The Two Assumptions of Heritability Studies

Evolution cannot occur without at least some traits being heritable. Heritability is a measure of parent-offspring resemblance; so a trait with high heritability has a high genetic component to it, right?
No.

This is one strange and paradoxical thing about heredity studies: heritability is defined as something specific to a certain population in certain circumstances. It is neither a property of an individual nor a property of a trait.

In parent-offspring correlation studies, heritability is simply the slope that measures the resemblance between the average trait value of the two parents and the trait value of the offspring. In twin studies, heritability is the correlation between the trait values of the twins.

From my reading about scientific and philosophical articles on heritability, I figured that two assumptions are implied in those studies without even being mentioned:
(1) Parents and offspring always resemble, instead of differ from, each other. (In other word, the slope of heritability on the parent-offspring graph is always positive.)
(2) Heritability can never equal to zero. This is because of how the math works out. Heritability is a correlation: a covariance over the product of two standard deviations. Covariances and standard deviations are always positive numbers.

Think about it: pick any measurable trait, its heritability will be a non-zero positive number, even if in reality it does not have genetic component. So what does heritability really tell us?

What would our evolution looks like if parents and offspring always tend to differ, not resemble, each other? Would we evolve backwards? Within one generation, the evolutionary response in the offspring will shift in the opposite direction as the selective pressure; however, the response will be cancelled in the grand-offspring generation under the same selective pressure. Just thinking about this weird scenario gave me a giggle fit in my philosophy-biology class.

The Third Eye of Lizards and Humans

How Humans Lost Our Chance at a Third Eye 

"The chemicals used for light perception in the lizard’s third eye are structurally similar to the psychedelic molecules humans use to open their 'third eye'."

Rita Chen

November 5, 2012

Seraphic Transport Docking on the Third Eye, 2004, Alex Grey

“Opening the third eye” is a cliché slogan and a concept of psychedelic users to describe the extended sight and mental perception under the influence of psychedelics that are beyond the normal state of mind. The phrase might be borrowed from Hinduism, in which the third eye is called ajna chakra and is believed to be positioned in the brain, between the eyebrows. The third eye, an asymmetrical structure, is usually not seen in bilaterians. Nevertheless, a news story published in September, 2012 talked about some species of lizards have a third eye, a little dot on the top of their heads, called the parietal eye. The chemicals that these lizards use to perceive light in the third eye are similar to some of the psychedelic drug molecules, which suggests a connection between the slogan and this ancient body structure.

A tuatara. The little dot on the top of its head is the parietal eye, the "third eye" of this species.

The “lizard species” with the third eye such as tuatara found in New Zealand are actually not lizards in the phylogenetic sense. The parietal eye is not as complex as the other two eyes, but does react to light. Evidence suggests that the structure is present in a shared ancestry of today’s mammals and reptiles. While some lizard species have the parietal eye, it is lost in most extant tetrapods. The development of a third eye is asymmetrical. In those reptiles, the left side of the brain becomes the parietal eye, while the right side of the brain becomes the pineal sac, which secretes melatonin that responds to light.

The position of the pineal gland relative to other brain structures in humans.

In humans, the homology to the pineal sac is the pineal gland, which stays down with the rest of the brain instead of situated at the top of the head. The pineal gland also secretes melatonin, the hormone that regulates biological rhythms. Melatonin is a derivative of serotonin, the major neurotransmitter responsible for the feeling of well-being, happiness, and normal sleeping patterns. The pineal gland secretes some serotonin.

The tryptamine psychedelics are structurally similar to serotonin and melatonin, and act as serotonin agonists when they enter the brain. Examples of naturally-occurring psychedelic tryptamines are DMT, 5-MeO-DMT, psilocin, psilocybin, and bufotenin. Except producing hallucinations, dissociation, and changes in mood and behavior, psychedelia is often associated with extended perception beyond the normal sight and spiritual experiences. According to the news story, because the pineal gland in humans has evolved to maintain a healthy state of mind, instead of light perception, having a third eye could be “at the expense of being able to sleep, be happy, and keep mentally fit.” Ironically indeed, while people attempt using psychedelics to achieve spirituality and see what is beyond the normal two eyes can see, their time and light perception is severely distorted, their mood can be affected in positive or negative ways, and even have a small chance to trigger underlying psychosis. People on psychedelics often see objects becoming more colorful, lights coming out of nowhere, or glowing objects. Since time perception is changed, the trip could feel much longer than it is. An extreme story is that a person feels that he has gone through a life-time experience in a bizarre, alternate universe during a 15-minute DMT trip.

Melatonin (N-acetyl-5-methoxytryptamine), the hormone secreted by the pineal gland to regulate biological rhythm, is a derivative of serotonin.

Serotonin (5-hydroxytryptamine), the neurotransmitter responsible for the feeling of well-being, happiness, and normal sleeping patterns.

Psilocin (4-hydroxy-dimethyltryptamine), a psychedelic tryptamine that is closely related to serotonin, occurred naturally in many species of mushrooms.

The chemicals used for light perception in the lizard’s third eye are structurally similar to the psychedelic molecules humans use to open their “third eye”. However, the pineal gland and melatonin have evolved to serve different functions in lizards and humans. Perhaps, “opening the third eye” is not merely gibberish spoken by people who are high, or a coincidence between the Indian and the Western interpretation on an altered state of mind, but a deep, ancient connection between the physiological structures of related lineages with an evolutionary explanation.