Sexual reproduction

How it works

The process depends on the formation and union of gametes: typically small, mobile sperms and larger, nutrient-rich eggs. Key biological steps include:

1. Meiosis, a special type of cell division that halves chromosome number and creates genetic recombination;
2. Gametogenesis, the differentiation of meiotic products into functional sperm or egg cells;
3. Fertilization, the fusion of two gametes to form a zygote that develops into a new individual.

Patterns and diversity

Sexual reproduction shows great variety. Fertilization can be external (common in many fish and amphibians) or internal (typical in birds and mammals). Some species are dioecious, with separate sexes, while others are hermaphroditic, carrying both male and female reproductive systems. Plants may use flowers, cones or gametophyte stages to produce gametes.

Advantages and consequences

By reshuffling genes each generation, sexual reproduction increases genetic variation in a population. This variation can enhance adaptation to changing environments and reduce the accumulation of harmful mutations. The process also has costs: it often requires more time, energy and the finding of a mate, and can reduce the number of offspring produced compared with asexual reproduction.

Examples across life

• Animals: many species produce distinct sperm and eggs and show sexual behaviours and selection.
• Plants: flowering plants combine pollen and ovules; some algae and nonvascular plants have alternating life stages.
• Fungi and protists: several groups exchange genetic material by gamete fusion or analogous mechanisms.

Notable facts and distinctions

Biological sex and the mechanics of sexual reproduction vary widely: some organisms change sex during life, some reproduce by self-fertilization, and sexual systems can involve more than two mating types in certain fungi and protists. These variations influence population genetics, ecology and evolution and are active topics in biological research and conservation.

For further reading, see resources on general animal and plant reproduction: animals, plants, and overviews of protists and fungi reproduction: protists, organisms.