Asexual and sexual reproduction

Asexual reproduction

The offspring are produced by cell division (mitosis) of the parent's cells. Because nothing is mixed with the genes of another organism, the offspring have exactly the same genes as the parent. They are genetically identical to it, and to each other.

Asexual reproduction can happen in several ways:

Sexual reproduction and the comparison

In sexual reproduction, two parents each make gametes: a sperm cell and an egg cell. These join in fertilisation to form a zygote. Each gamete carries half of its parent's genes, so the offspring get a new mix of genes. They are similar to their parents but not identical to them or to each other (except identical twins).

Advantages of asexual reproduction: it is fast, needs no mate, and a successful parent passes on its good features exactly.

Disadvantages: no variation, so if the environment changes or a disease arrives, all the offspring are equally affected.

Advantages of sexual reproduction: variation gives some individuals a better chance of surviving a change or a disease. Disadvantages: it is slower and uses more energy.

This fast doubling explains why an infection can spread so quickly: a few bacteria become thousands within hours. And because they are clones, they all share the same weaknesses, such as being killed by the same antibiotic.

Clones in farming, in nature and in the lab

A gardener in Co. Kilkenny wants more apple trees that give the same tasty apples. Planting seeds would give varied trees, since seeds come from sexual reproduction. Instead she takes cuttings (or grafts) from the best tree. Every new tree is a clone with the same tasty apples. The downside: a disease that kills one tree can kill them all.

Dolly's nucleus held all the genes of the sheep that donated it, so she was genetically identical to that donor. She is a clone, but she did not come from fertilisation: no sperm joined with an egg.

Cloning raises ethical concerns, such as animal welfare (many attempts fail and animals may have health problems), the worry about cloning humans, and loss of genetic diversity.

Variation, environment and natural selection

Which method is better? It depends on the environment.

• In a stable, favourable environment, asexual reproduction is very effective. Organisms that are already well suited to their surroundings pass on the same successful genes, and they multiply quickly.

• When the environment changes (a new disease, a drought, a colder climate), variation matters. In a varied population, some individuals may happen to have features that help them survive. They live longer, reproduce and pass on those genes. This is natural selection, and it needs variation to work on.

Aphids use both: they reproduce asexually in summer when food is plentiful, and sexually when conditions turn harsh.

Now we can close our opening question. In the 1840s, Irish potatoes were grown from tubers, so the plants were clones. The potato plants were genetically almost identical, so none had resistance and the crop failed across the country. A varied crop would probably have contained some resistant plants. Modern farmers reduce the risk by growing several varieties, rotating crops, breeding resistant varieties and using fungicides.

Summary