From my genes to yours

Updated: Apr 30



Satisfaction in one’s marriage can have immense bearing on physical and emotional well-being, especially for women [1]. Finding the right partner becomes important if one wants to avoid all the symptoms of depression associated with a dilapidated marriage.


For millions of Indians like me, this is no cause for concern. Our parents do it for us. But if you are not Indian, or if you are one of those “progressive” Indians who wants to find a similarly progressive partner who stands for liberal principles, using your “free will”, it might be harder than you think.


Your parents are the earthly incarnations of the divine. Your grandparents are holier. And your dead ancestors are practically gods. Your earliest ancestors were… well… Nucleic Acid Replicators, probably similar to RNA that lived in a proteinaceous ocean called the primeval soup about 4 billion years ago.


Replicators competed with each other and the losers became food for superior replicators. Replicators then started cooperating with each other to produce procreating machines that competed on their behalf - unicellular and multicellular life forms. That’s us. These replicators (now genes) only make their presence known by producing proteins.


"...do not look for them floating loose in the sea; they gave up that cavalier freedom long ago. Now they swarm in huge colonies, safe inside gigantic lumbering robots, sealed off from the outside world, communicating with it by tortuous indirect routes, manipulating it by remote control. They are in you and in me; they created us, body and mind; and their preservation is the ultimate rationale for our existence. They have come a long way, those replicators. Now they go by the name of genes, and we are their survival machines.”
― Richard Dawkins, The Selfish Gene

In many animals, genes produce behaviours that are hard-wired, with little wiggle room. Here's a fascinating example.


Digger wasps are a species of burrow dwelling, solitary wasps that immobilize their prey using a paralyzing sting. The female wasps then carry their prey on long flights back home, where they become food for developing larvae. The wasps display a curious sequence of behaviour before they drag the prey into their burrow.


They place the prey near the mouth of the burrow → go inside the burrow to examine it → satisfied, come out again → drag the prey inside with them.


The French naturalist J. Henri Fabré discovered that if the prey (placed near the entrance) was moved a little bit from the original spot while the wasp was inspecting its burrow, the wasp did not drag it in. The drag into burrow behaviour is only triggered in the wasp when it has examined its burrow and finds the prey at the exact spot where she had left it.



"When the wasp operates its domiciliary visit, I take the Cricket (prey), abandoned at the entrance of the dwelling, and place it a few inches away. The wasp rises again, utters its usual cry, looks in astonishment here and there, and seeing its game too far, it comes out of its hole to go and grab it and bring it back to the desired position. This done, he goes down (the burrow) again, but alone. Same maneuver on my part, same disappointment from the wasp when it arrived. The game is still brought back to the edge of the hole, but the wasp always descends alone; and so on, as long as my patience is not worn out. One after another, forty times, I repeated the same test on the same individual; his stubbornness overcame mine, and his tactics never changed."
J. Henri Fabré on Fixed Action Pattern in the Digger Wasp, Souvenirs Entomologiques

In insects, where flexibility in behaviour is a luxury, single genes exercise more deterministic control over behaviour. This means that single gene variations can account for huge differences in behaviour. For example, fire ant colonies with one variant of the gene Gp-9 have a single queen, while those with another variant have multiple queens [4].


Human behavior is much more flexible and subject to cognitive control. In humans, genes have relinquished deterministic power to create a learning machine. Ape brains are uniquely evolved to rewire, and allow for significant modulations in behavior. Human behaviour is also complex and rarely controlled entirely by one or a few genes. Numerous genes happily work together to determine behaviour, each exerting a minute influence.



However, genes are control-freaks when it comes to essential behaviour like breathing and eating. It is too risky to leave these functions to the whims of their procreating machines.


Likewise, sex is the only mechanism by which genes perpetuate themselves. Since this behaviour is much too essential, they want a say in who we date (much like, you guessed it... Indian parents!).


Darwinian Beauty


In Mario Vargas Llosa's novel The Way to Paradise, the painter Paul Gauguin is beset by paradoxical notions of the good life and beauty. He leaves Paris and travels to French Polynesia, where he lives with Maori girls. His overindulgence with the Maori lifestyle is interspersed by his longings to go back to Paris. This incoherence is paralleled in the way he thinks about European and Maori women.


“Motherhood had made her hips broader and rounder, and lent her belly a majestic heaviness that made you think of the bellies and hips of Ingres’s languid odalisques, and the queens and goddesses of Rubens and Delacroix.”
“So smooth and white was the skin of the youthful face, so melting her blue gaze, and so graceful and fragile her little body, which radiated innocence and goodness…”
― Mario Vargas Llosa, The Way to Paradise

The conception of beauty is considered to be subjective and as the case of Paul Gauguin indicates, variable over time. But, the science of attraction suggests that there is method to the madness. People consistently rate faces with average features (for any given population) and faces that are more symmetrical as being more attractive [6]. This strange proclivity is best explained by the “good gene” hypothesis.


The hypothesis was famously used by William Hamilton and Marlene Zuk to explain mate selection in birds.


Hamilton and Zuk theorized that the elaborate plumage seen in many North American birds evolved as an indicator of parasite resistance genes. Since a healthy plumage means greater immunity (a trait much sought after), the plumage serves as a litmus test for fitness. Individuals can reliably choose a mate with superior parasite resistance using this proxy. Their landmark study found that ornamentation or showiness in birds correlated positively with incidence of blood infections (making mate choice based on ornamentation advantageous).


Facial symmetry and averageness of facial features seem to serve as indicators of good genes in humans. Fluctuation from perfect bilateral symmetry indicates an inability to deal with stress during development and exaggerated facial features could be a sign of inbreeding [5].


Genes might be influencing mate choice in other, more bizarre ways.


Parasite Matchmakers


The major histocompatibility complex (MHC) are a group of genes that help organisms distinguish between self and non-self (parasite) elements in its body and initiate effective immune responses. MHC has been known to influence mate preference in mice since the pioneering study by K Yamazaki and colleagues in 1976. They discovered that, given a choice of mating with a female with similar MHC and one with dissimilar MHC, the males have a preference for the latter. Mice discriminate MHC profiles using olfaction, although the exact mechanism remains a mystery. It has been suggested that MHC-dissimilar parents produce offspring with certain immunological advantages.


Animals and organisms that parasitize them are engaged in a perpetual evolutionary arms race. Each gene in the MHC provides the animal with limited resistance to a specific group of parasites. The parasites, in-turn evolve mechanisms to help them diminish or bypass such resistance.


Rare Allele Advantage


"It is an advantage to the individual to possess a rare biochemical phenotype. For just because of its rarity it will be resistant to diseases which attack the majority of its fellows."
- Haldane J. B. S., 1949, Disease and evolution.

Since parasites are more likely to adapt to the most common type of resistance present in a population, rare MHC genes have disproportionately higher value. But, as soon as a gene propagates within a population, the parasites evolve to counter its effects.


Heterozygote Advantage


"It is an advantage to a species to be biochemically diverse, and even to be mutable as regards genes concerned in disease resistance.....the biochemically mutable species will not remain in a condition where it is resistant to all the diseases so far encountered, but an easy prey to the next one."
- Haldane J. B. S., 1949, Disease and evolution.

Under pressure from rapidly evolving parasites, slow-evolving hosts might benefit from a diverse range of resistance genes. It has been suggested that having dissimilar alleles (heterozygous condition) could lead to increased immunological surveillance [Doherty et al., 1975]. Attraction to MHC-dissimilar individuals could thus have evolved as a way to preserve diversity of immunocompetence -- a larger number of parasites identified and resisted by the immune system. But, this might be an overly rosy view. In the immunological warfare, evolution on the part of parasites takes place at a higher rate giving them, the upper hand.


Just one side of the story: MHC-disassortative mating can produce offspring with resistance to a wider range of parasites (but, evolution of parasites is completely ignored)

In a widely cited paper by Dustin Penn and Wayne Potts, the authors proposed that the MHC based mating preference could produce offspring that are sufficiently different biochemically as to provide the parasites with a "moving target". Since parasites have evolved to undermine the parents' MHC profiles, they have to catch up to a different profile in the offspring. This mechanism offsets the speed of evolution advantage in parasites and could favour more diverse, new combinations in each generation.


Love at first whiff


There is now robust evidence for MHC-mediated mate selection in mice. If not due to the widely held belief that romantic attraction is more than biological, then the misconception that olfaction in humans is inferior to that in mice, similar studies on humans are likely to be treated with suspicion.


In the famous "sweaty t-shirt study", Claus Wedekind and colleagues asked women to rate the pleasantness of the odour of t-shirts worn by men. This study [13] and a larger study [14] published two years later found that women rate the scent of men with more dissimilar MHC (compared to themselves) as more pleasant.


A replication study by Randy Thornhill and colleagues found no such effect, but found preference in women for the scent of men with a more heterozygous MHC profile. A more provocative finding in this area of research came from a study by Christine Garver-Apgar and colleagues, which found that sexual responsivity in women towards their partners increased as the number of shared MHC alleles decreased. Also, the number of extra-pair sexual partners and attraction to men other than their partners increased with increase in the number of shared MHC alleles.


Mate choice in humans, as in most animals is influenced by genes in small, but important ways. The magic of love is just genes jinxing us for evolutionary bargains.

FURTHER READING

REFERENCES

  1. Proulx, Christine & Helms, Heather & Buehler, Cheryl. (2007). Marital Quality and Personal Well-Being: A Meta-Analysis. Journal of Marriage and The Family - J MARRIAGE FAM. 69. 576-593.

  2. G.R. Else (1997). "Philanthus triangulum (Fabricius,1775)". The Bees Wasps & Ants Recording Society.

  3. Park, Bokri & Kim, Yonggyun. (2012). Genome size estimation of an endoparasitoid wasp, Cotesia plutellae, using quantitative real-time polymerase chain reaction. Journal of Asia-Pacific Entomology. 15. 349–353.

  4. Krieger MJ, Ross KG. Identification of a major gene regulating complex social behavior. Science. 2002;295(5553):328-332.

  5. Rhodes, G. (2006). The Evolutionary Psychology of Facial Beauty. Annual Review of Psychology, 57(1), 199–226.

  6. Rhodes, G., Proffitt, F., Grady, J. M., & Sumich, A. (1998). Facial symmetry and the perception of beauty. Psychonomic Bulletin & Review, 5(4), 659–669.

  7. Hamilton WD, Zuk M. Heritable true fitness and bright birds: a role for parasites?. Science. 1982;218(4570):384-387.

  8. Yamazaki, K., Boyse, E. A., Miké, V., Thaler, H. T., Mathieson, B. J., Abbott, J., Boyse, J., Zayas, Z. A., & Thomas, L. (1976). Control of mating preferences in mice by genes in the major histocompatibility complex. The Journal of experimental medicine, 144(5), 1324–1335.

  9. Bodmer, W. F. (1972). Evolutionary Significance of the HL-A System. Nature, 237(5351), 139–145.

  10. Doherty, P. C., & ZinkerNAGEL, R. M. (1975). Enhanced immunological surveillance in mice heterozygous at the H-2 gene complex. Nature, 256(5512), 50–52.

  11. Penn, D. J., & Potts, W. K. (1999). The Evolution of Mating Preferences and Major Histocompatibility Complex Genes. The American Naturalist 1999 153:2, 145-164.

  12. McGann J. P. (2017). Poor human olfaction is a 19th-century myth. Science (New York, N.Y.), 356(6338), eaam7263.

  13. Wedekind, C., Seebeck T., Bettens F., & Paepke A. J. (1995). MHC-dependent mate preferences in humans. Proc. R. Soc. Lond. B. 260, 245–249.

  14. Wedekind C, Füri S. (1997). Body odour preferences in men and women: do they aim for specific MHC combinations or simply heterozygosity?. Proc Biol Sci.;264(1387):1471-1479.

  15. Garver-Apgar CE, Gangestad SW, Thornhill R, Miller RD, Olp JJ. Major histocompatibility complex alleles, sexual responsivity, and unfaithfulness in romantic couples. Psychol Sci. 2006;17(10):830-835.