Biology of Sex

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

List of Figures
List of Boxes
Preface

1. Introduction
Key themes
1.1 The wider context: sex, gender, sexuality, and sexual orientation
1.1.1 Assigned sex and gender dysphoria
1.1.2 Sex as a biological construct and gender as a social construct
1.2 A caveat about biological determinism and ideology
1.3 The comparative approach
1.4 Biology as a part of natural science
1.5 Theories and the methodology of science
Summary
Further reading

2. Sex and Reproduction
Key themes
2.1 Does sexual motivation reveal its “purpose”?
2.1.1 Sex has diverse purposes through co-option
2.1.2 Homosexuality could be one manifestation of co-option
2.1.3 Sexual features can also be co-opted for non-social purposes
2.2 Reproduction: sexual, asexual, and non-sexual
2.2.1 Not all organisms reproduce sexually
2.2.2 Non-sexual reproductions of human cells is through mitosis and cytokinesis
2.2.3 Asexual reproduction in complex organisms
2.2.4 Switching between parthenogenesis and sexual reproduction
2.3 Sexual reproduction fundamentals
2.3.1 Female and male: Eggs and sperm
2.3.2 Primary versus secondary sex characteristics
2.4 From fertilization to the production of offspring
2.4.1 External versus internal fertilization
2.4.2 Other variations in patterns of sexual reproduction
Summary
Further reading

3 Sex and Inheritance
Key themes
3.1 How an Augustinian friar discovered the rules of sexual inheritance
3.1.1 Mendel’s peas were a “friendly” study system
3.2 Developing Mendel’s Law of Segregation
3.2.1 The second-generation offspring showed a surprising pattern of inheritance
3.2.2 The meaning of segregation in the making of gametes
3.3 Mendel’s Law of Independent Assortment
3.4 Updating Mendelian genetics
3.4.1 Using modern terminology
3.5 Applying Mendelian genetics to humans
3.5.1 Mendelian inheritance of blood groups in humans
3.6 Sex, DNA, and chromosomes
3.6.1 Chromosomes are the hereditary material
3.6.2 The human karyotype
3.6.3 DNA is the genetic code
3.6.4 How DNA encodes information
3.7 What happens to chromosomes in the making of gametes?
3.7.1 What is crossing-over?
3.7.2 The two divisions of meiosis
3.7.3 Sex and chromosomes that don’t crossover
3.7.4 How meiosis relates to Mendel’s laws
Summary
Further reading

4. Sex and Evolution
Key themes
4.1 A short history of evolutionary thought
4.1.1 Evolutionary theory is a natural result of the Enlightenment
4.1.2 Exploration, fossils, and a very old Earth also led to evolutionary theory
4.1.3 Jean-Baptiste Lamarck proposed a mechanism for evolution in 1809
4.1.4 Charles Darwin and the voyage of the Beagle
4.2 The principles of natural selection
4.2.1 The argument for natural selection
4.2.2 The concepts of fitness and adaptation
4.3 DNA as an evolutionary legacy
4.3.1 Mutations, and how alleles differ from one another
4.3.2 Chromosomal mutations can also contribute to evolution
4.3.3 Mutation rates in sperm and eggs
4.4 Thinking of breeding groups as gene pools
4.4.1 Human breeding groups can be thought of as gene pools
4.4.2 The ABO blood group gene pool
4.5 The evolutionary costs of sex
4.5.1 The numerical argument that sex is costly
4.5.2 Sexual reproduction also involves search costs
4.5.3 Sexual reproduction can entail health, injury, and mortality costs
4.6 The evolutionary benefits of sex
4.6.1 Advantages associated with a diverse gene pool
4.6.2 Inbreeding reveals the value of gene mixing through sex
4.6.3 The Red Queen hypothesis and Muller’s ratchet
4.7 The role of sex in the creation of species
4.7.1 Two different patterns of species evolution
4.7.2 How do gene pools become split at the start of speciation?
4.7.3 Pre-zygotic and post-zygotic reproductive barriers
Summary
Further reading

5. Sexual Selection
Key themes
5.1 Sexual selection is a sub-category of natural selection
5.1.1 Comparing examples of natural selection and sexual selection
5.1.2 Re-formulating the natural selection argument for sexual selection
5.1.3 Manifestations of sexual selection
5.2 Why are females usually the “limiting sex”?
5.2.1 Females usually invest more in reproduction
5.2.2 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 fitness
5.3.1 Genetic benefits in the runaway hypothesis: “Sexy sons”
5.3.2 Genetic benefits in the good genes hypothesis: Better survival
5.3.3 Benefits in the good resources hypothesis: Honest promises
5.4 Sex role reversal
5.4.1 Sex role reversal: Adjusting investments made by males and females
5.5 Not all intra-sexual competition involves fighting
5.5.1 Scrambles are a form of indirect competition
5.5.2 Endurance and subterfuge are also forms of indirect competition
5.5.3 Sperm competition
Summary
Further reading

6. Mating Systems
Key themes
6.1 There are five major types of mating systems
6.2 Monogamy
6.2.1 Distinguishing among types of monogamy
6.2.2 Hypotheses for monogamy
6.3 Polygyny is the most common form of polygamy
6.3.1 Two models for polygyny
6.3.2 Human polygyny
6.4 The benefits for females of mating with multiple males
6.4.1 Non-monogamous females hedge their bets against infertility
6.4.2 Non-monogamous females reap genetic benefits
6.4.3 Non-monogamous females can reap direct benefits
6.5 Polyandry
6.5.1 Polyandrous mating systems are much less common than polygynous ones
6.6 Polygynandry
6.6.1 Reproductive skew
6.6.2 Cooperative breeding
6.7 Promiscuity
6.7.1 Factors associated with promiscuity
6.7.2 Scramble competitions are usually promiscuous mating systems
6.7.3 “Lek polygyny” is a promiscuous system resembling hook-up culture
Summary
Further reading

7. Sexual Conflict
Key themes
7.1 Strategies for sexual success
7.1.1 A comment on terminology
7.1.2 Sexual strategies exist in the context of sexual conflict
7.2 Realms of sexual conflict
7.2.1 Sexual conflict before mating
7.2.2 Sexual conflict during mating
7.2.3 Sexual conflict after mating
7.2.4 Sexual conflict during parenting
7.3 Mating strategies can change with circumstances
7.3.1 Making the best of things
7.3.2 Life history theory argues for plastic mating strategies
7.3.3 Sex allocation
7.4 Same-sex parenting
Summary
Further reading

8. Sex Determination and Differentiation
Key themes
8.1 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-chromosomes
8.2.1 Using insects to discover the role of sex chromosomes
8.2.2 The X- and Y-chromosomes in humans
8.2.3 Little genetic differences between men and women, but big phenotypic differences
8.2.4 The SRY gene and transcription
8.3 Not all genetic sex determination relies on the XY system
8.3.1 Genetic sex determination without Y-chromosomes
8.3.2 Are females ever the heterogametic sex?
8.3.3 Chromosomal sex determination where hermaphroditism is a phenotype
8.3.4 Genetic sex determination in honeybees
8.4 Environmental sex determination
8.4.1 Temperature is a non-social sex-determination system
8.4.2 Environmental sex determination can override genetic sex determination
8.4.3 Can mothers use temperature to select their offspring’s sex?
8.4.4 Social systems of environmental sex determination
8.4.5 Anomalous sex determination caused by a member of a different species
8.4.6 Anomalous sex determination by ecotoxins
Summary
Further reading

9. Human Sexual Anatomy and Regulation
Key themes
9.1 Human sexual differentiation and function is highly dependent on hormones
9.1.1 The endocrine system regulates hormones in the bloodstream
9.1.2 Major hormones that regulate sexual development, function, and behavior
9.2 Sex differentiation is part of development
9.2.1 Human sexual differentiation before birth
9.3 The male reproductive system
9.3.1 Further male differentiation during the prenatal period
9.3.2 Male sexual development at puberty
9.3.3 Sexual anatomy of the human adult male
9.4 The female reproductive system
9.4.1 Further female differentiation during the prenatal period
9.4.2 Female sexual differentiation at puberty
9.4.3 Sexual anatomy of the human adult female
9.5 Anomalous sexual phenotypes in humans
9.5.1 Consequences of anomalies in the sex chromosomes
9.5.2 Intersex conditions that result from dysfunctional alleles
9.5.3 When girls become men at puberty
9.6 Sex linkage: Why some genetic disorders occur mostly in males
9.7 Cancers of sexually differentiated organs and tissues
9.7.1 Cancers associated with male organs
9.7.2 Cancers associated with female organs
Summary
Further reading

10. Human Fertility and Birth
Key themes
10.1 Key differences between making sperm and eggs
10.1.1 Spermatogenesis occurs from puberty to old age
10.1.2 Oogenesis occurs from the fetal stage to menopause
10.2 The menstrual cycle
10.2.1 Most mammals have an estrous cycle instead
10.2.2 Is women’s fertile period concealed?
10.3 Sexual arousal and response in men and women
10.4 Fertilization and the making of a zygote
10.4.1 Infertility
10.4.2 Contraception
10.5 Pregnancy
10.5.1 The placenta
10.5.2 Labor and delivery
10.6 Breast-feeding
10.7 Menopause
10.8 Sexually transmitted infections
10.8.1 A diversity of organisms cause STIs
10.8.2 Major bacterial STIs
10.8.3 Major STIs caused by viruses
10.8.4 Non-humans suffer sexually transmitted infections too
Summary
Further reading

Glossary
Index