Sexual reproduction sets in when resources become scarce. Under a wide range of conditions, the sexual species outcompete the asexual ones. Asexual species win only when survival conditions are harsh and death rates are high, or when resources are so little structured or consumer genotypes are so manifold that all resources are exploited to the same extent. ~ German evolutionary zoologists S. Scheu & B. Drossel
The first sex happened 1.2 billion years ago. Its evolution owes to a viral innovation.
Early on viruses developed a protein that lets 2 cell membranes fuse into 1, allowing combination of genetic material. This technique is essential for both viral and sexual reproduction.
Nature has a limited number of ways it can cause cells to fuse together into a single cell. The protein that first made sex possible – and is still used for sexual reproduction in many of Earth’s organisms – is identical to the protein used by certain viruses to enter human cells. This protein must have really put the spice in the primordial soup. ~ American cytologist William Snell
Having been handed the essential tool, early cells went their own way in becoming sexy. Sex chromosomes independently evolved in different lineages of eukaryotes.
In evolutionary terms, sex has been habit forming. Most protists are into sex, as are fungi, plants, and animals.
A few stopped having sex, including a class of freshwater rotifers (Bdelloidea). Despite parthenogenetic reproduction, these microscopic rotifers have evolved for tens of millions of years, differentiating into over 500 species, many with very distinct bodies. Bdelloidea demonstrate that sex is only one route to diversity.
Bdelloidea can survive desiccation: a necessity if you plan on living past your puddle drying up. Losing all the moisture in your body taxes the genome. Once wet again, Bdelloidea are able to repair the inevitable DNA damage from being bone dry.
This gene-patching skill is how Bdelloidea manage their multiplicity. Like viruses and prokaryotes, Bdelloidea are mavens of DNA manipulation. These rotifers put horizontal gene transfer to practical effect.
Parthenocarpic plants reproduce vegetatively and can produce fruit without fertilization. Sharks normally sexually reproduce but can dispense with the sex to parthenogenetically reproduce.
Sex is expensive. It necessitates additional organs which require developmental resources, and, more pressingly, taxes organisms with energies spent in mating and breeding.
Sex evolves when the need for adaptability is high, as sex more actively shuffles the genetic deck than asexual reproduction. Greater diversity in offspring raises the odds that some may survive to reproduce.
Variety’s the very spice of life, that gives it all its flavour. ~ English poet William Cowper
Tetrahymena are a genus of free-living ciliate protozoa found in freshwater ponds. Each is 1 of 7 possible sexes.
Tetrahymena have 2 nuclei: a regular soma nucleus, which has a gene for its own sex, and a 2nd germline nucleus for reproduction. The germline nucleus carries incomplete versions of all 7 sex genes.
During growth, the somatic nucleus handles all gene transcription while the germline nucleus stays silent.
When it comes time to mate, the germline nucleus is sorted out until a single mating sex is determined. To maximize genetic diversity, Tetrahymena mate with another of a different sex. Offspring may be any 1 of the 7 sexes.
Many microorganisms practice sex. Some algae and fungi reproduce sexually, including the mold that produces penicillin. Yet asexual reproduction works exceedingly well for many microbes. The littlest ones possess rather astonishing adaptability thanks to their ability to selectively pick up genic bits from the environment and each other (transformation and horizontal gene transfer, respectively).
Given that complex life forms are chock full of rapidly adapting commensal microbes which facilitate digestion and help fight disease, sexual reproduction is not a convincing mechanism for optimizing adaptability, especially considering the energy cost involved. Whatever selective advantage may be conferred by sexual reproduction, its results have often been emergence of 2 sexes so different in appearance and behaviors as to easily mistake sexes for species.
Sex-biased genes are those that have different expression depending upon sex. Both sexes have the same genes, but sex-limited ones express only in 1 sex, often at a specific period of development. Sex-biased genes are responsible for sexual dimorphism.
The unique properties of sex-limited genes led to the widespread assumption that they are the product of sexual selection and sexual conflict. Darwinists took for granted – as lynchpin proof of “survival of the fittest” – that sexual selection via male mating rights contests provides a sure means for delivering the best male genomes to females. Actuality is more complicated.
Promiscuous birds that must fight for mating rights have genomes that evolve faster than monogamous birds which pair for life. Adaptive rapidity accounts for the supernormality in males that may arise quickly, and so is instrumental in conflict-based sexual selection.
The males that win the mating battles may not carry the best genome. Sex-biased genes geared to winning fights are not necessarily best for survival in other ways, which is why females of such sex-selective species often have extra-pair matings with also-rans. Instead of proving Darwinism, sexual conflict illustrates how deviously convoluted Nature is.
A male may be attractive to a female and fight hard to mate with her, but he doesn’t deliver at the genetic level. As a result, his descendants will be less fit. ~ English evolutionary geneticist Judith Mank
Polyandry is a mating system of 1 female and multiple males. Polygyny is the converse (1 male, multiple females).
Polyandry is common among mice. Females can tell the difference between a healthy male and an unhealthy one by their scents. Yet, while preferring healthy mice for their primary sex partners, they still participate in extra-pair mating with those not so robust.
Females living under a conflict-based sexual selection mating system hedge their bets when they can, as a mating system rigged to competitive contests is no guarantee of overall genomic quality in the winners.
Mating-competitive males seldom contribute much to raising offspring. Consequently, species with such males compete for the available females, while females exercise discrimination because offspring are a considerable investment which limit a female’s reproductive output.
Getting worked up over sex takes its toll. The mate competition that males undergo lessens lifespan, whether vinegar fly or furless primate. On top of that, any physical dominance afforded by sexual dimorphism carries a cost in longevity. Sex extracts a price.
Gray Tree Frogs
Gray tree frog males have a mating call that is a string of 20–40 pulses per call. These frogs produce 5–15 calls per minute. There is a trade-off in call duration and repetition. The longer the call, the fewer that are made each minute.
Tree frog females prefer a male with shorter but complex calls, as a rapidly robust croak indicates a quick mind. Sensory bias may come into play: preferring a trait which is not necessarily an indication of fitness but instead a preference that is an inherited disposition.
Some guppies have a fondness for orange foods. The preference varies by population.
In those with a sunny disposition, males acquire orange pigments from the carotenoids in their food. They incorporate the orange coloring into tail tattoo patches.
Females fall for a male with big orange tattoos. This signal is meaningful, as it indicates a male as a successful forager.
Conspecific Male Plant Competition
Most biologists agree that males fighting in competition for females has no scope in plants. ~ Argentinian botanist Andrea Cocucci et al
Male mating competition is not confined to animals. Several conspecific plants compete to mate, as pollen battle their way to an awaiting ovule. Via chemical conflict, wild radish pollen grains duke it out for the privilege of pollination.
Milkweed reproduce by hooking sacs of pollen grains – pollinaria – to the bodies of birds and other pollinators. Pollinaria are unwittingly dropped into another flower to complete pollination.
It is possible for multiple pollinaria to entangle due to the limited number of attachment points on the pollinator. The adaptive solution was horns that prevent pollen sacs from hooking together. Horns favor the flower that a pollinator visits first. Just as beetle and ungulate stag horns are the implements of male competition, pollarium horns force flower competition to the fore.
Neither self-propulsion nor well-developed sensory perception are required for sexual selection to take place through intrasexual struggles. Apparently, only physical contact is enough to influence the reproductive success of competitors and to promote the evolution of defensive and attack weaponry. ~ Andrea Cocucci et al
The marbled crayfish (aka marmokreb) is a 4–8 cm freshwater decapod discovered in the pet trade in Germany in the late 1990s. (Marmokreb is German for “marbled crayfish.” They are one of the most popular crayfish in the international pet trade.) 10-legged decapods are the crustacean order of crayfish, crabs, lobsters, prawns, and shrimp.
The marmokreb is parthenogenetic: the only decapod known to reproduce without sex. Their being uniquely triploid – having 3 chromosome sets – may be the reason. All marbled crayfish are female clones, and they lay lots of eggs.
Only 1 in 10,000 animals species comprise cloning females. Many studies suggest that parthenogens are rare because they don’t last long. Lacking genetic diversity, a single disease might readily wipe out a population.
Marmokrebs are obviously creatures of saltation. Their DNA indicates that they arose 1,250 years ago, but they were never seen before the late 20th century.
Having been set loose in lakes and streams by pet owners, marmokrebs have gone invasively wild, merrily walking hundreds of meters to reach new water sources. Feral populations have been spotted throughout eastern Europe (north and south), in Madagascar, and in Japan. Marmokrebs have been able to thrive in a variety of habitats with just their single genome.
Asexuality is a fantastic short-term strategy. ~ American biologist Abraham Tucker
Hypotheses on Sex
There have been many hypotheses about the origin and maintenance of sex. While sex remains a contentious area of theoretical evolutionary biology, competing views pretty much answer the same question the same way.
What compensates for the sex tax? The answer invariably involves adaptability.
Broadening genetic diversity renders a population more robust: a concept with several facets, including the ability to evade and overcome disease. Sex may well have evolved by virtue of presenting a moving target to pathogens. Sex and a microbiome-based immune system were the evolutionary insurance policies taken out to raise the odds of a population surviving nefarious microbes.
Paradoxically, though theorists agree that adaptability through diversity accounts for sex, their theories do not account for that very thing. Instead, many reproductive-advantage theories, beginning with Darwin, foolishly focused on competition. Darwin believed that sexual selection was a powerful force: a special case of natural selection that emphasized mating success for males. Propagation of a male’s genes were secured by maximizing progeny, whereas a female’s concern was optimizing quality: raising the odds of success to maturity and reproduction, not merely the number of offspring.
Darwin’s sexual selection theory was treated skeptically for decades. English evolutionary biologist Ronald Fisher invigorated the theory in the 1930s with the observation that supernormal ornaments could arise simply from female choice.
Nature sometimes equips animal males with mate-attracting ornaments and display behaviors that go so far as to be life-threatening: hence the idea of supernormality leading to survival of the fittest. If a male can flash self-endangering bling and survive, he must be well endowed where it counts; or so it was supposed. The implication was that, in evolutionary time, traits desired by females would become further exaggerated. Preference for long tails would result in longer and longer tails. This became known as the runaway hypothesis.
The Darwin/Fisher runaway hypothesis assumed that supernormality correlates with fitness. But a female-desired trait is not necessarily an evaluation of fitness. It may simply be that females favor a certain trait for its aesthetic value.
Seemingly capricious choices may become quite widespread in a population simply by being desirous. The senses are often positively responsive to supernormal features, such as men liking lips enhanced by red lipstick. In this regard, sensory biases toward supernormal features are biologically favorable, at least in stimulating the desire for copulation.
Females that mate with males because of their popular ornaments may benefit because they produce sons with similarly charming features that can attract mates, and so perpetuate the lineage.
Sexual selection theories evolved after Fisher. Many centered on the iffy proposition that the desirous trait was indicative of a male’s quality. While appearances are often an honest signal of a male’s overall health, there are a multitude of exceptions where appearance does not equate with reproductive fitness or success, including humans. From an evolutionary perspective sex is not straightforward.
Sexual Selection Intensity
In many animals, male reproductive success is more dependent upon competitive mating pressure than it is for females. Hence, sexual selection commonly acts with greater intensity on males than females. The result shows itself in a syndrome of traits, including behaviors.
In courtship, males commonly expend more time and energy, and take greater risks. Males generally court and attempt copulation with a wider range of partners than females. Males are more likely to resort to violence against sexual rivals and to force copulations upon resist-ing partners.
The sexual selection syndrome also includes life-history variables. Because of the sexual competition they face, males take longer to attain sexual maturity. Males tend to have a higher mortality owing to sexual competi-tion.
Then there are the morphological traits: anatomical specializations for aggression, including horns, antlers, enlarged canine teeth, and larger bodies than are ecologi-cally optimal. Males commonly sport more spectacular sexual advertisements, both visual (elaborate and brightly colored adornments) and tactile (complex genitalia).
Male displays and ornaments are not necessarily for females alone. They also signal other males. Many mating exhibitions are assessment displays which allow females to choose a mate based upon a male’s place in the dominance hierarchy of the social group.
At least in some ways, competitive males are fit. Prowess in battle is a better indicator of overall health than a long tail.
Male elephant seals are huge: 3 to 4 times the size of a female. The very largest and strongest dominate, gathering harems on breeding beaches. In some breeding seasons, a mere 4% of the males in the group perform 85% of the matings.
Because elephant seal males greatly outsize females, most any male can mount a female, which can offer little physical resistance. A female elephant seal can object to being mounted by loudly calling to alert other males. If a subordinate male has mounted, he is soon sent on his way by a more dominant male.
The females of many species prefer dominant males. A domestic hen favors a dominant cock.
Some females are more immediately practical, preferring what a male can do to rather than how he looks. Female pied flycatchers prefer males that secure prime real estate at the nesting grounds. Their dominance is well grounded.
Complex genitalia can result from different evolutionary mechanisms, though sexual selection is increasingly regarded as the primary force behind the evolution of genital diversity. Females respond to manipulating male strategies with behavioural counterstrategies to retain control over fertilization. ~ American evolutionary biologist Patricia Brennan et al
Nature has been inventive with genitals. They come in various shapes, sometimes with adornments.
Drakes have dicks shaped like corkscrews. Females have correspondingly long corkscrew vaginas, spiraling in the opposite direction.
Certain male ducks, especially mallards, are rare among birds in regularly being rapists. 40% of duck matings are forced.
The Lake Duck has the longest relative penis of vertebrates. Typically coiled when flaccid, the penis grows to half-duck length or longer when erect. This enormous spiny organ likely evolved in response to competitive pressure in these highly promiscuous birds: to the sperm of previous matings much like a bottle brush.
As an evolutionary counterstrategy, a female duck fends off sexual violence with maze-like genitals – featuring twists, pouches, and dead ends – that help her retain control of who fathers her offspring. Generally, there is such a poor fit between the male and female ducks’ genitals that a female must be relaxed and willing for mating to succeed.
The males of most birds forgo penises altogether. Instead, they have a cloaca: an opening much like a female’s. Mating is attained with a cloacal kiss.
Certain sessile barnacles pack the longest cock relative to body size. Among mobile animals, the deep-water greater hooked squid have the longest relative penis: as long as the mantle, head, and arms combined.
Many mammal penises, including mice, chimps, and felines, have keratinized penile spines. A cat withdrawing its penis rakes the walls of a female’s vagina, prompting ovulation.
Males of many insects have spiked penises that puncture and wound a female. Bed bugs and bat bugs bypass the vagina altogether. Instead, they pierce a female’s abdomen, directly inseminating the blood. Sperm then swim to the ovaries, guided by pheromones.
This has prompted counter-evolution. Female African bat bugs have paragenitals: a defense mechanism that limits damage by guiding a male’s sharp prick into a spongy structure.
Bat bugs are rabidly randy. A male may rape another male. Because of that, male bat bugs also evolved protective paragenitals.
Having a penis does not necessarily nix motherhood. Male seahorses and pipefish – not females – get pregnant.
Role reversal can be complete. In 4 closely related species of booklice, female possess a prickly penis (gynosome), while males have vaginas. The female is in charge of mating. A female inserts her erect gynosome into a male. Penal spines anchor her inside him. Unlike conventional penises, a gynosome is no squirter. Instead, it vacuums up a male’s sperm, which a female uses to fertilize her eggs.
Booklice can copulate for 70 hours non-stop. Besides sperm, male spermatophores provide nutrients. The female does not to let go until she has had a satisfying meal.
Female booklice packing penises is highly irregular, but not altogether unique. An extinct species of mite had a similar genitalia scheme, though without the anchoring spines.
Across the animal kingdom, sex is often an emotionally rewarding experience. Rejection is hard to take.
Vinegar flies have an elaborate courtship ritual. A male may vibrate a love song on one wing, tap his intended’s abdomen with his foreleg, and touch her genitalia with his proboscis (a smell/taste sense organ).
If she is put in the mood, his attempts at copulation are not rejected. Afterward, the 2 flies part: she goes her way to lay eggs, he to court again.
If instead a female rejects a male’s advances, a male may grow despondent and drown his sorrows in the sauce (fermented fruit).
Rejection or deprivation of sex leaves male flies in a state that increases the preferential consumption of ethanol. ~ American biologist Troy Zars
Though the differences are less pronounced than among many other mammals, considerable anatomical and behavior evidence indicates that men have been subject to stronger sexual selection pressure than women.
Men are larger than women. Boys reach sexual maturity later than girls, and men age more rapidly than women.
Aggression and violent competition among males is more common and pronounced than it is among females. Male sexual coercion is far more frequent than the reverse. Rape and sexual assault by men remain common in almost all modern societies.
Polygyny is much more common than polyandry. Women are typically more selective than men in their choice of sexual partners.
In one respect, humans reverse the usual pattern between more and less sexually selected genders: men are more concerned than women with the physical attractiveness of a potential sexual partner.
The physical beauty of the female receives more explicit consideration than does the handsomeness of the male. The attractiveness of the man usually depends predominantly upon his skills and prowess rather than upon his physical appearance. ~ American anthropologist Clellan Ford & American ethologist Frank Beach
There is an evolutionary reason for the strong appeal of men to a woman’s physical attractiveness: it correlates well with fecundity. Owing to menopause, there is much more age-related variance in fertility among women than men. The sexiest women are those in their prime child-bearing years.
Menopause is somewhat common among invertebrates, fish, birds, and mammals. According to the grandmother hypothesis, menopause serves to lengthen a female’s life so that she may assist her descendants, thus enhancing her legacy prospects. This has been observed in women and orcas. More broadly, having experienced females about may enhance group survival prospects.