Reproduction

Without reproduction, every species would die out. Living organisms have evolved two very different strategies for producing offspring: sexual reproduction and asexual reproduction. Both are used widely, often by the same organism at different times.

Sexual reproduction

Sexual reproduction is the production of offspring by the fusion of two gametes (sex cells) from two different parents

Key features:

• Two parents are required.
• Each parent makes specialised sex cells called gametes. Gametes have half the normal number of chromosomes (in humans, 23 instead of 46), so they are described as haploid. They are made by a special kind of cell division called meiosis.
• When the male gamete fuses with the female gamete at fertilisation, the resulting cell (a zygote) gets a full set of chromosomes back. The zygote then divides by mitosis and develops into an embryo.
• The offspring inherit a mix of DNA from both parents, so they are genetically different from either parent and from each other.

The big advantage of sexual reproduction is that it produces genetic variation in the offspring. Variation is the raw material on which natural selection acts, helping the species adapt to changing conditions and survive new diseases. The price paid is that sexual reproduction is slow and needs two parents, which is expensive in energy and effort.

Asexual reproduction

Asexual reproduction is the production of offspring from one parent only, with no fusion of gametes

Key features:

• Only one parent is needed.
• No gametes are made. The parent's cells divide by mitosis, producing daughter cells that are genetic copies of each other and of the parent.
• All offspring are genetically identical to each other and to the parent. They are called clones.
• Reproduction can be very fast and does not depend on finding a mate.

The big advantage of asexual reproduction is speed and efficiency. A single individual can produce many offspring quickly. The price is that all the offspring are genetically identical, so if a disease or environmental change can kill one of them, it can probably kill all of them.

Side-by-side comparison

Some organisms use both strategies depending on conditions. Plants, for example, often reproduce sexually in good conditions (producing seeds that can be dispersed widely), but also asexually through runners or bulbs (filling out the local area quickly). Bacteria, yeast and aphids reproduce mainly asexually when conditions are good but switch to sexual reproduction when conditions become harsh.