Every living thing on the planet came from something that reproduced. A cell split. A seed germinated. An egg hatched. Reproduction is the one process that turns a single life into a lasting line.
The word covers two very different methods. In sexual reproduction, two cells fuse, and the young carry a mix of genes from both parents. In asexual reproduction, one parent produces young on its own, and the young are close copies of it.
Both methods are everywhere. Bacteria divide in half, strawberries send out runners, and dandelions make seeds without pollination. Fish spawn in open water, and flowering plants move pollen on the bodies of insects. The rest of this article explains how each route works, and why sex survives when cloning is cheaper.
What Reproduction Has to Do
Reproduction is not growth. A tree adds wood every year and stays one tree. A hydra grows a bud that breaks off as a second animal.
It is not repair either. Healing a cut restores the same body. Making a new one continues the line. The test is simple: if the new part can live on its own, and carries the same kind of genetic material, the parent has reproduced.
One measure matters more than any other: generation time. Bacteria can double in twenty minutes, an aphid can produce a daughter in days, and an oak may take twenty years to set its first acorn. Fast generations fill a space quickly. Slow ones hold it for longer.
A second measure is the number of young. A cod releases millions of eggs and leaves them. A whale calves once every few years and feeds the calf. Each pattern answers the same question: how many young can this parent afford to lose?
Two Routes: Sexual and Asexual
Sexual reproduction starts with gametes: an egg and a sperm, or two cells of the same kind. The two fuse, and the fused cell is a zygote. The zygote has two sets of chromosomes, one from each parent. It divides by mitosis, and every cell of the new body carries that mixed set.
Asexual reproduction skips the fusion. One parent copies its own DNA and divides, or grows a bud, or sends out a shoot. The young inherit one set of instructions, so they resemble the parent closely.
| Question | Sexual reproduction | Asexual reproduction |
| Parents needed | Two, usually of different mating types | One |
| How the young start | Two gametes fuse into a zygote | A cell, bud, fragment or unfertilized egg |
| Genes in the young | A mix from both parents | A copy of the parent, barring mutation |
| Speed | Slower, because a mate has to be found | Fast, because one parent can start at once |
| Where it wins | Changeable places, and against disease | Stable places, and empty space |
Sexual and asexual reproduction compared: parents, the start of the young, their genes, speed, and where each route wins.
Neither column is better. Each is a bet on what the next generation will face, and many species place both bets in one lifetime.
What Meiosis Does That Mitosis Cannot
Ordinary cell division, mitosis, copies every chromosome and gives each daughter cell a full set. That keeps the number the same from one cell to the next.
Meiosis halves it. The cell copies its DNA once, then divides twice, and four cells come out with one set of chromosomes each. In animals those cells become gametes. In plants they become spores.
Two events in the first division do the mixing. In crossing over, paired chromosomes swap matching pieces, so a single chromosome can carry genes from both grandparents. Then the pairs line up and separate at random, so every gamete gets a different mixture.
This is why two siblings differ. They share parents, but each egg and each sperm carried a different hand of cards.
Meiosis has to halve the set. If gametes carried a full set, each generation would double its chromosomes, and a fertilized egg would hold twice what its parents held.
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| The main events of meiosis. One copy of the DNA is followed by two divisions, and four cells with half the chromosome number come out. Image: National Institutes of Health, public domain, via Wikimedia Commons |
Why Sex Persists When Cloning Is Cheaper
The arithmetic looks bad for sex. A female that clones passes all her genes to every daughter. A female that mates passes half of them, and only half of her young carry any given gene. That gap is called the twofold cost of sex.
If cloning is twice as productive, sex should lose. It has not. Most animals and most flowering plants reproduce sexually, at least some of the time.
Several explanations are still being tested. One is that mixing genes helps a species keep up with fast-changing enemies, such as parasites and diseases that adapt to whatever is common. Another is that sex combines useful mutations and sheds harmful ones. A third is that a varied population can use more kinds of habitat.
None of them settles the question on its own. What is clear is that variation is worth a real price, paid in every generation.
Asexual Reproduction, Mechanism by Mechanism
Asexual reproduction appears in many forms, and they work in different ways. They share one feature: no fusion, and no second parent.
Binary fission is the simplest. A bacterium copies its single chromosome, grows, and pinches into two cells. There is no nucleus to divide, because bacteria have none. Under good conditions a population can double every twenty minutes.
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| Binary fission. The cell copies its chromosome, lengthens, and divides into two daughter cells. Image: Mcstrother, CC BY 3.0, via Wikimedia Commons |
In budding, a new individual grows from the body of the parent and then breaks away. Yeast does this, and so does Hydra. A Hydra bud starts as a small outgrowth, grows tentacles, and detaches when it is complete.
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| Budding in a hydra. The daughter grows from the body wall of the parent and separates as a clone. Image: SadiesBurrow, CC BY-SA 4.0, via Wikimedia Commons |
Fragmentation uses pieces. A flatworm cut into sections can regrow a whole animal from each piece, and some starfish do the same from an arm. Many algae and some corals spread this way.
Plants add a slower version. A strawberry sends out a stolon, a stem that runs along the ground and roots at its tip, and a new plant grows there. A potato tuber is a swollen underground stem with buds, and each bud can become a shoot. A daffodil bulb divides into offsets. Gardeners rely on all of these, because a cutting or a division gives a plant identical to the one they chose.
Spores are another route. A fern makes haploid spores by meiosis, and each spore can grow into a small plant. Fungi release spores in huge numbers. A spore is not a gamete: it needs no partner, and it grows directly.
Apomixis is the strangest case. A plant makes a seed, but the seed forms without fertilization, from the mother's own tissue. Dandelions do this, which is why a lawn can be covered by one genetic individual. Each seed still travels on the wind. It is cloning with a dispersal kit.
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| A dandelion seed head. Each seed can form without fertilization, so the plant clones itself and spreads at the same time. Image: Avenue, CC BY-SA 4.0, via Wikimedia Commons |
| Mechanism | What happens | An example |
| Binary fission | One cell copies its DNA and splits in two | Bacteria |
| Budding | A new individual grows from the parent and detaches | Hydra, yeast |
| Fragmentation | A piece regrows into a whole animal | Flatworms, some starfish |
| Vegetative growth | A stem, root or leaf makes a new plant | Strawberry runner, potato tuber |
| Spore formation | Haploid cells grow into a new individual | Ferns, fungi |
| Apomixis | A seed forms without fertilization | Dandelion |
| Parthenogenesis | An unfertilized egg develops into an animal | Aphids, whiptail lizards |
The mechanisms of asexual reproduction, what each one does, and an organism that uses it.
Parthenogenesis: Young From an Unfertilized Egg
In parthenogenesis, an egg develops without a sperm. Some aphids use it for most of the year. In temperate regions they switch to sex in autumn, when shorter days, falling temperatures or poorer food trigger the birth of males and of females that can mate.
More than eighty species of reptiles consist only of females, and they reproduce this way. Whiptail lizards are among them. Their eggs double the chromosomes before dividing, so each young gets two sets from its mother.
It also turns up in animals that normally mate. Komodo dragons, some sharks and a few birds have produced young from unfertilized eggs, usually when no male was present.
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| A Komodo dragon. Females of this species have produced young from unfertilized eggs, and those young were male. Image: Charles J. Sharp, CC BY-SA 4.0, via Wikimedia Commons |
The sex of those young follows the animal's chromosome system. In the ZW system, which birds and Komodo dragons use, the female is ZW and the male is ZZ. An egg that doubles its chromosomes becomes ZZ, so the offspring are male.
Parthenogenesis is not the same as cloning. The doubling shuffles the mother's genes and turns her mixed pairs into identical ones, so the young are less varied than she is. They are close copies, not exact ones.
Sexual Reproduction Across the Living World
Sex needs two gametes to meet. How they meet depends on the habitat.
In water, many animals release eggs and sperm into the current and let them find each other. Corals go further and spawn together. On many reefs, whole colonies release bundles on the same few nights, timed by the moon and by water temperature.
A synchronized spawn overwhelms the fish that eat eggs, and it raises the chance that eggs and sperm from different colonies meet. A spawn spread over weeks would be eaten and diluted.
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| Coral spawning at Flower Garden Banks. Colonies release eggs and sperm on the same few nights each year, a week or so after a full moon. Image: National Marine Sanctuaries, public domain, via Wikimedia Commons |
On land, gametes dry out and cannot swim far. Internal fertilization solves that for animals, and pollen solves it for plants. A pollen grain is a tiny male gametophyte. It lands on a stigma, grows a tube down the style, and delivers sperm cells to the egg inside the ovule. No water is needed between the two plants.
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| A honey bee collecting pollen. An animal that visits flowers moves pollen from one plant to another, which is pollination. Image: Anson Aswat, CC BY-SA 4.0, via Wikimedia Commons |
Alternation of Generations
Plants and many algae have two bodies in one life cycle, and the two alternate.
The diploid body is the sporophyte. It makes spores by meiosis. Each spore grows into a haploid body, the gametophyte, which makes gametes by mitosis. Two gametes fuse, and the zygote grows into a new sporophyte.
The relative size of the two bodies varies. In mosses the gametophyte is the leafy plant, and the sporophyte is a stalk on top of it. In ferns both are independent, and the sporophyte is the larger. In flowering plants the gametophyte is reduced to a few cells inside the flower.
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| The plant life cycle alternates between a diploid sporophyte and a haploid gametophyte. Image: Peter coxhead, public domain, via Wikimedia Commons |
This matters for students. The plant you call a fern, with fronds and roots, is the sporophyte. The small flat body you might find under it is the gametophyte, and it is a separate organism.
Selfing, Cross-Pollination and Two Sexes in One Flower
Many flowers carry both male and female parts, so a plant could fertilize itself. Many prevent it. Some ripen their pollen and their stigma at different times. Others reject their own pollen at the surface of the stigma.
Selfing keeps a good set of genes intact, and it works when a plant stands alone. Cross-pollination brings in new combinations, which is the point of sex.
Some plants separate the sexes entirely. Holly and willow carry male flowers on one plant and female flowers on another, so a holly berry needs a male nearby.
Reproduction, Species and Ecosystems
Reproduction also decides what a species is. The usual definition is a group of organisms that can breed with one another and produce fertile young.
That definition works because of reproductive isolation: barriers that stop two groups from mixing. Some act before fertilization. Two frogs may breed in different months, or sing different calls, or live on opposite sides of a river. Others act after. A mating may produce a hybrid that dies, or one that grows up sterile, like a mule.
| Barrier | When it acts | An example |
| Habitat | Before mating: the two never meet | Two insects on different host plants |
| Timing | Before mating: different seasons | Two frogs that breed in different months |
| Behavior | Before mating: the signals do not match | Two birds with different songs |
| Mechanical | Before fertilization: the parts do not fit | Two flowers built for different visitors |
| Hybrid inviability | After fertilization: the young dies | Some frog crosses |
| Hybrid sterility | After fertilization: the young is sterile | Mule, from a horse and a donkey |
Barriers that keep species apart, whether each one acts before or after fertilization, and an example.
Isolation is not a wall. It leaks. Where two species meet, some hybrids form, and the boundary shifts.
Isolation lets small differences build up. Given enough time and enough separation, two populations become two species, and each one carries its own set of answers.
When One Clone Covers a Coast
Clones can be vast. In Shark Bay, Western Australia, a seagrass meadow of about 200 square kilometers turned out to be one plant: Posidonia australis.
Samples taken up to 180 kilometers apart had the same genotype. The meadow grew by sending rhizomes across the seafloor, at perhaps 35 centimeters a year, for at least 4,500 years.
That plant is old and huge, and it is also uniform. One disease, one heat wave or one change in water chemistry meets the same genome everywhere.
Why Genetic Diversity Is Insurance
A clone has one answer to every problem. A varied population has many, and that matters when conditions change.
The banana in most shops is the Cavendish, grown from clones. It replaced an earlier commercial variety that Panama disease wiped out in the mid-twentieth century. A new strain of the same fungus now threatens the Cavendish.
The potato is the harsher example. Ireland's potato crop in the 1840s was grown from a narrow set of clones. When late blight arrived, it spread through them, and about a million people died.
Sex is the opposite bet. It spreads the risk across many combinations, so a disease that beats one plant may fail against its neighbor. That is the trade the twofold cost describes: a price paid in every generation, in exchange for options.
Reproduction looks like one subject and turns out to be several. Fission, budding and runners copy a parent. Meiosis, gametes and fertilization mix two. Nearly every living thing uses one route, the other, or both, and each choice carries its own risk.
Key Terms in Plain English
These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.
| Term | What it means |
| Reproduction | The making of new people from existing ones, so a line of life continues. |
| Sexual reproduction | Reproduction in which two gametes fuse, so the young carry genes from both parents. |
| Asexual reproduction | Reproduction from one parent, without the fusion of gametes. |
| Gamete | A reproductive cell with half the usual chromosome number, such as an egg or a sperm. |
| Zygote | The cell formed when two gametes fuse; it grows into a new individual. |
| Fertilization | The joining of two gametes into one zygote. |
| Mitosis | Cell division that copies every chromosome, so both daughter cells get a full set. |
| Meiosis | Two divisions that halve the chromosome number and mix the genes, producing gametes or spores. |
| Crossing over | The swap of matching pieces between paired chromosomes during meiosis. |
| Genetic diversity | The range of different gene versions carried by a population. |
| Clone | An individual that is genetically identical to its parent, or a group of such people. |
| Binary fission | Division of a single cell into two, as bacteria do. |
| Budding | Asexual reproduction in which a new individual grows from the parent and then detaches. |
| Fragmentation | Asexual reproduction in which a piece of an organism regrows into a whole one. |
| Vegetative reproduction | A new plant grown from a stem, root or leaf of the parent, as with runners and tubers. |
| Spore | A single cell that can grow into a new individual without fusing with another cell. |
| Parthenogenesis | Development of an unfertilized egg into an animal. |
| Apomixis | The formation of a seed without fertilization, so the seed is a clone of the mother plant. |
| Alternation of generations | A life cycle that switches between a diploid and a haploid body. |
| Sporophyte | The diploid body in a plant life cycle, which makes spores by meiosis. |
| Gametophyte | The haploid body in a plant life cycle, which makes gametes by mitosis. |
| Pollination | The transfer of pollen from one flower to another, or to another part of the same plant. |
| Reproductive isolation | Any barrier that stops two groups from interbreeding and producing fertile young. |
| Hybrid | The young of two different species or varieties, such as a mule. |
Terms used in this article, defined in plain words.
Key Takeaways
- Reproduction makes new people, while growth enlarges one and repair restores one.
- Sexual reproduction fuses two gametes into a zygote, so the young carry a mix of genes from both parents.
- Asexual reproduction starts from a single parent, by fission, budding, fragmentation, runners, spores, apomixis or parthenogenesis.
- Meiosis halves the chromosome number, and crossing over plus random separation mix the genes on the way.
- The twofold cost of sex means a cloning female passes twice as many genes per young, so variation must be worth the price.
- In parthenogenesis an unfertilized egg develops, and in ZW animals such as the Komodo dragon the young that result are male.
- Plants alternate between a diploid sporophyte and a haploid gametophyte, and the two differ greatly in size.
- Pollen carries sperm to the ovule without water, which is what let seed plants reproduce away from a pond.
- Reproductive isolation, whether it acts before or after fertilization, keeps species apart and lets new ones form.
- Uniform clones such as the Cavendish banana and the Irish potato crop are productive and fragile, so diversity works as insurance.
Frequently Asked Questions
What is reproduction in simple words?
It is the process by which living things make new people like themselves. A bacterium splits in two, a plant grows from a seed, and an animal produces young. Without it, a species would end when its last individual died.
What is the main difference between sexual and asexual reproduction?
Sexual reproduction needs two parents and fuses their gametes, so the young are genetically mixed. Asexual reproduction needs one parent and no fusion, so the young are close copies of that parent.
Is parthenogenesis a form of cloning?
Not exactly. The egg carries only the mother's genes. But the doubling that makes it diploid turns her mixed gene pairs into identical ones, so the young are less varied than she is. They are close copies, not exact ones.
Why is meiosis important for sexual reproduction?
Meiosis halves the chromosome number, so a fertilized egg gets one set from each parent instead of double the usual number. It also mixes the genes, through crossing over and the random separation of chromosome pairs.
Do all plants need pollinators?
No. Some release pollen into the wind, including grasses and many trees. Some, such as dandelions, form seeds without fertilization at all. Animal pollinators matter most for flowers that offer nectar or pollen as a reward.
What is alternation of generations?
It is a life cycle with two multicellular bodies: a diploid sporophyte that makes spores by meiosis, and a haploid gametophyte that makes gametes by mitosis. Mosses, ferns and flowering plants all do it, with the two bodies very different in size.
Why is genetic diversity important for a species?
It gives a population more than one answer to a new disease, a heat wave or a change in food. A clone has a single answer, so a threat that beats it beats every individual.
Can a species switch between sexual and asexual reproduction?
Many do. Aphids clone themselves for most of the year and mate in autumn. Some plants spread by runners and also set seed. Switching lets a species grow fast when conditions are good and mix its genes when they turn.
Key references
• Reproduction — the process in outline (Wikipedia)
• Sexual reproduction — gametes, the twofold cost, and why sex persists (Wikipedia)
• Asexual reproduction — fission, budding, fragmentation and spores (Wikipedia)
• Meiosis — the two divisions and crossing over (Wikipedia)
• Parthenogenesis — unfertilized eggs and ZW sex determination (Wikipedia)
• Alternation of generations — sporophyte and gametophyte (Wikipedia)
• Pollination — pollen tubes and animal visitors (Wikipedia)
• Reproductive isolation — pre-zygotic and post-zygotic barriers (Wikipedia)
• Apomixis — seed formation without fertilization (Wikipedia)
• Vegetative reproduction — runners, tubers and offsets (Wikipedia)
• Cavendish banana — a clonal crop and Panama disease (Wikipedia)
• Posidonia australis — the Shark Bay clone (Wikipedia)
Sources: the Wikipedia article on reproduction, for the definition and for the distinction between reproduction, growth and repair; the article on sexual reproduction, for gametes, the zygote, and the twofold cost of sex with its 50 percent fitness disadvantage; the article on asexual reproduction, for binary fission in prokaryotes, budding, fragmentation, vegetative growth, spore formation, apomixis and parthenogenesis, and for aphids alternating between clonal and sexual generations; the article on meiosis, for the two divisions, crossing over and the halving of the chromosome number; the article on parthenogenesis, for the more than eighty all-female reptile species, for parthenogenesis in Komodo dragons and sharks, and for the ZW system in which a parthenogenetic egg yields a male; the article on alternation of generations, for the sporophyte and gametophyte bodies and for their relative size in mosses, ferns and seed plants; the article on pollination, for the pollen tube and the delivery of sperm to the ovule; the article on reproductive isolation, for the pre-zygotic and post-zygotic barriers and Mayr's division of them; the article on apomixis, for dandelions and the formation of seed without fertilization; the article on vegetative reproduction, for runners, tubers and offsets; the article on the Cavendish banana, for a clonal crop with very low genetic diversity and its vulnerability to Panama disease; and the article on Posidonia australis, for the single Shark Bay clone, its 200 square kilometers, its 180 kilometer spread and its age of at least 4,500 years. The New Mexico whiptail and aphid articles were also read, for parthenogenesis in whiptail lizards and for the autumn sexual phase in aphids. Each source was opened and read in full. Figures are from Wikimedia Commons under the credit named in each caption. This article is written for education.
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