Exact Copies, or Copies Worth Varying

Every organism has a definite life span: it is born, grows, matures, reproduces and eventually dies. Reproduction is the biological process by which living beings produce new individuals of their own kind, and it's how life on Earth continues to exist at all, a mango tree dies but its seeds grow into new mango trees, cows give birth to calves, humans to children. Living beings reproduce in two fundamentally different ways. Asexual reproduction involves a single parent producing offspring that are almost exact copies of that parent. Sexual reproduction involves two parents, with offspring inheriting a mix of characteristics from both, and this mixing can produce small differences between parents and their young, differences that, accumulated over many generations, help species adapt to changing environments and sometimes even give rise to entirely new species. This chapter explores how reproduction actually works, across plants, animals and humans, and why so many organisms have settled on one strategy or the other, or sometimes both.

Asexual reproduction appears across unicellular organisms like bacteria, amoeba and yeast, simple multicellular organisms like hydra and sponge, and many plants. Plants with fleshy underground stems, like potato and ginger, sprout new plants without seeds; money plant and sugarcane grow from stem cuttings; Bryophyllum leaves sprout tiny plantlets. All of these are vegetative propagation, new plants arising from a plant's vegetative, growing parts, involving only one parent and producing genetically identical individuals. Scientists and horticulturists have adapted this natural process into deliberate agricultural methods: cutting (planting a prepared piece of stem), grafting (joining a cutting from one plant onto a rooted plant of another variety), layering (burying part of a flexible twig until it roots, then separating it), and tissue culture (mass-producing healthy, often virus-free plantlets from a shoot tip). In yeast, small round outgrowths called buds emerge from the parent cell and eventually separate to live independently, a process called budding, also seen in hydra, where multiple buds can grow on the parent's body at once. Fungi reproduce through spores, tiny, usually single-celled structures produced in huge numbers, often millions from a single mould colony, that float through air currents and germinate quickly into a new individual once they land somewhere with enough moisture and nutrients; experiments by Louis Pasteur proved that new life always comes from pre-existing life, disproving the older idea that living things could arise spontaneously from non-living matter. Across every one of these organisms, the underlying process behind asexual reproduction is mitosis, a cell division producing two daughter cells genetically identical to the parent cell, which is exactly why the offspring produced this way are called clones.

Sexual reproduction involves two parents contributing genetic material to the offspring, but if each generation received a full set of chromosomes from both parents, the chromosome number would double every generation. This problem is solved by meiosis, a special cell division that forms gametes by reducing a parent cell's chromosome number from diploid (the full number) to haploid (half). In humans, cells normally carry 46 chromosomes, as 23 pairs, but sperm and eggs, the gametes, carry only 23. During meiosis, the chromosomes of each pair separate so each gamete receives only one chromosome from every pair, and because this separation happens somewhat randomly across all the pairs at once, the number of possible combinations is enormous: with just three pairs of chromosomes carrying different traits, eight different combinations are already possible, and with the human body's actual 23 pairs, the number of possible combinations is astronomically larger. This random mixing is exactly why children are genetically different from both their parents and their siblings, a source of variation that helps some individuals adapt better to changing environments, some people tolerating low oxygen at high altitude better than others, for instance, and over time, this variation is what evolution actually works on.

A complete flower has four parts: sepals (the outermost whorl, usually green, protecting the flower in the bud stage), petals (often coloured and fragrant, attracting pollinators), stamens (the male part, each with a filament and an anther that produces pollen grains containing male gametes), and the pistil (the female part, with a stigma at the tip, a style connecting it to the ovary, and an ovary containing ovules, each with an egg cell). The transfer of pollen from anther to stigma is called pollination, essential for fruit and seed formation; when pollen moves to the stigma of the same flower or another flower on the same plant, it's self-pollination, and when pollen moves from one plant's anther to a different plant's stigma, it's cross-pollination. Pollination depends on external pollinators: wind carries the light, small, abundantly produced pollen of plants like wheat, maize and rice, whose stigmas are long and feathery to trap it; water carries pollen in aquatic plants like Vallisneria and Hydrilla; insects like bees and butterflies pollinate brightly coloured, nectar-producing, fragrant flowers like sunflower and hibiscus, whose pollen is large, sticky or spiny to cling to an insect's body; and birds like sunbirds pollinate flowers like the coral tree.

Once pollen reaches a compatible stigma, it produces a pollen tube that grows down through the style into the ovary; the male gamete travels through this tube and fuses with the egg cell in the ovule, a fusion called fertilisation, marking the beginning of a new life. The fertilised egg, now a zygote, develops into an embryo, while the ovary enlarges into a fruit and the ovules develop into seeds; seeds disperse by wind, water or animals, and germinate into new plants when conditions are favourable. Wind-pollinated grasses like maize and wheat release an enormous 500,000 to 1,000,000 pollen grains per flower but form only 50 to 200 seeds on average, while insect-pollinated plants like sunflower release a far smaller 20,000 to 40,000 pollen grains yet form 800 to 1,000 seeds, a dramatically more efficient pollen-to-seed ratio. Producing an enormous number of pollen grains, even at low individual efficiency, can still be an effective strategy overall, since it compensates for how much pollen wind simply wastes compared to a targeted insect delivery.

In many aquatic animals, frogs and most fish among them, fertilisation occurs outside the body: the female releases eggs into water and the male releases sperm over them, a method called external fertilisation. Large numbers of eggs are laid this way, but many are destroyed by water currents or eaten by predators, so survival of young is generally low. In reptiles, birds and mammals, fertilisation instead takes place inside the female's body, called internal fertilisation, and the fertilised egg or embryo is far better protected, giving generally higher survival. Fish, amphibians and insects lay hundreds to thousands of eggs at once, each with just enough yolk to produce a larva rather than a fully formed young one; the larva then feeds itself on organic waste to grow, eventually transforming into the adult form, as seen in butterflies and frogs. Reptiles and birds instead lay eggs with enough yolk to nourish the embryo all the way through to hatching, while in mammals, the zygote develops directly inside the female's body instead of inside an external egg at all. How long an embryo develops inside an egg or the mother's body broadly determines how much post-birth care the young ones need afterward; mammals in particular typically feed their young via breast milk for some time after birth.

Sperm are produced in a pair of testes housed in the scrotum, which keeps them slightly cooler than body temperature, necessary for sperm formation; sperm travel through the vas deferens to the urethra, gaining nourishing fluids from glands like the seminal vesicles and prostate along the way. Eggs are produced in a pair of ovaries; the oviducts connect each ovary to the uterus, a bag-like structure where a foetus develops, which opens into the vagina through the cervix. Gametogenesis, the formation of gametes, happens in the testes and ovaries via meiosis, halving the chromosome number to 23 so that when sperm and egg combine, the resulting zygote has the normal 46; males produce millions of tiny, motile sperm continuously, while females form a single, large, non-motile egg roughly once a month. From puberty onward, one mature egg is usually released every month in a process called ovulation; before this, the uterus lining thickens in preparation. During sexual intercourse, millions of sperm enter the female body, and if one succeeds in fusing with the egg in the oviduct, a zygote forms, undergoes several mitotic divisions while travelling to the uterus, and implants into the thickened uterine lining, marking the beginning of pregnancy. If an egg is not fertilised, it degenerates within about a day, the thickened uterine lining is no longer needed and sheds along with some blood through the vagina, a process called menstruation, and this entire cycle of ovulation, uterine preparation and menstruation typically repeats every 21-35 days, beginning at puberty and continuing until menopause around age 50.

Human pregnancy lasts about nine months, divided into three trimesters: in the first, the embryo forms and major organs begin developing; by the ninth week it's called a foetus, which continues growing through the second trimester, when movements become noticeable; in the third, rapid growth prepares the baby for life outside the womb. During childbirth, uterine muscle contractions push the foetus out through the birth canal; the mother's nutrition, rest and avoidance of harmful substances during pregnancy directly affect the baby's growth and safety. As adolescents grow, sexual maturity, the capability for reproduction, develops gradually alongside overall body growth, but emotional maturity, the ability to handle feelings, communicate clearly and make thoughtful decisions, takes longer to develop; being ready for sexual activity involves both, not physical readiness alone. Responsible choices help prevent unplanned pregnancies and sexually transmitted infections, or STIs, infections including gonorrhoea, herpes, syphilis, genital warts and HIV that spread through sexual contact, some not yet curable; condoms help prevent both STI transmission and pregnancy. Different contraceptive methods exist beyond condoms: hormonal oral pills that alter egg release, intra-uterine devices like copper-T, and surgical methods that block the vas deferens or fallopian tubes; abortion, the surgical removal of an unwanted pregnancy, is generally only performed within the first trimester, and prenatal sex determination is strictly prohibited by Indian law to prevent sex-selective abortion and the resulting imbalance in society's sex ratio.