Biology of Sex

Alex Mills is Associate Lecturer in the Department of Biology at York University.

• List of FiguresList of BoxesPreface1. IntroductionKey themes1.1 The wider context: sex, gender, sexuality, and sexual orientation1.1.1 Assigned sex and gender dysphoria1.1.2 Sex as a biological construct and gender as a social construct1.2 A caveat about biological determinism and ideology1.3 The comparative approach1.4 Biology as a part of natural science1.5 Theories and the methodology of scienceSummaryFurther reading2. Sex and ReproductionKey themes2.1 Does sexual motivation reveal its "purpose"?2.1.1 Sex has diverse purposes through co-option2.1.2 Homosexuality could be one manifestation of co-option2.1.3 Sexual features can also be co-opted for non-social purposes2.2 Reproduction: sexual, asexual, and non-sexual2.2.1 Not all organisms reproduce sexually2.2.2 Non-sexual reproductions of human cells is through mitosis and cytokinesis2.2.3 Asexual reproduction in complex organisms2.2.4 Switching between parthenogenesis and sexual reproduction2.3 Sexual reproduction fundamentals2.3.1 Female and male: Eggs and sperm2.3.2 Primary versus secondary sex characteristics2.4 From fertilization to the production of offspring2.4.1 External versus internal fertilization2.4.2 Other variations in patterns of sexual reproductionSummaryFurther reading3 Sex and InheritanceKey themes3.1 How an Augustinian friar discovered the rules of sexual inheritance3.1.1 Mendel’s peas were a "friendly" study system3.2 Developing Mendel’s Law of Segregation3.2.1 The second-generation offspring showed a surprising pattern of inheritance3.2.2 The meaning of segregation in the making of gametes3.3 Mendel’s Law of Independent Assortment3.4 Updating Mendelian genetics3.4.1 Using modern terminology3.5 Applying Mendelian genetics to humans3.5.1 Mendelian inheritance of blood groups in humans3.6 Sex, DNA, and chromosomes3.6.1 Chromosomes are the hereditary material3.6.2 The human karyotype3.6.3 DNA is the genetic code3.6.4 How DNA encodes information3.7 What happens to chromosomes in the making of gametes?3.7.1 What is crossing-over?3.7.2 The two divisions of meiosis3.7.3 Sex and chromosomes that don’t crossover3.7.4 How meiosis relates to Mendel’s lawsSummaryFurther reading4. Sex and EvolutionKey themes4.1 A short history of evolutionary thought4.1.1 Evolutionary theory is a natural result of the Enlightenment4.1.2 Exploration, fossils, and a very old Earth also led to evolutionary theory4.1.3 Jean-Baptiste Lamarck proposed a mechanism for evolution in 18094.1.4 Charles Darwin and the voyage of the Beagle4.2 The principles of natural selection4.2.1 The argument for natural selection4.2.2 The concepts of fitness and adaptation4.3 DNA as an evolutionary legacy4.3.1 Mutations, and how alleles differ from one another4.3.2 Chromosomal mutations can also contribute to evolution4.3.3 Mutation rates in sperm and eggs4.4 Thinking of breeding groups as gene pools4.4.1 Human breeding groups can be thought of as gene pools4.4.2 The ABO blood group gene pool4.5 The evolutionary costs of sex4.5.1 The numerical argument that sex is costly4.5.2 Sexual reproduction also involves search costs4.5.3 Sexual reproduction can entail health, injury, and mortality costs4.6 The evolutionary benefits of sex4.6.1 Advantages associated with a diverse gene pool4.6.2 Inbreeding reveals the value of gene mixing through sex4.6.3 The Red Queen hypothesis and Muller’s ratchet4.7 The role of sex in the creation of species4.7.1 Two different patterns of species evolution4.7.2 How do gene pools become split at the start of speciation?4.7.3 Pre-zygotic and post-zygotic reproductive barriersSummaryFurther reading5. Sexual SelectionKey themes5.1 Sexual selection is a sub-category of natural selection5.1.1 Comparing examples of natural selection and sexual selection5.1.2 Re-formulating the natural selection argument for sexual selection5.1.3 Manifestations of sexual selection5.2 Why are females usually the "limiting sex"?5.2.1 Females usually invest more in reproduction5.2.2 Does inter-sexual selection produce payoffs?5.2.3 Does intra-sexual selection produce payoffs?5.3 How mate choice based on ornaments increases female fitness5.3.1 Genetic benefits in the runaway hypothesis: "Sexy sons"5.3.2 Genetic benefits in the good genes hypothesis: Better survival5.3.3 Benefits in the good resources hypothesis: Honest promises5.4 Sex role reversal5.4.1 Sex role reversal: Adjusting investments made by males and females5.5 Not all intra-sexual competition involves fighting5.5.1 Scrambles are a form of indirect competition5.5.2 Endurance and subterfuge are also forms of indirect competition5.5.3 Sperm competitionSummaryFurther reading6. Mating SystemsKey themes6.1 There are five major types of mating systems6.2 Monogamy6.2.1 Distinguishing among types of monogamy6.2.2 Hypotheses for monogamy6.3 Polygyny is the most common form of polygamy6.3.1 Two models for polygyny6.3.2 Human polygyny6.4 The benefits for females of mating with multiple males6.4.1 Non-monogamous females hedge their bets against infertility6.4.2 Non-monogamous females reap genetic benefits6.4.3 Non-monogamous females can reap direct benefits6.5 Polyandry6.5.1 Polyandrous mating systems are much less common than polygynous ones6.6 Polygynandry6.6.1 Reproductive skew6.6.2 Cooperative breeding6.7 Promiscuity6.7.1 Factors associated with promiscuity6.7.2 Scramble competitions are usually promiscuous mating systems6.7.3 "Lek polygyny" is a promiscuous system resembling hook-up cultureSummaryFurther reading7. Sexual ConflictKey themes7.1 Strategies for sexual success7.1.1 A comment on terminology7.1.2 Sexual strategies exist in the context of sexual conflict7.2 Realms of sexual conflict7.2.1 Sexual conflict before mating7.2.2 Sexual conflict during mating7.2.3 Sexual conflict after mating7.2.4 Sexual conflict during parenting7.3 Mating strategies can change with circumstances7.3.1 Making the best of things7.3.2 Life history theory argues for plastic mating strategies7.3.3 Sex allocation7.4 Same-sex parentingSummaryFurther reading8. Sex Determination and DifferentiationKey themes8.1 Are male and female bodies the only two options in sex determination?8.2 The familiar method of sex determination relies on X- and Y-chromosomes8.2.1 Using insects to discover the role of sex chromosomes8.2.2 The X- and Y-chromosomes in humans8.2.3 Little genetic differences between men and women, but big phenotypic differences8.2.4 The SRY gene and transcription8.3 Not all genetic sex determination relies on the XY system8.3.1 Genetic sex determination without Y-chromosomes8.3.2 Are females ever the heterogametic sex?8.3.3 Chromosomal sex determination where hermaphroditism is a phenotype8.3.4 Genetic sex determination in honeybees8.4 Environmental sex determination8.4.1 Temperature is a non-social sex-determination system8.4.2 Environmental sex determination can override genetic sex determination8.4.3 Can mothers use temperature to select their offspring’s sex?8.4.4 Social systems of environmental sex determination8.4.5 Anomalous sex determination caused by a member of a different species8.4.6 Anomalous sex determination by ecotoxinsSummaryFurther reading9. Human Sexual Anatomy and RegulationKey themes9.1 Human sexual differentiation and function is highly dependent on hormones9.1.1 The endocrine system regulates hormones in the bloodstream9.1.2 Major hormones that regulate sexual development, function, and behavior9.2 Sex differentiation is part of development9.2.1 Human sexual differentiation before birth9.3 The male reproductive system9.3.1 Further male differentiation during the prenatal period9.3.2 Male sexual development at puberty9.3.3 Sexual anatomy of the human adult male9.4 The female reproductive system9.4.1 Further female differentiation during the prenatal period9.4.2 Female sexual differentiation at puberty9.4.3 Sexual anatomy of the human adult female9.5 Anomalous sexual phenotypes in humans9.5.1 Consequences of anomalies in the sex chromosomes9.5.2 Intersex conditions that result from dysfunctional alleles9.5.3 When girls become men at puberty9.6 Sex linkage: Why some genetic disorders occur mostly in males9.7 Cancers of sexually differentiated organs and tissues9.7.1 Cancers associated with male organs9.7.2 Cancers associated with female organsSummaryFurther reading10. Human Fertility and BirthKey themes10.1 Key differences between making sperm and eggs10.1.1 Spermatogenesis occurs from puberty to old age10.1.2 Oogenesis occurs from the fetal stage to menopause10.2 The menstrual cycle10.2.1 Most mammals have an estrous cycle instead10.2.2 Is women’s fertile period concealed?10.3 Sexual arousal and response in men and women10.4 Fertilization and the making of a zygote10.4.1 Infertility10.4.2 Contraception10.5 Pregnancy10.5.1 The placenta10.5.2 Labor and delivery10.6 Breast-feeding10.7 Menopause10.8 Sexually transmitted infections10.8.1 A diversity of organisms cause STIs10.8.2 Major bacterial STIs10.8.3 Major STIs caused by viruses10.8.4 Non-humans suffer sexually transmitted infections tooSummaryFurther readingGlossaryIndex