NSW HSC Biology

Reproduction is how a species continues once its individuals die. Organisms do it in two ways that differ in what they do to genetic variation, and most of the differences between groups come down to how that variation is traded against speed and energy cost.

Asexual and sexual reproduction

In asexual reproduction one parent produces offspring without gametes and without fertilisation. The offspring are produced by mitosis, so they are genetically identical to the parent and to each other (clones), apart from any new mutations. Because no mate has to be found and no gametes have to meet, populations can grow very quickly when conditions suit the parent. The disadvantage is that a population of clones has almost no variation, so a change such as a new disease or a shift in climate can affect every individual in the same way.

In sexual reproduction two gametes, usually from two parents, fuse in fertilisation to form a zygote. Gametes are made by meiosis, which shuffles alleles, and fertilisation combines alleles from two parents, so the offspring differ from each other and from both parents. That variation is what natural selection acts on, and it is why sexually reproducing species are better placed to survive when the environment changes. The cost is that it is slower and uses more energy, because gametes must be made, a mate found and the gametes brought together.

Animals: external and internal fertilisation

Animals that reproduce sexually are divided by where the gametes meet.

External fertilisation

In external fertilisation the female releases eggs and the male releases sperm into the surrounding water, and they meet outside the body. It is found in most fish and amphibians (frogs are a familiar example) and in many aquatic invertebrates such as sea urchins. Water is needed so that gametes do not dry out and so that sperm can swim to the eggs.

Its advantages are that a very large number of eggs can be released at once, so a large number of offspring can result, and that no courtship, contact or copulation is needed, which saves energy. Releasing gametes at the same time as others of the species, often triggered by temperature, day length or chemical signals, raises the chance that they meet. The disadvantages follow from the exposure: gametes are diluted and washed away, eggs and larvae are eaten by predators, and the chance that any one egg is fertilised and survives is low. A species that uses this method must therefore produce very large numbers of gametes, and parental care is usually limited or absent.

Internal fertilisation

In internal fertilisation sperm are placed inside the female's reproductive tract and fertilisation happens inside her body. It is found in mammals, birds, reptiles and some fish and invertebrates. It works on land because the gametes are kept moist inside the body.

Its advantages are that a far higher proportion of eggs are fertilised, so fewer gametes need to be made, and that the zygote and embryo are protected from drying out and from predators, either inside the mother or inside a shell. The embryo can also be supplied with food, by yolk or across a placenta, and parents often care for the young, which improves survival. The disadvantages are that fewer offspring are usually produced at a time, finding a mate and mating cost energy and time, and pregnancy or egg-laying puts demands on the female.

Asexual reproduction in animals

Some animals also reproduce asexually. A hydra forms a bud, a small outgrowth that develops into a miniature copy and then detaches. A flatworm can split into pieces that each regrow the missing parts, which is fragmentation. In parthenogenesis an egg develops without being fertilised, as it does in aphids and in some lizards and sharks. Aphids use this to build up numbers very quickly in spring, when conditions are favourable.

Fungi: budding and spores

Fungi are mostly made of threads called hyphae, and they reproduce in two main asexual ways.

Budding is used by yeasts, which are single-celled. A small outgrowth forms on the parent cell, the nucleus divides by mitosis and one nucleus moves into the bud, and the bud then grows and breaks away as a new cell. In good conditions, such as a warm sugar solution, yeast populations multiply quickly.

Most other fungi, including moulds and mushrooms, make spores. Spores are tiny cells, each capable of growing into a new fungus. Asexual spores are formed by mitosis in structures such as sporangia, so they are genetically identical to the parent. They are very light and produced in huge numbers, which means they are carried by wind, water or animals, and each one that lands somewhere warm, moist and with a food source germinates into new hyphae. Many fungi can also reproduce sexually when hyphae from two individuals fuse. The resulting spores are made by meiosis and are genetically varied, which gives a fungus a way of producing new combinations when conditions change.

Bacteria: binary fission

Bacteria are prokaryotes and reproduce asexually by binary fission. The steps are:

1. The circular chromosome is replicated, starting from a single point, and the two copies attach to different parts of the cell membrane.
2. The cell elongates, which moves the two chromosomes apart. Plasmids are also copied.
3. The membrane and cell wall grow inwards at the middle, forming a dividing wall (a septum).
4. The cell splits into two daughter cells, each with one copy of the chromosome.

Under ideal conditions some bacteria, such as Escherichia coli, can divide about every 20 minutes, so one cell can give rise to a huge population in a day. The daughter cells are clones, but variation still arises through mutation, which happens at a low rate per division but adds up because there are so many divisions, and through the transfer of plasmids between cells. This is part of why bacterial populations can become resistant to antibiotics so quickly.

Protists: binary fission and budding

Protists are a varied group of mostly single-celled eukaryotes, and their asexual reproduction involves mitosis followed by division of the cytoplasm.

• In binary fission the nucleus divides by mitosis and the cell then splits into two equal daughter cells. Amoeba and Paramecium reproduce this way.
• In budding a small outgrowth containing a copy of the nucleus separates from the parent. The daughter cell starts smaller than the parent and grows.
• Some protists divide into many cells at once (multiple fission). Plasmodium, the parasite that causes malaria, does this in the liver and in red blood cells of its human host.

Many protists can also reproduce sexually. In Paramecium, two cells join and exchange genetic material, after which each divides again, so the new cells carry a combination of the two parents' genes. Asexual reproduction is the usual way these organisms build up their numbers, and the occasional sexual event brings in variation.