Reproduction and Life Cycles

Two ways to reproduce

In asexual reproduction, one parent makes offspring on its own. The young get a copy of that parent's genetic information, so they are genetically identical to it, apart from rare copying mistakes. Biologists call such copies clones.

In sexual reproduction, two special cells, usually from two parents, join together. Each parent passes on half of its genetic information, so the offspring are a new mix and differ from both parents and from one another.

Plenty of living things can do both. Many plants grow from seeds but also spread by runners or underground stems, and aphids switch between the two methods with the seasons.

Asexual reproduction

Asexual reproduction happens in several ways. Here are six you will meet in biology class.

• Binary fission. The parent splits into two roughly equal halves. Bacteria reproduce this way, as the Microorganisms page explains. Some sea anemones and coral polyps also reproduce by splitting.
• Budding. A small outgrowth, the bud, forms on the parent's body and grows into a new individual. A hydra's bud breaks off and lives on its own. In many corals the buds stay attached, which is how a colony grows. Yeast, a single-celled fungus, buds too.
• Fragmentation. The body breaks into pieces, and a piece grows into a new individual. Many sea stars do this: an arm that breaks off can grow into a whole new sea star. People who once tried to kill sea stars on oyster beds by cutting them in half and throwing them back only ended up with more sea stars.
• Regeneration. This is regrowing lost body parts. In some animals it is so complete that a small piece can rebuild a whole body. Planarian flatworms are famous for it: cut one in two, and each piece can grow the parts it is missing.
• Vegetative propagation. A new plant grows from a stem, root, or leaf of the parent, without a seed. Strawberries send out runners that root a short way off. A potato is a swollen underground stem called a tuber, and each “eye” can sprout a new plant. Gardeners use the same idea when they grow plants from cuttings.
• Parthenogenesis. An egg develops into a new individual without being fertilized. Aphids, water fleas, and some stick insects can do this. In honeybees, drones (males) hatch from unfertilized eggs. It has even been recorded in a few reptiles and sharks kept in zoos, where a female living without males laid eggs that hatched. Scientists are still finding out how often it happens in the wild.

Sexual reproduction

Sexual reproduction depends on gametes, also called sex cells. In animals the female gamete is an egg and the male gamete is a sperm. Gametes are made by a special kind of cell division called meiosis, which gives each one half the usual number of chromosomes. When an egg and a sperm join, which is called fertilization, the fertilized egg (a zygote) ends up with a full set again: half from each parent. You can read how chromosomes and genes pass from parents to young on the Heredity and Genetics page.

Inside or outside the body?

In external fertilization, eggs and sperm meet outside the body, usually in water. Most fish, frogs, oysters, and sea urchins reproduce this way. Many eggs never meet a sperm, and many young are eaten, so these animals tend to release huge numbers of eggs.

In internal fertilization, the sperm reaches the egg inside the female's body. This is common in land animals, including insects, reptiles, birds, and mammals, where eggs would dry out in open air. These animals usually have fewer young, and more of them survive. After internal fertilization, birds, many reptiles, and two kinds of mammals (the platypus and echidnas) lay eggs, while most mammals carry their young inside the body until birth.

Humans reproduce sexually, with internal fertilization. A human egg cell and sperm cell join to form a zygote with 46 chromosomes, in 23 pairs. The baby then develops inside the mother's uterus, a pregnancy that usually lasts about 40 weeks, or just over nine months.

Why variation matters

Because each offspring gets a different mix of its parents' genes, brothers and sisters differ. That variation is useful when conditions change. If a new disease arrives or the climate shifts, some offspring may happen to have traits that help them survive it. A population of clones would all share the same weaknesses.

Pros and cons

Neither way is simply better. Each has trade-offs, which is part of the reason so many living things use both.

Animal life cycles

Some young animals look like small copies of their parents. Others go through metamorphosis, a big change in body form as they grow. Insects show two main patterns.

Complete metamorphosis

Butterflies, beetles, bees, and ants have four stages: egg, larva, pupa, and adult. A monarch butterfly egg hatches in about four days into a larva, the caterpillar, which spends its time eating and growing. The caterpillar then forms a pupa, which for a butterfly is called a chrysalis. The pupa does not eat. Inside it, the larva's body is rebuilt into a winged adult.

Incomplete metamorphosis

Grasshoppers, praying mantises, and dragonflies have three stages: egg, nymph, and adult. There is no pupa. A nymph often looks like a small adult, but its wings are only short wing pads that cannot fly yet. The pads grow a little with each molt, and working wings appear only at the final molt. It sheds its hard outer skin several times as it grows. Dragonfly nymphs, called naiads, live underwater for months or even a few years before they become flying adults.

Frogs: a change from water to land

Frogs and toads usually go through metamorphosis too. Most frogs mate in water, with fertilization outside the body. The eggs are laid in jelly-coated clumps or strings, and they often hatch in about 10 to 14 days. A tadpole breathes with gills, swims with its tail, and mostly eats algae. Over weeks or months, back legs grow, then front legs. Lungs develop, the tail shrinks, and the young froglet climbs out onto land.

Direct development

Mammals, birds, and reptiles do not go through metamorphosis. Their young hatch or are born looking like smaller versions of the adults and simply grow larger. This is called direct development. A chick that hatches from a fertilized egg already has a beak, legs, and feathers. A few amphibians skip the swimming larva stage as well. The young of the slimy salamander, for example, develop inside the egg and hatch as tiny adults.

Plant life cycles

Flowers, pollination, seeds, and spores are covered on the Plants and Photosynthesis page. This section adds a few ideas about how plant lives play out over time.

From seed to seed

A seed holds a tiny young plant, the embryo, with a store of food. Many seeds stay dormant, alive but inactive, for months, years, or even longer, until conditions are right. Germination begins when the seed takes in water. The first thing to push out is usually the young root, which anchors the seedling and starts taking in water. Then the shoot grows up toward the light. The plant grows, flowers, and makes fruit with seeds inside, and the cycle starts again. The drawing at the top of this page shows this cycle.

Annuals, biennials, and perennials

• Annuals go from seed to seed in a single growing season and then die. Many garden flowers and weeds are annuals.
• Biennials take two seasons. A carrot spends its first year growing leaves and storing food in its root. In its second year it flowers, makes seeds, and dies.
• Perennials live for many years. Trees and shrubs are perennials. Most flower year after year, though some, such as many bamboos, grow for decades, flower once, and then die.

Alternation of generations

Plants have an unusual life cycle that biologists call alternation of generations. Put simply, a plant's life cycle switches between two forms. One form makes spores, and the other makes gametes. In mosses, the soft green carpet you see is the gamete-making form. In ferns and seed plants, the plant you notice is the spore-making form, and the gamete-making form is small. The details get complicated, and you will meet them again in high school biology.

How long do living things live?

A life span is how long an individual lives. Some animals die of old age within days. A few trees have been alive for thousands of years. Very long life spans are hard to measure, so many of the numbers below are estimates, and record holders change as new evidence turns up.

• Mayflies. Mayflies are known for short adult lives, but most of their life is spent as a nymph in water, for a few weeks to about two years. Adults cannot feed. Few species live more than two days as winged adults, and the adult females of one North American species, Dolania americana, live less than five minutes.
• Monarch butterflies. A summer monarch lives about two to five weeks as an adult. Monarchs that hatch at the end of summer migrate south and can live up to about nine months.
• Tortoises. Jonathan, a giant tortoise on the island of Saint Helena, is thought to have hatched around 1832, which would make him about 194 years old in 2026. No one recorded his hatching. The estimate is based on him being fully grown when he arrived on the island in 1882.
• Greenland sharks. These slow-moving sharks of the cold North Atlantic and Arctic may be the longest-lived vertebrates (animals with backbones) known. A 2016 study estimated that the largest shark it measured was about 392 years old, give or take 120 years.
• Bristlecone pines. A Great Basin bristlecone pine in California known as “Methuselah” was dated by counting the tree rings in thin cores taken in 1957. Those counts put it at more than 4,800 years old today. Its exact location is kept secret to protect it.
• The “immortal” jellyfish. Turritopsis dohrnii is only about 4.5 millimeters across. When it is injured, starving, or stressed, it can turn back into an earlier life stage, a polyp, and grow up again. Scientists say it may never die of old age, but that does not make it immortal. In the wild, most are eaten or die of disease, and no one knows how long one could last.

For comparison, the oldest person whose age has been fully verified, Jeanne Calment of France, lived to 122.

Why life spans vary

Life span depends partly on genes, which differ from species to species, and partly on the environment: food, temperature, disease, and predators. Scientists who compare hundreds of animal species have spotted some patterns, although every one has exceptions.

• Body size. Large animals tend to live longer than small ones.
• Escaping danger. Animals that can fly, live in trees, or burrow often live longer than others their size. Animals that face fewer predators may gain more from a body that lasts.
• Life cycle strategy. Some living things put everything into one burst of reproduction and then die, like the adult mayfly or a bamboo that flowers once. Others reproduce again and again over a long life.
• The season you are born in. Even within one species, life span can change. The long-lived autumn monarchs and the short-lived summer monarchs are the same species.

Who figured this out?

A few milestones in the study of reproduction and life cycles.

Key words

• Asexual reproductionone parent making offspring that are genetically identical to it
• Sexual reproductiontwo gametes joining to make offspring with a new mix of genes
• Gametea sex cell, such as an egg or a sperm, with half the usual number of chromosomes
• Fertilizationthe joining of an egg and a sperm
• Zygotea fertilized egg, the first cell of a new individual
• Buddinga new individual growing out of the parent's body
• Parthenogenesisan egg developing without being fertilized
• Vegetative propagationa new plant growing from a stem, root, or leaf, without a seed
• Metamorphosisa big change in body form as an animal grows
• Larvathe young, feeding stage in complete metamorphosis, such as a caterpillar
• Pupathe resting stage in which a larva is rebuilt into an adult
• Nympha young insect in incomplete metamorphosis that looks partly like the adult
• Germinationa seed starting to grow
• Life spanhow long an individual lives

Game: Put the Life Cycle in Order

You will see seven living things, one at a time. Build each life cycle by choosing the stage that comes first, then the next one, until the cycle is complete. Watch out: some choices belong to a different kind of life cycle. A stage you get right on your first try earns 1 point, for a total of 27. Your best score is saved on this device.

Quick quiz

Six questions. Pick one answer for each, then press Check my answers.