Asexual vs Sexual Reproduction in Animals

Asexual reproduction produces offspring from a single parent with no fusion of sex cells, so the offspring are genetically identical, while sexual reproduction combines sex cells (gametes) from two parents and produces genetically varied offspring. In brief: a hydra that grows a bud, a flatworm that regrows from a fragment and a female aphid that produces clones are examples of asexual reproduction in animals. Humans, dogs, birds and most other animals reproduce sexually through the fusion of a sperm and an egg.

The two modes of reproduction have different costs and benefits, and many animals use both at different times. This guide defines each type, describes the main methods in animals, explains fertilization and why variation matters, compares the strategies in a table, works through a population example and corrects common mistakes.

What is asexual reproduction?

In asexual reproduction, a single parent produces offspring by mitosis. No gametes are involved, so there is no fertilization. The offspring are genetically identical to the parent, apart from rare mutations, and form a clone.

The main advantages are speed and efficiency. An animal does not need to find a mate, and every individual can produce offspring, so a population can grow rapidly in a favorable environment. The main disadvantage is the lack of genetic variation: if the environment changes or a new disease appears, all individuals may be equally vulnerable.

What are the methods of asexual reproduction in animals?

Asexual reproduction is common among simpler animals, but rare among vertebrates.

• Budding: a small outgrowth develops on the parent, grows into a miniature copy and then detaches. Hydra and sponges reproduce this way.
• Fragmentation and regeneration: a piece breaks off and regrows into a complete animal. Flatworms (planarians) and starfish can do this. Some starfish can regenerate from a single arm that contains part of the central disc.
• Binary fission: the body divides into two similar parts. Sea anemones, some corals and some worms can do this.
• Parthenogenesis: an egg develops without being fertilized. Aphids, water fleas (Daphnia), some bees (drones are produced this way), some lizards, such as whiptails, and rare sharks and snakes reproduce this way. Aphids often reproduce by parthenogenesis in summer and switch to sexual reproduction in autumn.

What is sexual reproduction?

Sexual reproduction involves two types of specialized sex cells, gametes. They are produced by meiosis, a type of cell division that halves the number of chromosomes. In humans, body cells have 46 chromosomes (diploid), and gametes have 23 (haploid). The male gamete is a sperm, which is small and motile. The female gamete is an egg (ovum), which is much larger and carries nutrients.

When a sperm fuses with an egg during fertilization, the diploid number is restored in the new cell, the zygote. The zygote has half of its chromosomes from each parent and divides to form the embryo. Because meiosis shuffles the genes, and fertilization combines gametes from two different individuals, offspring differ from each other and from the parents. This variation is the raw material of evolution.

Sexual reproduction costs more. An animal must find and attract a mate, and in species with two sexes, only half of the population (the females) can produce offspring directly. This is called the two-fold cost of sex. The benefit of variation is thought to outweigh the cost in changing environments, for example because it helps populations to resist parasites that evolve to attack the most common genotype.

How does fertilization occur?

There are two main types of fertilization in animals.

• External fertilization: the gametes are released into water and fuse outside the body. Many fish and amphibians reproduce this way. A large number of eggs and sperm are released, because many do not meet or survive. Parental care is usually limited.
• Internal fertilization: sperm are placed inside the female’s body, and the fertilized egg is protected. This is usual in reptiles, birds and mammals, as well as in insects. Fewer eggs are produced, but each has a better chance of survival, and parental care is more common.

After internal fertilization, development can take place in an egg with a shell (oviparity, as in birds and most reptiles), in an egg that hatches inside the mother (ovoviviparity, as in some sharks and snakes) or within the mother’s body with nourishment from her (viviparity, as in most mammals).

Asexual and sexual reproduction compared

Worked example: growth of an asexual and a sexual population

Compare two imaginary populations, each starting with 10 females. In population A, each female reproduces asexually and produces 2 daughters, and every individual in the population is female. In population B, there are 5 females and 5 males, each female produces 2 offspring, and half are female and half are male. Assume that each generation is distinct and that parents die after reproducing.

1. Population A: generation 0 has 10 females. Generation 1 has 10 × 2 = 20 females. Generation 2 has 20 × 2 = 40. Generation 3 has 80.
2. Population B: generation 0 has 5 females and 5 males, a total of 10. Generation 1: 5 females × 2 = 10 offspring, of which 5 are female and 5 are male. The total remains 10.
3. Comparison: after three generations, population A has 80 individuals, and population B still has 10.

The two-fold cost of sex is shown by the model: when only half of the individuals produce offspring, the population does not grow at the same rate. In nature, sexual species often produce more than 2 offspring per female, and species compensate through other advantages. The model also ignores death, competition and food limits, which would eventually stop the growth of population A. Asexual reproduction gives rapid growth, but sexual reproduction gives variation.

How were these processes understood?

In 1740 Abraham Trembley described the regeneration and budding of hydra, one of the first demonstrations of asexual reproduction in animals. Charles Bonnet found in the 1740s that aphids reproduce without males. The role of sperm and egg in fertilization was clarified in the nineteenth century, when Oscar Hertwig observed the fusion of nuclei in sea urchin eggs in 1876. August Weismann argued in the 1880s that sexual reproduction creates variation on which natural selection can act.

Which mistakes are common?

Mistake 1: Saying that asexual offspring are “similar” to the parent

They are genetically identical, except for rare mutations.

Mistake 2: Believing that all animals reproduce sexually

Many invertebrates, and a few vertebrates, reproduce asexually.

Mistake 3: Confusing mitosis and meiosis

Asexual reproduction uses mitosis. Gametes for sexual reproduction are made by meiosis.

Mistake 4: Saying that sexual reproduction is better in every situation

Asexual reproduction is advantageous in stable environments and where mates are scarce.