- Aman Deep


Once there was a man who was living in a small town with his happy family. He was an avid reader and wanted to become the best father in the world. He had two curious kids. His innate desire was to give them the best upbringing in the world. Recently, he read about the concept of forest bathing in one of the books. Forest bathing is a Japanese practice of immersing oneself mindfully in a forest environment to promote relaxation and health. One day, he took his kids to the forest to reap the benefits of forest bathing. As they were enjoying the splendid beauty of the forest, one of his kids saw a beautiful peacock. The bright, heavy, and colorful tail caught the attention of a kid. He immediately asked his father, Dad, why do peacocks have extravagant features like bright colored tails that reduce their chances of survival? His father replied, "It's an interesting question." Even though natural selection is the deepest known theory that explains the variety we see around us. Still, it can't answer all the questions. Peacocks have bright colored tails due to sexual selection. Sexual selection! Can you please explain in detail?, asked the boy to his Dad.
Don't worry, my dear son, let's dive deep into evolution to understand this mystery.
Dad: Darwin realized that some traits can't be explained by natural selection. A peacock's tail is heavy, bright colored, and energetically costly. Natural selection should eliminate such traits, yet they exist. He proposed that sexual selection is one of the most powerful forces shaping life, responsible for many traits that otherwise seem useless, costly, or even harmful to survival. The fierce competition among males and the female preference for certain males are due to sexual selection.
Son: How do males evolve to display their traits like strength, wealth, intelligence, dominance, mate guarding, status seeking, etc., and concomitantly, females evolve to have a preference for certain males?
Dad: Two theories that answer your question are:
Parental Investment Theory
Good gene hypothesis
Parental Investment Theory: The different costs of reproduction create different strategies for males and females. Females produce a few expensive eggs, often invest heavily in their offspring, like gestation and nurturing. They have more to lose from mating with a poor-quality male. Therefore, females are choosy. On the other hand, males produce many cheap sperm, have more to gain by mating frequently. Hence, males compete for mating opportunities.
Good gene hypothesis: Females pick those males that indicate health, resistance to parasites, a strong immune system, etc. All these traits indicate good genetic quality. It's an indirect signal to females that tells them they have genes that help them to survive despite handicaps. These traits serve as honest signals that demonstrate only a strong male can afford such handicaps.
Son: Does the good gene hypothesis mean the same as the Handicap principle?
Dad: Yes, they are the same. Let me explain it in detail.
A trait that is costly to survive with is a reliable signal of genetic fitness, also known as the Handicap principle. The strong and healthy male can afford such handicaps as:
A giant peacock's tail
Bright color attracts predators
Big antlers that take huge energy to grow
These traits become a trustworthy indicator that males have good genes. It works because if the trait is not costly, a weak or unhealthy male could easily fake it. Only a high-quality male can pay for the handicap. Therefore, the handicap becomes the honest advertisement of fitness. Females prefer males who show that they can survive despite the burden. This ensures their offspring inherit the good genes of a male who can pay for the cost.
Sexual selection can override natural selection, leading to flamboyant traits. Female choice drives evolution as much as survival pressures do. Many human traits, such as creativity, intelligence, humor, etc., may function as a mental handicap costly signal of evolutionary fitness. The handicap idea becomes the foundation for explaining that human intelligence evolved partly as a sexual display.
Son: How do intelligence, creativity, humor, etc., evolve as a sexual display?
Dad: Ronald Fisher proposed a theory known as the Runaway sexual selection. It's an evolutionary process in which a female's preference for a trait and the male trait itself reinforce each other, causing the trait to become exaggerated over generations, sometimes to an extreme that harms survival.
Runaway sexual selection works as follows:
Females prefer a male with a slightly longer tail. These males get more mates and produce more offspring.
Their sons inherit the long tails, and their daughters inherit the preference for long tails.
The trait and the preference increase together. More females prefer long tails, and males evolve even longer tails. The preferences get stronger in each generation.
The feedback loop runs away. Each generation magnifies the trait until it becomes extremely exaggerated.
The trait can become maladaptive. The trait can grow so huge that it reduces survival. A tail so huge that it makes flying harder, colors so bright that they attract predators, and antlers so big that they break easily. But as long as sexual advantage(more mates) outweighs survival advantage, the trait continues to evolve. Sexual selection often overpowers natural selection. The same logic is extended to human traits like intelligence, creativity, humor, art, and music.
Son: Evolution works on the principle of least effort. Sex is extremely inefficient compared to asexual reproduction. In sex, an individual passes only half of their genes and has to find a partner. Mating exposes him to predators, energy, cost, and disease. Males contribute no resources in many species except sperm. In asexual reproduction, every individual can reproduce, passes all their genes, no need for mates and would double their growth rates. Cloning is superior. Why is it not preferred by Natural selection?
Dad: Haha! An interesting insight. Natural Selection favored sex because of the Muller Ratchet. It's an evolutionary concept that explains why asexual reproduction tends to accumulate harmful mutations and why sex and recombination help avoid this problem.
This idea was proposed by geneticist Herman Muller. In asexual organisms, harmful(deleterious) mutations accumulate irreversibly over generations because there is no recombination. Asexual organisms pass on their genome unchanged (except for new mutations). Once all members of the population acquire at least one harmful mutation, it becomes impossible to return to a "mutation-free" state. This irreversible accumulation of bad mutations is the ratchet -- it clicks forward but never backward.
In asexual reproduction, the harmful mutations can be added but can't be removed. Imagine an asexual population where the best genome has zero harmful mutations. Over time, new mutations appear randomly. Some individuals with perfect genomes are lost due to chance events like predation, accidents, and small population size. If all zero-mutation individuals disappear, the population's best is now one harmful mutation. This is the click of a ratchet.
When all 1-mutation individuals are lost, the best remaining genome has two mutations. The ratchet keeps clicking. When harmful mutations accumulate, fitness drops, fertility decreases, survival weakens, and extinction risk rises. It leads to genetic meltdown. Without a mechanism to remove mutations, asexual reproduction faces a chance of long-term extinction.
How does sexual reproduction solve this problem?
Sex involves recombination, which allows harmful mutations to be separated and offspring to inherit cleaner combinations. Natural selection helps eliminate individuals with heavy mutation loads. Through recombination, two individuals with some bad mutations can produce offspring with fewer harmful mutations. Sex can reverse the ratchet, but asexual reproduction can't.
Asexual reproduction is like buying 100 lottery tickets with the same number, but sexual reproduction is like buying 100 lottery tickets with different numbers. If the environment stays stable, clones outperform, and if it changes, genetic variability increases a species' chance that at least some offspring will survive. Asexual reproduction is efficient but fragile, and sexual reproduction is inefficient but resilient.
Sexual reproduction is very effective against parasites. A Red Queen principle in biology says species must constantly evolve to keep pace with their parasites, predators, and competitors. If a species stops evolving, it doesn't stay the same, but it falls behind and risks extinction. Sex exists because it helps organisms keep up in this endless evolutionary race.
Hosts and parasites chase each other in evolutionary cycles. Parasites adapt to exploit the host's most common genotype. That genome becomes vulnerable. Hosts that reproduce sexually produce new, rare genotypes. Parasites are less effective against an unfamiliar combination of genes. Selection favors hosts with rare combinations. Parasites evolve to catch up again. This arms race has no finish line but only constant motion. You have to keep running just to survive. This idea is extended to human progress also. You have to keep innovating to stay in the race. Change is the only constant.
Infidelity, violence, aggression, jealousy, infanticide, etc., are the evolved reproductive strategies of sexual selection. The reproductive success of males is limited by access to females, not resources. Females prefer to mate with the male having good genes, whereas the male wants to mate with as many females as possible to increase his numbers in the gene pool. Male-male competition can lead to lethal fights in animals for access to females. Natural selection often favors reproductive success over morality, niceness, and fairness. It creates psychological adaptations, not moral rules.
Son: Humans are far more intelligent than necessary to survive in the wild. Other primates survive with smaller brains. We spend enormous energy on growing huge brains. Brains are expensive. They burn glucose, require long childhoods, and complicate birth. Why did evolution favor such costly traits?
Dad: Human intelligence evolved partly to attract mates. Just as peacock tails are costly displays of fitness, mental skills signal underlying genetic quality. Creativity, humor, storytelling, music, and art are analogous to mental peacock tails. These skills are hard to fake and costly in terms of energy and learning time, making them an honest signal of fitness. Males who displayed these traits reproduced more successfully. Over generations, this led to ever more exaggerated mental abilities for males and preferences for females. It mirrors the Fisherian runaway process observed in mammals.
Conclusion: Humans spent a vast portion of their evolutionary history in the savannah, where certain behaviors were adaptive and helped our ancestors survive and reproduce. Today, our environment has changed dramatically, but many of those ancient behavioral tendencies remain. When we encounter behaviors we label as immoral, it is often because they are rooted in evolutionary adaptations that no longer fit modern life. Behaviors such as infidelity, jealousy, aggression, competition, and violence are not simply moral failings; they once served specific functions in our evolutionary past. We do not transcend these tendencies by suppressing them, but by understanding them. Viewing these behaviors as adaptive responses rather than personal flaws allows us to gain deeper insight into human relationships. This awareness gives us the ability to respond more consciously, build healthier relationships, and shape a more cooperative and compassionate society. By understanding where we come from, we can choose how we want to live now.
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Photo by Praswin Prakashan on Unsplash


