They often are.)

3.) Neither natural nor sexual selection necessarily leads to an improvement in “species becoming better adapted to their environments over time”. Selection most often operates on genes that affect the reproductive output of their carriers, but that needn’t improve the adaptation of a species to its environment. For example, a mutation that increases the number of a bird’s offspring, but has no other effect, will simply increase the number of young birds in the population, which isn’t an improvement in adapting to the environment. In fact, this could ultimately lead to a depletion of food that could drive a population extinct. Likewise, the mechanism of “meiotic drive”, in which one mutant gene simply kills the other genetic variants during gamete formation, is a form of natural selection that can and probably has driven populations extinct. Throughout the article, Prum seems to conflate adaptation (a phenomenon of genes and individuals) with the survival of a species or its adaptation to the exigent environment. While this can happen, it’s not a necessary connection.

Those sentences would surely mislead a reader who wasn’t acquainted with evolutionary biology.

I could go on, but I’ll give just one more example of misleading prose in Prum’s short piece:

Of course, females do not harm their own survival by choosing males with attractive songs the costs are deferred to their sons and daughters. Although their daughters will inherit more awkward wing bones, their sons will inherit sexually attractive songs, resulting in more grandchildren.

In the absence of direct costs to the choosers, the population will not be saved by natural selection. Because the cost is deferred, the whole population can ease further and further into maladaptive dysfunction, generation by generation.

Evolved decadence may turn out to be common.

. . . The wing songs of the club-winged manakin teach us that adaptation by natural selection does not control everything that happens in evolution. Some of the evolutionary consequences of sexual desire may not be adaptive. Rather, they can be truly decadent. Despite the ubiquity of natural selection, organisms are not always getting better at surviving. Natural selection is not the only source of design in nature.

This is deeply misleading, and not just by confusing reproductive “fitness” with survival alone. Deferring the “cost” of choosing a clumsy-flying male by one generation doesn’t throw the population into a death spiral. In that next generation, if the cost of female choice outweighs the benefits of choosing those males, females will evolve in the reverse direction, choosing males that can fly better. There is no inherent force in sexual selection that will lead both males and females to evolve beyond their fitness optima. And if natural selection (including sexual selection) isn’t the only source of design in nature, what is? How does “decadence” produce design?

I hope Prum’s book is better than this excerpt. I showed his NYT piece to another evolutionist, who allowed that it was profoundly confusing. But the average reader, not deeply acquainted with sexual selection theory, will think that Prum has hit on some new principle of evolution. And in that way his article does the reader a disservice, for what he says is a mixture of stuff that evolutionary biologists already know, confusing and misleading characterizations of selection, and a neglect of competing and unresolved explanations for female choice—explanations that haven’t all been examined in manakins.

Evolutionarily Stable Strategies and Behavior

Evolutionary biologists imagine a time before a particular trait emerges. Then, they postulate that a rare gene arises in an individual, and they ask what circumstances would favor the spread of that gene throughout the population. If natural selection favors the gene, then the individuals with the genotypes incorporating that particular gene will have increased fitness. A gene must compete with other genes in the gene pool, and resist any invasion from mutants, to become established in a population’s gene pool.

In considering evolutionary strategies that influence behavior, we visualize a situation in which changes in genotype lead to changes in behavior. By ‘the gene for sibling care’ we mean that genetic differences exist in the population such that some individuals aid their siblings while others do not. Similarly, by ‘dove strategy’ we mean that animals exist in the population that do not engage in fights and that they pass this trait from one generation to the next.

At first sight, it might seem that the most successful evolutionary strategy will invariably spread throughout the population and, eventually, will supplant all others. While this does occur, it is far from always being so. Sometimes, there is no single dominant strategy. Competing strategies may be interdependent in that the success of one depends upon the existence of the other and the frequency with which the population adopts the other. For example, the strategy of mimicry has no value if the warning strategy of the model is not efficient.

Game theory belongs to mathematics and economics, and it studies situations where players choose different actions in an attempt to maximize their returns. It is a good model for evolutionary biologists to approach situations in which various decision makers interact. The payoffs in biological simulations correspond to fitness—comparable to money in economics. Simulations focus on achieving a balance that evolutionary strategies would maintain. The Evolutionarily Stable Strategy (ESS), introduced by John Maynard Smith in 1973 (and published in 1982), is the most well known of these strategies. Maynard Smith used the hawk-dove simulation to analyze fighting and territorial behavior. Together with Harper in 2003, he employed an ESS to explain the emergence of animal communication.

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An evolutionarily stable strategy (ESS) is a strategy that no other feasible alternative can better, given that sufficient members of the population adopt it. The best strategy for an individual depends upon the strategy or strategies that other members of the same population adopt. Since the same applies to all individuals in that particular population, a mutant gene cannot invade an ESS successfully.

The traditional way to illustrate this problem is by simulating the encounter between two strategies, hawk and dove. When a hawk meets a hawk, it wins on half of the occasions, and it loses and suffers an injury on the other half. Hawks always beat doves. Doves always retreat against hawks. Whenever a dove meets another dove, there is always a display, and it wins on half of the occasions. Under these rules, populations of only hawks or doves are no ESS because a hawk can invade a population made up entirely of doves, and a dove can invade a population of hawks only. Both would have an advantage and would spread in the population. A hawk in a population of doves would win all contests, and a dove in a population of hawks would never get injured because it wouldn’t fight.

However, it is possible for a mixture of hawks and doves to provide a stable situation when their numbers reach a certain proportion of the total population. For example, with payoffs as winner +50, injury -100, loser 0, display -10, a population comprising hawks and doves (or individuals adopting a mixed strategy of alternating between playing hawk and dove strategies) is an ESS whenever 58,3% of the population are hawks and 41,7% doves or when all individuals behave at random as hawks in 58,3% of the encounters and doves in 41,7%. The percentages (the point of equilibrium) depend on costs and benefits (or the pay-off, which is equal to benefits minus costs).

Evolutionarily stable strategies are not artificial constructs. They exist in nature. The Oryx, Oryx gazella, have sharp pointed horns, which they never use in contests with rivals, except in a ritualised manner, and only in defense against predators. They play the dove strategy. They hawk strategy is rarely seen in nature except when competing for mating partners. However, up to 10% per year of Musk Ox, Ovibos moschatus, adult males die because of injuries sustained while fighting over females.

An ESS is a modified form of a Nash equilibrium. In most simple games, the ESSes and Nash equilibria coincide perfectly, but some games may have Nash equilibria that are not ESSes. Furthermore, even if a game has pure strategy Nash equilibria, it might be that none of those pure strategies are ESSes. We can prove both Nash equilibria and ESS mathematically (see references).

Peer-to-peer file sharing is a good example of an ESS in our modern society. Bit Torrent peers use Tit-for-Tat strategy to optimize their download speed. They achieve cooperation exchanging upload bandwidth with download bandwidth.

Evolutionary biology and sociobiology attempt to explain animal behavior and social structures (humans included), primarily in terms of evolutionarily stable strategies.


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Featured image: The traditional way to illustrate Evolutionarily Stable Strategies is the simulation of the encounter between two strategies, the hawk and the dove.

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The evolutionarily stable remating probability

When the remating benefit equals γt, the ESS remating probability q* = 0 as long as the ESS stability condition holds true (A9). The factor T(μPμ0) is a measure of the how much the mortality rate increases during mating. Take a sample of nF single females and equally many just formed pairs so that nF = nP to begin with. When mating has ended, the difference in numbers is (taking logarithms) ln (nF) − ln (nP) = T(μPμ0). Because this is caused by higher predation pressure on pairs, we can interpret it as an ecological cost of remating. To this is added β, which measures effects of remating on female mortality that are unrelated to predation. For example, β > 0 could be due to harmful ejaculate substances, or infection by parasites/pathogens ( Stockley, 1997 Reinhardt et al., 2003 ).

The costs of remating in eqn (8) is weighted by the ratio ɛ0/μ0, which is the life-time fitness i.e. the oviposition rate (ɛ0) times the life-span (1/μ0), of an average female in a population without remating. This puts the absolute fitness benefit γ in relation to the average population fitness at the point q = 0.

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Zeeman, E. C., 1979: Population dynamics from game theory. Proc. Int. Conf. Global Theory of Dynamical Systems, Evanston, Northwestern

Mechanisms and evolution of deceptive pollination in orchids

The orchid family is renowned for its enormous diversity of pollination mechanisms and unusually high occurrence of non-rewarding flowers compared to other plant families. The mechanisms of deception in orchids include generalized food deception, food-deceptive floral mimicry, brood-site imitation, shelter imitation, pseudoantagonism, rendezvous attraction and sexual deception. Generalized food deception is the most common mechanism (reported in 38 genera) followed by sexual deception (18 genera). Floral deception in orchids has been intensively studied since Darwin, but the evolution of non-rewarding flowers still presents a major puzzle for evolutionary biology. The two principal hypotheses as to how deception could increase fitness in plants are (i) reallocation of resources associated with reward production to flowering and seed production, and (ii) higher levels of cross-pollination due to pollinators visiting fewer flowers on non-rewarding plants, resulting in more outcrossed progeny and more efficient pollen export. Biologists have also tried to explain why deception is overrepresented in the orchid family. These explanations include: (i) efficient removal and deposition of pollinaria from orchid flowers in a single pollinator visit, thus obviating the need for rewards to entice multiple visits from pollinators (ii) efficient transport of orchid pollen, thus requiring less reward-induced pollinator constancy (iii) low-density populations in many orchids, thus limiting the learning of associations of floral phenotypes and rewards by pollinators (iv) packaging of pollen in pollinaria with limited carry-over from flower to flower, thus increasing the risks of geitonogamous self-pollination when pollinators visit many flowers on rewarding plants. All of these general and orchid-specific hypotheses are difficult to reconcile with the well-established pattern for rewardlessness to result in low pollinator visitation rates and consequently low levels of fruit production. Arguments that deception evolves because rewards are costly are particularly problematic in that small amounts of nectar are unlikely to have a significant effect on the energy budget of orchids, and because reproduction in orchids is often severely pollen-, rather than resource-limited. Several recent experimental studies have shown that deception promotes cross-pollination, but it remains unknown whether actual outcrossing rates are generally higher in deceptive orchids. Our review of the literature shows that there is currently no evidence that deceptive orchids carry higher levels of genetic load (an indirect measure of outcrossing rate) than their rewarding counterparts. Cross-pollination does, however, result in dramatic increases in seed quality in almost all orchids and has the potential to increase pollen export (by reducing pollen discounting). We suggest that floral deception is particularly beneficial, because of its promotion of outcrossing, when pollinators are abundant, but that when pollinators are consistently rare, selection may favour a nectar reward or a shift to autopollination. Given that nectar-rewardlessness is likely to have been the ancestral condition in orchids and yet is evolutionarily labile, more attention will need to be given to explanations as to why deception constitutes an 'evolutionarily stable strategy'.

Why do men have beards? An inquiry from an evolutionary biology perspective

One of the most easily recognizable features of sexual dimorphism in humans is the fact that males grow beards whereas women don’t. But what is the point of having a beard in the first place, evolutionary-speaking?

Do beards make men more attractive?

Whenever there are important physiological differences between males and females of a species, these features are more often than not due to the evolutionary pressure of sexual selection — the process that favors traits that promote mating opportunities.

Charles Darwin proposed the concept of sexual selection 150 years ago in On the Origin of Species by Means of Natural Selection, but his definitive work on sexual selection was undoubtedly covered in ones of his lesser-known works: The Descent of Man, and Selection in Relation to Sex, which was published in 1871. Although Darwin wrote extensively about sexual selection and offered ample evidence to support his thesis, this simple quote from the book illustrates the concept quite clearly:

“We are, however, here concerned only with that kind of selection, which I have called sexual selection. This depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction.”

Essentially, Darwin argued that sexual selection drove variation in traits such as skin and hair color, and also shaped many differences between men and women. According to Darwin, such traits help, not with the struggle for survival (natural selection), but with the struggle for reproduction.

However, determining the effects of sexual selection in humans is very tricky because our behavior is also largely driven by culture. It may be difficult to identify a human complex behavior that is completely independent of culture or social learning. For instance, we dress in fashionable clothes to attract the opposite sex — and fashion always changes with the times and varies depending on the geographical location. Footbinding in ancient China and neck rings in the Kayan are some extreme examples of such behavior.

So what does all of this have to do with beards? Being a defining feature of men, it stands to reason that beards evolved to attract mates. However, studies have been rather inconclusive in this respect.

It’s not the beard, bro. Credit: Pixabay.

One 2013 study found that “women judged faces with heavy stubble as most attractive and heavy beards, light stubble and clean-shaven faces as similarly less attractive.” However, a 1996 study reached the opposite conclusion, finding that men with “facial hair were perceived as more aggressive, less appeasing, less attractive, older, and lower on social maturity than clean-shaven faces.”

To complicate things even further, research suggests that in times when beards are fashionable, being clean-shaven is more attractive, while if there are many clean-shaven men, beards become more attractive simply by contrast.

Some women really like beards, while others can’t stand them. There’s no universal preference for beards across the board.

The lack of consistent evidence and the fact that most studies are performed with Westerner participants makes a poor case that men’s beards serve to attract females. However, we’re not out of sexual selection territory yet.

Beards as a signal of dominance for other men rather than an attraction cue for women

Traits favored by sexual selection do not necessarily serve to attract, they can also improve reproductive outcomes by making men appear more dominant, hence more able to fend off competition for mates.

Studies suggest that men with beards are perceived as older, stronger, and more aggressive than those that are clear-shaven.

Credit: Pixabay.

One interesting study that assessed British facial hair styles between 1842 and 1971 found that beards and moustaches became more fashionable during times when there was a great proportion of single men competing for fewer women.

A 2015 study, which was published in the journal Behavioral Ecology, found that perceptions of men’s dominance increased with features of masculinity (lower-pitched voices and greater beard growth). Beards didn’t appear to affect a man’s attractiveness rating.

“Together, these results suggest that the optimal level of physical masculinity might differ depending on whether the outcome is social dominance or mate attraction. These dual selection pressures might maintain some of the documented variability in male physical and behavioral masculinity that we see today,” the authors wrote.

Beards to soften the punch?

Aside from enhancing traits of dominance (and providing the perfect breeding grounds for bacteria and other germs), beards may also serve a very practical purpose.

A recent study, which was published in April 2020 in the journal Integrative Organismal Biology, suggests that growing a thick beard offers protection for the human jaw from the impact of blunt force.

Previous research suggested that human hands evolved to be used as weapons and the human face is naturally developed to withstand blunt force.

The new study suggests that the beard can also offer men an edge during physical confrontations with other males. The researchers covered a human skull with fiber epoxy composite and grafted a beard made of untrimmed sheepskin.

Their trials found that the faux beard absorbed 37% more energy than hairless models. What’s more, beard-covered skulls broke bones only 45% of the time, compared to hair-free skulls that broke almost all of the time.

“These differences were due in part to a longer time frame of force delivery in the furred samples. These data support the hypothesis that human beards protect vulnerable regions of the facial skeleton from damaging strikes,” the authors wrote.

Bottom line: it’s highly unlikely that beards are some fluke of evolution. Instead, they’re likely the result of evolutionary pressures meant to enforce dominance hierarchies, perhaps enabling some men to intimidate competitors for mates. They may also aid in physical confrontations with other men by softening the impact of blunt force. In the end, unfortunately (or maybe fortunately for you), there is limited evidence that beards make men more attractive.


The female reproductive tract is where competition between the sperm of different males takes place, aided and abetted by the female herself. Intense postcopulatory sexual selection fosters inter-sexual conflict and drives rapid evolutionary change to generate a startling diversity of morphological, behavioural and physiological adaptations. We identify three main issues that should be resolved to advance our understanding of postcopulatory sexual selection. We need to determine the genetic basis of different male fertility traits and female traits that mediate sperm selection identify the genes or genomic regions that control these traits and establish the coevolutionary trajectory of sexes.

Extra–pair mating, male plumage coloration and sexual selection in yellow warblers (Dendroica petechia)

Extra–pair mating has been proposed as a source of sexual selection responsible for secondary sexual traits that are common among socially monogamous birds, although supporting evidence is scant. In the socially monogamous yellow warbler, males are larger than females, and unlike females, have extensive reddish streaking on their breasts. Using DNA fingerprinting we show that within–pair parentage was positively related to male size, and that extra–pair mating success was positively related to the amount of streaking on the breast. To our knowledge, this is the first intraspecific evidence of an association between a male plumage ornament and gains of extra–pair paternity that is apparently independent of age. This study confirms that extra–pair mating can be an important mechanism of sexual selection even when the most successful sires are commonly cuckolded, and refutes a previous hypothesis that the variation in plumage and behaviour among male yellow warblers is an example of alternative, equally successful, evolutionarily stable strategies (ESS). More generally, the demonstrated independence of within–pair and extra–pair success and their associated traits indicates that where animals have multiple secondary sexual traits, different traits may be selected by different mechanisms that contribute to total reproductive success.


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