Asexual and sexual reproduction
Reproduction means making new living things of the same kind. Without it, a species would die out.
Asexual reproduction: one parent
- Budding: a hydra or a sponge grows a bud that becomes a new animal.
- Fission: some flatworms split in two.
- Fragmentation and regeneration: a piece of a starfish arm or a planarian can grow into a whole animal.
- Parthenogenesis: a female makes young from eggs that are not fertilised. Aphids do this in summer; male honeybees (drones) come from unfertilised eggs.
Asexual reproduction is fast and needs no partner, but the young are copies. If a disease or a change in weather hits one, it can hit all.
Sexual reproduction: two gametes
A gamete is a sex cell. The male gamete is the sperm (small, moves with a tail). The female gamete is the egg or ovum (large, stores food). They are made in the testes and ovaries. When they join, the new cell is a zygote. The zygote divides again and again to form an embryo.
Because genes from two parents are mixed, every young one is a little different. This variation helps a species survive change.
Some animals, such as earthworms and many snails, are hermaphrodites: one animal has both male and female organs. Most still pair up, so genes still mix.
External and internal fertilisation
Fertilisation is the joining of a sperm and an egg.
External fertilisation
Eggs and sperm are released into water and meet there. Examples: frogs, toads, most bony fish, sea urchins. Water carries the sperm and stops the eggs drying out. Many eggs are eaten or never meet a sperm, so these animals release hundreds or thousands of eggs. Parents usually give little care.
Internal fertilisation
The male places sperm inside the female's body. Examples: insects, reptiles, birds, mammals, sharks. More eggs get fertilised and the embryo is protected, so fewer eggs are needed. Parents often give more care.
| External | Internal | |
|---|---|---|
| Where | Outside, in water | Inside female body |
| Number of eggs | Very many | Few |
| Examples | Frog, fish | Hen, cow, human, insect |
Eggs, the amniotic egg and where the embryo grows
Animals that lay eggs are oviparous (hen, frog, snake). Animals that give birth to young are viviparous (cow, dog, human). Some, such as many sharks and some snakes, keep eggs inside until they hatch (ovoviviparous).
The egg matches the habitat. Fish and frog eggs are soft and jelly-covered because water keeps them wet. On land an egg needs protection, so reptiles and birds lay the amniotic egg:
- Shell: hard or leathery, stops drying, but has pores for air.
- Amnion: a bag of fluid around the embryo, like a private pond and shock absorber.
- Yolk sac: holds the food (yolk).
- Allantois: stores waste and helps with gas exchange.
- Chorion: the outer membrane under the shell; exchanges oxygen and carbon dioxide.
Reptiles, birds and mammals are called amniotes because their embryos have an amnion. In most mammals these membranes help form the placenta, which feeds the embryo from the mother's blood.
Development: direct, indirect and metamorphosis
In direct development the young look like small adults (birds, mammals, spiders). In indirect development there is a larva that looks very different and later changes. This change of body form is metamorphosis.
Complete metamorphosis (4 stages)
Egg → larva (caterpillar, maggot, grub: eats and grows) → pupa (resting case where the body is rebuilt) → adult. Butterflies, moths, beetles, flies, bees.
Incomplete metamorphosis (3 stages)
Egg → nymph (a small adult without full wings) → adult. Grasshoppers, cockroaches, dragonflies.
Moulting and hormones
Insects have a hard outer skeleton that cannot stretch, so they shed it to grow. This is moulting. Hormones control it: one hormone (ecdysone) starts each moult; another (juvenile hormone) keeps the insect young. When juvenile hormone drops, the next moult makes the pupa or adult.
Frogs also show metamorphosis: tadpoles breathe with gills and have a tail; the thyroid hormone makes them grow legs, lose the tail and develop lungs.
Parasites with more than one host
Some parasites need two hosts. The tapeworm lives as an adult in the human gut (final host, where sexual reproduction happens) and as a larva in a pig or cow (intermediate host). Cooking meat well breaks the cycle.
Key formulas and definitions
- Gamete: sex cell (sperm or egg)
- Sperm + egg → zygote (fertilisation) → embryo
- Oviparous: lays eggs; viviparous: gives birth to young
- Complete metamorphosis: egg → larva → pupa → adult
- Incomplete metamorphosis: egg → nymph → adult
- Amniotic egg membranes: amnion, chorion, allantois, yolk sac
- Parthenogenesis: young from an unfertilised egg
Worked examples
1. A frog lays about 2000 eggs but a hen lays one egg at a time. Explain the difference.
Step 1: Frogs use external fertilisation in water. Step 2: Many eggs are never fertilised or are eaten by fish. Step 3: To leave at least some young, the frog must release very many eggs. Step 4: The hen uses internal fertilisation and protects the egg with a shell and by sitting on it, so fewer eggs survive better.
2. Classify: housefly, cockroach, silkworm moth, dragonfly — complete or incomplete metamorphosis?
Step 1: Ask: is there a pupa? Housefly (maggot then pupa): complete. Silkworm moth (caterpillar, cocoon): complete. Step 2: Cockroach and dragonfly have nymphs and no pupa: incomplete.
3. A gardener finds only females in an aphid colony in July, yet the colony keeps growing. How?
Step 1: Growth means young are made. Step 2: No males means no sperm, so the eggs are not fertilised. Step 3: Aphids use parthenogenesis: females make young from unfertilised eggs. Step 4: The young are copies of the mother, which lets the colony grow very fast in good weather.
4. Why can a lizard lay eggs in dry sand but a frog cannot?
Step 1: Frog eggs have only jelly, no shell, so they dry out on land. Step 2: Lizard eggs are amniotic: a leathery shell stops water loss. Step 3: The amnion keeps the embryo in its own fluid, the yolk feeds it and the chorion and allantois handle gases and waste. Step 4: So the lizard does not need a pond.
Common mistakes
- Thinking fertilisation and reproduction are the same thing. Fertilisation is just the joining of sperm and egg; reproduction is the whole process of making young.
- Saying a pupa is dead or resting doing nothing. Inside the pupa the body is being rebuilt into an adult.
- Calling all animals that lay eggs 'external fertilisers'. Hens and snakes lay eggs, but fertilisation happened inside the body.
- Thinking hermaphrodites always fertilise themselves. Earthworms are hermaphrodites but still swap sperm with a partner.