THE ORIGIN OF THE OPPOSITE SEX APPEAL
We don’t know for sure whether external beauty really represents core biological qualities. Is it a manifestation of good genetics, or is it simply a random selection?
Charles Darwin didn’t like peacocks. In a letter to his friend in 1860, Darwin said that they made him feel nauseous. Their massive feathers, many times their body length, and their eye-catching array of colors annoyed Darwin because he couldn’t explain them. At that time Darwin’s Theory of Evolution said that natural selection would eliminate redundant and keep what is productive, and male peacocks seemed to be the exception. Not only is it costly in terms of resources to form and maintain, this plumage also makes peacocks more vulnerable to predators, home to pathogens, and highly vulnerable. This self-destructive beauty made Darwin doubt how peacocks could have evolved in such a direction, until he found a more plausible answer in 1871.
1. Beauty orientation and the origin of “feminism” in the natural world
It was then that Darwin realized that evolution through natural selection was not only related to survival but also closely related to reproduction. In this reproductive relationship, since females spend more energy on egg production, pregnancy and rearing, they also play a decisive role in mate selection, taking extra care to ensure their investment is paid off. In “The Descent of Man,” he writes: “[The conflict between males] is not a struggle for existence, but a struggle between the males for the possession of the females.” Thus in the biological world, the majority of males are more beautiful than females, and the evolution of this appearance is “selected” by the very “preferences” of the females. For the right to mate, males in the wild must compete in many ways, from fighting with opponents, possessing large and colorful plumage (in birds, or fins in fish), dancing, using songs… Darwin said that birds are the most aesthetic of all animals, famous for their plumage and singing, they are so attractive that humans are also impressed by them. In fact, the dinosaur ancestors of birds are also thought to have noticed an early use of form to attract females, such as triceratops, a three-horned dinosaur with an enlarged bony frill in the neck for the purpose of attracting a mate, even though it looks like a self-defense mechanism [1]. To account for all this, he proposed a new concept besides natural selection, that is, sexual selection.
Basically, when we approach the Theory of Evolution, we need to know that evolution does not happen at the level of individuals, but only at the level of species. Humans will evolve over time, but an individual will not evolve during his entire short life. This entire evolutionary process is simply the result of a process in which a few advantageous individuals continue to survive, reproduce, and leave their genes behind; while others disappear. Thus, the genetic resources of a population or species continuously change over time, and indirectly cause the phenotype of each individual within that species to change as well. For example, a hundred blocks of all shapes and sizes, when passing through a mold that only allows squares to pass through, then through another mold that only allows smaller squares… in the end, the remaining cubes only consist of small squares. Evolution works in a similar way: some have been eliminated and some have been selected, what remains determines the phenotype of the species.
Thus, evolution will take place as long as the individuals in the population ensure that:
- Survive long enough.
- Complete their reproduction purpose.
- Make sure that their offspring can survive and complete their reproduction purpose.
In this whole process, organisms will be influenced by natural selection. For example, suddenly there is an event that causes the climate to become colder, wolves with thin furs will freeze to death before they can pass on their genes, while other wolves with thick furs survive successfully, over time, this wolf population will only consist of individuals with thick fur and thin fur genes may even disappear. Of course, these events are not intended to help wolves evolve, and have the potential to drive the entire wolf species to extinction (such as too much and too sudden temperature drops), as many dinosaurs became extinct because of their unsuitable size when disaster struck, and only a few small-sized species were able to continue surviving.
But survival itself does not play a full role in evolution, because evolution only works when there is a process of “leaving genes”, i.e. successful reproduction. Therefore, assuming no unusual winter happens, and wolves still having a wide variety of thick and thin fur, but if female wolves only prefer to mate with individuals with thick fur, then all wolf populations may also evolve into thick-fur wolves. If there is a pattern, regardless of whether it is a mold created by the environment or the preferences of the females, evolution continues because of selection. The mold created by the females is the sexual selection as Darwin refers to.
We should talk a little bit about history, back in Darwin’s time, knowledge of genes and heredity was very poor. He invoked the “aesthetic in higher animals” that causes organisms to choose to mate with individuals possessing certain beauties to explain sexual selection, which is vague. As a result, his ideas were disregarded or even ridiculed for decades afterward. People talk a lot about natural selection, but rarely talk about sexual selection, because it is not believed that the fate of a species can be determined by the “will” of the females. But why not, because it is the females themselves that decide which males to mate with, that is, the selection element that holds the key factor in deciding which genomes will be passed on.
It was not until 1930 that sexual selection was taken seriously again after the statistician Ronald Fisher restated it based on a combination of Mendelian genetic knowledge and mathematics. Fisher’s work not only explains why many species evolve with a series of decorative accessories that are useless for survival, but also explains why males and females in species have different shapes, and sometimes as extreme as in birds.
He argues that phenotypic differences can arise randomly within populations through mutation and may or may not affect survival. Assuming it doesn’t affect survival, for example a peacock has a slightly longer tail, it will immediately attract the attention of other female peacocks because this male peacock relatively stands out to the rest of the population. This attraction need not come from the female peacocks viewing it as “beautiful” or consciously appreciating it. Since Fisher assumed that any difference enough to make comparisons with the rest would create prominence and trigger a process of sexual selection because females are very sensitive when comparing other males. Thus, this male peacock will be selected by a female peacock most sensitive to male differences (long tail feathers), and produce a generation of male peacocks like the father (with long tail) and mother-like females (sensitive to males with long tail feathers). Over time, both the female peacock’s selection bias and the male peacock’s tail feathers will be widely inherited and amplified until the behavior becomes widespread throughout the population. At this point, the peacock population will only include males with long tail feathers, and females prefer to mate with partners with colorful feathers. Thus, not only male and female peacocks are increasingly phenotypically different, but also determine reproductive behavior. This process will become increasingly extreme as long as it does not violate the interests of natural selection, i.e. the male peacock doesn’t have a tail too long to survive, or the female is strict enough to the point of refusing to mate. This whole process is called the “Fisherian runaway,” and Fisher has created a formula to show that the rate of phenotypic development increases exponentially, hence the massive plumage of birds with many layers and complex color schemes are not too difficult to understand, as long as it is within the limits of natural selection [2].
It should be noted that this whole process does not necessarily need to be triggered by a “conscious” cause, such as “female peacocks choose males with long tail feathers because this phenotype represents the reproduction ability and good genetic resources”, because how did the peacock have a basis to associate this with in the first place if peacocks did not have long tail feathers before? Fisher also emphasizes that the apparent advantage of the ornaments is not so important, that is, whether or not it is a proxy for fitness, but whether or not it does, has no effect on the Fisherian runaway, or at first it does, but this extreme evolutionary process will quickly lose its connection to that purpose. For example, a beautiful plumage may be a reliable indicator of fitness, but why does this plumage continue to evolve toward more and more massiveness just to perform the same old task? We can only explain this with a mathematical model based on statistical probability and the randomness of the Fisherian runaway.
You can understand the relationship between males and females like flowers and insects. In the beginning, flowers were pollinated by wind, but some were accidentally helped by insects and pollinated more efficiently. These flowers are more and more widespread and have evolved qualities to attract insects, while in the opposite direction, insects are more sensitive in identifying these flowers for the sake of nutrition. So the flowers become more and more colorful and fragrant. The same is true for male birds.
Following the view of Darwin and Fisher, in 2017, ornithologist Richard O. Prum published the book “The Evolution of Beauty” to describe more closely, in more detail the results of sexual selection and selection of the female in the process of shaping the beauty of the male. He also put more emphasis on random factors, as well as the irrational selection bias of sexual selection. In this book, Prum points out that the feathers of birds as they are today most likely date back to the time of the dinosaurs, which were originally intended to be “decorative” and their presence triggered the Fisherian runaway process to help modern birds acquire their full plumages with the ability to fly. Prum also says that physical disparities, especially beauty, don’t necessarily make the species or individuals better. Since evolution doesn’t have to be a ladder, it doesn’t lead to the best choice after all, and arbitrary sex selection makes things more complicated than ever.
For example, the female’s selection of males with certain characteristics may adversely affect her own offspring, but this is maintained as long as the decline is met by natural selection (i.e. capable of survival and reproduction). As in the Club-winged manakin, a small passerine bird, is evolving towards worse flight capability as the males develop wing structures suitable for dance but not for flight. The data suggest that the wing bones of this species in both males and females have been modified, as the females, through a process of their own ‘favorite’ selection, have made subsequent generations (including both males and females) have more wings unsuitable for flight. Females from the next generation will therefore have a harder time living, but their genes are still inherited because males of the same bloodline have a higher reproductive advantage. This could theoretically cause extinction, if not halted by natural selection because of cost asymmetries [3]. He also said that while wide hips in women are a good phenotype for fertility, people still like or dislike them today regardless of whether this advantage is known or not, and prefer other features regardless of whether they represent fertility or not. Prum says the evidence that wide hips are actually a reproductive advantage is frugal, and that surveys in men may be the result of teenagers mistakenly believing what was shown in the media is also a universal norm or human nature [4].
Prum’s book entered the top 10 best books of 2017 voted by the New York Times, as well as a finalist of the Pulitzer Prize for non-fiction, but has not been appreciated by colleagues in the world.
2. Several billion years ago
Although sexual selection can be purely coincidental, it can also originate from causal events. For example, in some fishes that live in low-light waters, the male possessing scales that are able to reflect light of certain wavelengths, make it easier to be seen by female fishes are also more likely to become selected and pass on their genes. Some birds have patterns and colors related to the chemical contents of a certain nut, so the size of the pattern also indicates their ability as well as the nutritional health of the male bird. Regarding the story of peacocks, Israeli biologist Amotz Zahavi’s “handicap principle” disagrees with Fisher, who believes that because the male peacock’s plumage brings so many disadvantages, it is an evidence shows that male peacocks must be very strong to survive at such a disadvantage, and that is a reliable signal because weak peacocks will not be able to imitate them [5] .
Going deeper into the principle of efficiency, Geoff Hill, an evolutionary biologist at Auburn University in Alabama, has suggested that the cause may even stem from the mitochondria — an internal component responsible for ATP synthesis (the energy for all living activities of organisms), through which beautiful feathered peacocks are peacocks carrying a cell system with effective mitochondria.
According to current theory, two billion years ago mitochondria were bacteria swimming freely in the primordial waters of a world full with other single-celled bacteria. Because all life at that time was unicellular, evolution was so slow that it was called “the most boring period of Earth’s history”, which lasted for billions of years (Boring Billion). But this boredom ended when a random event happened: a single-celled bacterium ate another, and the two went on to survive to develop into a primitive eukaryotic cell — the basis for all complex life forms today. After 4 billion years of living with boring competitive, cooperative relationships that couldn’t accelerate evolution, this random event finally opened the history of all life on Earth, with the formation of cell nuclei containing DNA. Inside this simple cell, same as in today’s complex cells, mitochondria (derived from a free bacterium) performed a single task: respiration to synthesize energy for the cell, using oxygen to burn food to make ATP.
ATP is the unit of energy that ensures all life. From increasing cell size, making proteins, copying DNA to growing a massive plumage, it all takes ATP. Thus, a peacock with a large plumage must have cost a lot of ATP, which means having an efficient mitochondrial system. The system must be so efficient that there is enough excess energy for a peacock to grow and maintain its plumage, while ensuring other functions more important for survival must still be maintained. Thus, Hill said, a female peacock, when selecting a male peacock with massive plumage, is selecting the gene that has built an efficient mitochondrial system. The males that spend a lot of energy growing feathers and dancing are trying to prove that they are willing to do so because they always have a lot of energy to spend, and the weaker ones cannot imitate — this conclusion is similar to “handicap principle”.
Hill’s theory is supported at the genetic level because (1) mitochondrial potential is heritable, and (2) unlike other species, male birds have more genetic influence on subsequent generations. Although mitochondrial inheritance is complex (the genetic information is so perturbed that it is impossible to determine whether the output will be positive or negative relative to the input), because the sex chromosomes in birds are ZZ (male) and ZW (female), instead of XX (female) and XY (male), just as Z is the larger sex chromosome, carrying more genes than W, so the offspring is always certain definitely inherit Z from its father [this knowledge can help in understanding Fisherian runaway]. In particular, there are at least three genes on the Z chromosome in birds that are involved in mitochondrial ATP production, suggesting that the inheritance of mitochondrial advantage may not be overly perturbed. However, this theory of Hill has not been tested, and although to date no evidence has been found to disprove it]. There has been a few more supporting pieces of evidence, but we will need some time to see if his theory is accepted. In addition, it should be noted that Hill’s theory does not show why this process happened in the first place, which has been well explained by Fisher, nor can it give a universal answer but just confined to birds while mitochondria are a universal subject in biology.
So, after all, what can humans learn from the lessons of the formation of beauty in living things?
The takeaway, up to this point, is that we don’t know for sure whether external beauty really represents core biological qualities. Is it a manifestation of good genetics, or is it simply a random selection?
In an interview with The Verge about his book “The Evolution of Beauty”, he said: “By reframing the biology of sex in terms of the subjective experience of individuals, I want to reframe in some sense the way in which we think about our own sexualities. People today, especially adolescents, are growing up in a culture in which these ideas have become so popular that they see each one of their individual flaws or variations as somehow an honest indicator of their true, objective quality. This is a tragedy because I think it affects how people think about themselves, that other people are actually in some way objectively genetically better than they are. That leads to anorexia, that leads to plastic surgery, it leads to all sorts of unpleasantness.”
Not to mention the complex cultural factors in human society that also play an important role in determining beauty, we need to know that even biologically, phenotypes that are considered beautiful are sometimes evaluated and selected very randomly, rather than a straightforward and objective measure of good qualities. We are forced to acknowledge beauty as an advantage, but we need to be careful when trying to figure out the rationale behind this appreciation, especially before ascribing this choice to objective or universal reasons.
Don’t underestimate the element of randomness and focus too hard on finding meaning where it doesn’t exist. Because even our origins stem from a very random event a few billion years ago, and natural selection itself also doesn’t have any specific reasons.
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References:
[1] “Triceratops | Description, Size, Fossil, Diet, & Facts | Britannica,” Encyclopædia Britannica. 2021, Accessed: Sep. 28, 2021. [Online]. Available: https://www.britannica.com/animal/Triceratops.
[2] “Runaway selection hypothesis | biology | Britannica,” Encyclopædia Britannica. 2021, Accessed: Sep. 28, 2021. [Online]. Available: https://www.britannica.com/.../runaway-selection-hypothesis.
[3], [4], [7] A. Chen, “How beauty might have evolved for pleasure, not function,” The Verge, May 19, 2017. https://www.theverge.com/.../richard-prum-evolution... (accessed Sep. 28, 2021).
[5] A. Zahavi, “Mate selection — A selection for a handicap,” Journal of Theoretical Biology, vol. 53, no. 1, pp. 205–214, Sep. 1975, doi: 10.1016/0022–5193(75)90111–3.
[6] Alex Riley, “Why do the most flamboyant males have the evolutionary edge? | Aeon Essays,” Aeon, 2016. https://aeon.co/.../why-do-the-most-flamboyant-males-have... (accessed Sep. 28, 2021).
Further Reading:
How Beauty Is Making Scientists Rethink Evolution — https://www.nytimes.com/.../beauty-evolution-animal.html
