Book summary “Evolution's Rainbow”: summary

Introduction

You live in a culture that often views biological sex and gender diversity as modern deviations from a natural binary. When you look at standard evolutionary biology textbooks, you find an assumption that males are naturally promiscuous and females are naturally choosy. This framework makes it difficult for you to understand the vast spectrum of sex roles, same-sex sexuality, and gender expressions found across the natural world and human history. You might wonder why conventional science labels these diverse expressions as anomalies or evolutionary mistakes.

Joan Roughgarden challenges this entire paradigm by demonstrating that biological diversity in gender and sexuality is a normal, adaptive feature of life. Biological diversity in gender and sexuality is a natural, adaptive norm across animals and humans that invalidates rigid sexual selection and medical pathologization. You will discover a new evolutionary framework called social selection theory that replaces traditional views on competition and mate choice. The following pages explore animal rainbows, the true function of same-sex behavior, human neurobiology, medical definitions of normality, and global cultural variations.

• How does social selection theory rewrite our understanding of animal societies?
• Why do animals engage in same-sex matings that do not produce offspring?
• What do human brains and hormones actually reveal about gender binaries?
• How does medicine transform natural biological variations into diseases?

1. Challenging Darwinian Sexual Selection

Darwin claimed that males and females follow universal templates where males are passionate and females are coy. Joan Roughgarden argues this theory is inaccurate in detail and inadequate in scope, serving primarily to validate male philandering, aggression, and genetic elitism, and should be replaced.

Darwin's sexual selection theory fails to explain real-world animal diversity and relies on rigid universal templates.

The peacock is noted as a species where males have showy ornaments directly used in courtship, representing the narrow class of species where sexual selection might apply. Social selection theory replaces sexual selection by viewing animals as participating in a biological labor market for mutual assistance. This new theory proposes that animals evolve social-inclusionary traits to gain entry into groups that control resources for reproduction and safe living spaces. Reproductive opportunity functions as a currency within this market, explaining traits not covered by traditional sexual selection.

The penis of female spotted hyenas is cited as a social-inclusionary trait possessed exclusively by females and left unexplained by standard theories.

Undergraduate premedical programs currently focus on organic chemistry, biochemistry, genetics, and cell biology, limiting students' exposure to only 7 species: the limited set of organisms on which most medical laboratory studies and professional training are based. This narrow training leaves physicians unequipped to handle natural biological diversity, causing them to treat normal variations in human anatomy and sexuality as pathological disorders. A young doctor might experience undue alarm upon encountering a baby with unusual genital plumbing, a masculine woman, a feminine man, or a patient with a same-sex partner, even though such phenomena are common across vertebrate biology.

To reform this training, you must implement specific educational changes:

• Require instruction in biological diversity, particularly in gender and sexuality, in the undergraduate curriculum for premedical students.
• Enforce this requirement as a condition for medical school admission.

2. Animal Rainbows and Social Selection

Social selection theory replaces sexual selection by viewing animals as participating in a biological labor market for mutual assistance and resource sharing.

Many animal species feature multiple genders within a single biological sex. Morphological differences dictate distinct life histories and behaviors. You can observe these variations through specific traits:

• Size
• Color
• Posture
• Lifespan
• Real estate preferences
• Parental care

Plainfin midshipman fish have two male genders: large territorial singers and small silent sneakers. Large males possess sound-producing muscles that are six times larger in mass than those of small males. Their Z-lines are twenty times wider. A large male can produce a low humming sound for a maximum duration of fifteen minutes.

Bluegill sunfish possess three distinct male genders and one female gender. Spawning males consist of large controllers, small end-runners, and medium cooperators. Rather than being deceived by female-colored medium males, large controllers employ them as cooperators or marriage brokers to help facilitate mating.

Bluegill sunfish studied at Lake Opinicon in Ontario and Lake Cazenovia in New York show that 85 percent of spawning males are either small or medium, while 15 percent are large controllers. A large male is 17 centimeters long and eight years old. A medium male is 10 centimeters long and four years old. A small male is 7 centimeters long and three years old. A female bluegill sunfish is 12 centimeters long and six years old.

During a one-day spawning episode, a large male accumulates 30,000 eggs. Territory structures vary significantly across the population. Eighty-five percent of territories consist of one male with one female, 11 percent consist of two or more males and one female, and 4 percent consist of one male and two females.

Barry Sinervo and colleagues tracked population density cycles and offspring quantity/quality games among male lizards in California rocky outcroppings. Male side-blotched lizards exhibit a rock-paper-scissors dynamic of alternative mating strategies driven by natural selection and density cycles. Three distinct male throat-color phenotypes—orange, blue, and yellow—engage in a cyclical frequency-dependent game where each strategy beats one and loses to another. The carrying capacity figures for orange and yellow females are 0.70 and 1.18 respectively.

The traditional biological concept of female mimicry and deceit in animals is an unfounded myth driven by human prejudice. Labeling feminine males as deceptive she-males or sexual parasites relies on implausibly low cognitive assumptions about dominant males and ignores clear evidence of cooperation.

Studies on red-sided garter snakes in Manitoba showed that 20,000 snakes gather in a single den during the winter. The ratio of males to females near den entrances after emergence is 10 to one, and 15 percent of 200 mating balls consist of a perfumed male surrounded by courting males. All males carry female lipid perfumes upon emerging from winter dens, engaging in group sex and cooperative socialization rather than deceit.

3. Same-Sex Sexuality and Relationship Management

Traditional evolutionary theory dismisses homosexuality as an impossibility because it yields no offspring. You will find that evidence shows animals use sexual contact to maintain social bonds, reduce tension, and establish coalitions. Mating is not restricted to between-sex encounters.

Mating and breeding are frequently decoupled in nature, as same-sex matings serve to manage and negotiate social relationships rather than merely transfer sperm for reproduction.

A comprehensive review published in 1999 documented same-sex courtship and genital contact in over three hundred species of vertebrates across 751 pages.

All-female asexual whiptail lizard species engage in elaborate same-sex courtship rituals that synchronize hormonal cycles and significantly increase reproductive output. Despite reproducing clonally without males, asexual whiptail lizards perform intricate sexual rituals where one female assumes a male role with high progesterone and the other a female role with high estradiol, alternating roles as their hormone cycles switch.

Asexual female whiptail lizards housed with another female whose hormonal state leads to male behavior laid an average of 2.6 batches of eggs per season, compared to only 0.9 batches when housed alone. In nature, these asexual females lay 2.3 batches of eggs on average each season.

Almost all male bighorn sheep participate in homosexual courting and copulation, adopting lordosis postures identical to females. Conversely, males that live exclusively with ewes and refuse male mounts are labeled aberrant and subject to endocrinological scrutiny. Male bighorn rams grow up to a weight of 300 pounds, and rams cost between 350 to 4,000 dollars.

Researchers at the U.S. Sheep Experimental Center in Idaho coined the term dud stud for domestic rams attracted exclusively to other rams, testing them in stanchions to determine sexual preference. A specific domestic sheep study found that 8 out of 94 males tested as gay.

Bonobos utilize frequent same-sex and between-sex sexual contact, including genito-genital rubbing, to facilitate food sharing, resolve conflicts, integrate newcomers, and form female coalitions. Bonobo females maintain strong friendships and control access to resources through genital rubbing. This high degree of female cooperation and reduced threat from males leads to an earlier age of first reproduction and higher lifetime reproductive success.

Bonobos engage in sexual contact before dining on figs, after capturing prey, or when introduced to a new cardboard box toy, using sex to prevent squabbling and facilitate sharing. Each female participates in GG-rubbing once every two hours or so, and a sexual encounter lasts 10 to 15 seconds. The Pan and human lineages split apart 8 million years ago.

4. Human Brains, Hormones, and Genetics

Human biological diversity spans overlapping neural clusters, shared hormonal suites, and complex genetic networks that subvert the traditional gender binary.

Historical attempts to define male and female purely through distinct sex chemicals failed when female hormones were found in male urine and male hormones induced female traits. Both sexes synthesize all sex hormones from cholesterol, and the final physiological effect depends on both the hormone level and the availability of cellular receptors.

Testosterone transplanted into female rabbits induced uterine growth, and stallions were found to carry large amounts of female hormone. An 1939 authoritative summary asserting two distinct sets of sex hormones followed earlier 1927 female hormone extraction from pregnant women's urine and 1931 male hormone extraction from men's urine.

Human brain anatomy exhibits massive overlap between the sexes, with only minor, statistically valid differences in specific nuclear clusters and cell packing. Unlike birds, human male and female brain anatomies are remarkably similar, resembling two heavily overlapping bell curves. Minor differences include slightly larger total brain size in males, neuron density variations in the cerebral cortex, and microscopic cell clusters like the SDN-POA, BSTc, and VIP-SCN in the hypothalamus.

Adult males have a total brain size 120 to 160 grams larger, with a 5 percent difference in brain weight by sex in newborns of equal weight. The SDN-POA in the human hypothalamus contains about 5,000 cells at birth in both sexes, peaking at 50,000 by age four, and declining in females to about 25,000 by adulthood. Other structures show distinct proportions:

• The male BSTc measures 2.5 cubic millimeters with 35,000 cells versus 1.75 cubic millimeters with 20,000 cells for the female.
• The male VIP-SCN contains 2,500 cells versus 1,000 cells in the female.
• The cerebral cortex holds 115,000 neurons per cubic millimeter in males versus 100,000 in females.

Postmortem neurobiological studies reveal that male-to-female transsexual individuals possess neuron numbers in specific limbic brain nuclei that align with female typical values. Research led by Zhou, Swaab, and Kruijver examined the bed nucleus of the stria terminalis and other hypothalamic structures, demonstrating neuroanatomical correlates of gender identity that correspond with the experienced gender rather than the birth sex.

Kruijver and Swaab analyzed postmortem brain tissue from male-to-female transsexuals and found female typical neuron numbers in the central division of the bed nucleus of the stria terminalis. This work appeared in volume 85 on pages 2034 through 2041 of the Journal of Clinical Endocrinology and Metabolism in 2000.

Data from the Netherlands Brain Bank show that the BSTc in transgender women matches the size and neuron count of nontransgender women, while a transgender man falls squarely in the male range, pointing to a neurobiological basis established during early brain development.

A reference collection of thirty-four preserved brains at the Netherlands Brain Bank was analyzed across heterosexual men, homosexual men, heterosexual women, and transgender individuals. Straight males exhibit a BSTc relative size 150 percent larger than females, measuring 2.5 cubic millimeters with 33,000 neurons compared to 1.75 cubic millimeters with 19,000 neurons in straight females. The analysis included six brains of transgendered women and one brain of a transgendered man.

5. Disease versus Diversity in Medicine

Medical dictionaries fail to scientifically define the concepts of normal and function, which allows societal norms and social values to masquerade as objective medical science.

Traditional medical definitions of disease rely on impairment of a normal state and interrupted function. They fail to establish cutoff points for how common a trait must be. They also fail to observe how people actually function in their natural environments rather than a doctor office.

In biological science, a true genetic defect must meet two strict criteria. It must be extremely rare through mutation-selection equilibrium. It must also be deleterious under all conditions.

Traits that are advantageous in some contexts or relatively common cannot be classified as genetic defects. A threshold frequency of 1 in 50,000 is used as the convention for a trait causing a 5 percent reduction in Darwinian fitness.

Aggregated clinical data compiled by Blackless and colleagues demonstrate that the total frequency of individuals born with intersex anatomical traits is substantially higher than traditional medical dogmas suggest. By reviewing anomalies across chromosomal, gonadal, and genital traits, researchers calculated that non-normative sexual anatomy occurs in approximately one percent of live births. This synthesis reviewed specific anatomical variations showing a reported frequency rate of 0.5797 per 1,000.

Medical practices regarding intersex infants rely on social panic to force immediate surgical interventions and gender assignments that violate children rights. The birth of an intersex child is treated as a social emergency by multidisciplinary medical teams who measure phallus size with devices like phallometers and perform irreversible cosmetic surgeries.

The Intersex Society of North America created a phallometer tool where a phallus length under 1.5 centimeters leads to immediate, irreversible female assignment. Typical male penis length at birth ranges from 2.5 to 4.5 centimeters, while phallus measurements less than 1.5 plus or minus 0.7 centimeters lead to female rearing.

When you evaluate medical protocols for intersex infants, you must recognize specific clinical interventions:

• Measure the phallus size of a newborn during a simulated erection.
• Compare the measurement against established millimeter cutoffs on a phallometer rule.
• Assign the infant sex based on the phallus length and surgically trim or sculpt the genitalia accordingly.

Transitioning requires navigating medical protocols, social coming out, and the crucial requirement of passing for safety and productive living. The transition process involves working with therapists, taking hormones, and undergoing a real-life test. Passing is essential to avoid public hostility, harassment, or attack, though it forces you to weigh heavy personal losses against gains.

A transgendered man set up an appointment with his boss, explained his gender dysphoria diagnosis, and successfully outlined the Standards of Care to retain his job.

When you pursue clinical transition steps, you follow structured guidelines:

• Work with a behavioral therapist for at least three months under the Standards of Care protocol.
• Obtain a referral from the therapist for a physician to prescribe hormones.
• Live full-time in the gender of identity during a one-year trial period called the real-life test.
• Apply for sex reassignment surgery upon referral by two behavioral therapists after the trial period.

6. Cultural Rainbows and Transgender Lives

Human cultures across history and geography exhibit a wide range of gender and sexuality variations, from Native American Two-Spirits to Indian hijras. Different societies accommodate gender variation through distinct institutionalized roles. Native Americans identify individuals as two-spirits, Polynesia has the mahu experiencing tension with Western transgender concepts, and India hosts over one million hijra within a total population of one billion Indians. European history also records transgender figures like Cybelean priestesses, transvestite saints, and Joan of Arc.

A comprehensive transgender political agenda outlines the fundamental rights and social inclusions required for transgender individuals to live safely and productively.

The author outlines six points of the trans agenda focused on social normalization, unfiltered narratives, bodily dignity, legal protections against hate crimes, equal access to social institutions, and comprehensive healthcare coverage for gender-transition services:

• Be cherished as a normal part of human diversity.
• Demand the freedom to offer unfiltered narratives and own voices.
• Be treated with courtesy, dignity, and respected as people rather than bodies.
• Demand that the killing of transgendered people stop by extending anti-hate crime legislation.
• Demand equal participation in public social institutions including employment, education, housing, marriage, adoption, military service, and religious life.
• Demand full-service health plans to cover gender-transition medical services similar to pregnancy benefits.

Anthropological studies by Will Roscoe and Walter Williams document historical figures such as Osh-Tisch among the Crow and the Navajo weaver and healer Klah, illustrating how indigenous societies accommodated third and fourth genders. Walter Williams and Will Roscoe chronicled the life of Osh-Tisch, a Zuni or Crow Two-Spirit individual celebrated for bravery and traditional craftsmanship.

Native American tribes practiced public transition ceremonies that allowed communities to endorse and bless young two-spirited individuals based on spiritual inclination. The Navajo held a ceremony where a youth stood in a circle and danced like a woman upon hearing hidden songs, while the Papago asked a youth to enter a burning brush enclosure and choose between a bow and arrow or a basket.

Youths participated in structured public transition rites:

• Youth is led into a public circle.
• Hidden singer begins performing songs.
• Youth dances with intensity if two-spirited.
• Youth is bathed and receives feminine clothing.
• Youth announces new feminine name to the crowd.

Hijras in India constitute a traditional institutionalized third-gender community of individuals who undergo ritual castration and fulfill ceremonial roles during births and marriages. Ethnographic research by Serena Nanda and Kira Hall explores the social organization, linguistic practices, and political emergence of hijras, highlighting both traditional reverence and contemporary social marginalization. Anthropologist Serena Nanda recorded oral narratives from prominent hijras such as Meera, Sushila, and Salima regarding their community life and spiritual traditions.

The hijra genital surgery known as nirvan is a rational, highly beneficial choice that provides bodily feminization, social authenticity, and spiritual power. Pre-operative hijras view their male genitals as useless. The surgery allows them to align their bodies with their female gender identity, fulfill husbands' expectations, and gain the authoritative right to bless in the name of the Goddess. Kamladevi paid her guru Meera a massive bride-like fee of 27 saris, 20 petticoats, 27 blouses, 2 dance dresses, 1 tin box, 9 stone nose rings, and 200 rupees paid by Kamladevi for her operation to undergo the nirvan operation.

Candidates for this transformation follow precise community rites:

• Candidate is apprenticed to a guru and sponsored for initiation.
• Dai ma or midwife performs the surgery by removing testicles and penis with diagonal cuts.
• Patient undergoes a six-week convalescence period.
• Initiate is reborn as a true hijra and disciple of the Goddess.

Conclusion

You can now view biological diversity in gender and sexuality as an adaptive norm across nature rather than a deviation from a universal rule. The biological, psychological, and cultural threads fit together to show that variation is fundamental to animal and human societies. You must apply this unified framework of natural diversity to reform rigid institutional practices.

1. Abandon rigid sexual selection models in favor of social selection theories that account for cooperation and mutual benefit.
2. Separate mating from breeding when evaluating animal behavior and recognize the social functions of same-sex interactions.
3. Treat human gender and sexual variations as normal genetic and neurological diversity rather than medical pathologies.
4. Reject the medical pathologization of intersex and transgender individuals by demanding scientific definitions of normal biological function.
5. Respect historical and cross-cultural gender categories such as Two-Spirits and hijras as legitimate expressions of human organization.

10 Key Ideas

1. Biological diversity in gender and sexuality is a natural, adaptive norm across animals and humans that invalidates rigid sexual selection and medical pathologization.
2. Social selection theory replaces sexual selection by viewing animals as participating in a biological labor market for mutual assistance and resource sharing.
3. Many animal species feature multiple genders within a single biological sex, exhibiting morphological differences that dictate distinct life histories and behaviors.
4. Mating and breeding are frequently decoupled in nature, as same-sex matings serve to manage and negotiate social relationships rather than merely transfer sperm for reproduction.
5. Human biological diversity spans overlapping neural clusters, shared hormonal suites, and complex genetic networks that subvert the traditional gender binary.
6. Aggregated clinical data compiled by researchers demonstrate that the total frequency of individuals born with intersex anatomical traits is substantially higher than traditional medical dogmas suggest.
7. Medical dictionaries fail to scientifically define the concepts of normal and function, which allows societal norms and social values to masquerade as objective medical science.
8. Human cultures across history and geography exhibit a wide range of gender and sexuality variations, from Native American Two-Spirits to Indian hijras.
9. The hijra genital surgery known as nirvan is a rational, highly beneficial choice that provides bodily feminization, social authenticity, and spiritual power.
10. A comprehensive transgender political agenda outlines the fundamental rights and social inclusions required for transgender individuals to live safely and productively.