Asexual reproduction and the cost of sex

Mitosis makes genetically identical cells, and meiosis makes haploid cells that differ from each other. Whether a species makes offspring with or without gametes decides how much its offspring differ from each other and from their parents.

Two ways to make offspring

In sexual reproduction, two haploid gametes fuse at fertilization to form a diploid cell called a zygote. In plants and animals, the zygote then divides by mitosis, over and over, to build a body of many diploid cells. Meiosis makes the gametes; mitosis builds the body.

In asexual reproduction, offspring come from a single parent, with no fusion of gametes. The offspring are usually genetically identical to the parent.

Asexual reproduction has clear advantages. There’s no need to spend energy finding a mate or building body parts used only for reproduction. It’s most common in species that live in very stable environments, where there’s little pressure for offspring to vary.

Kinds of asexual reproduction

Eukaryotes reproduce asexually in several ways. The first four don’t involve gametes at all; the rest use eggs, seeds, or other special structures.

Parthenogenesis can produce haploid or diploid offspring. A queen honeybee’s unfertilized haploid eggs become haploid males, called drones, while her fertilized eggs become diploid females. In some reptiles and fish, a diploid female makes diploid eggs that develop without sperm.

Check yourselfA gardener roots a stem cut from a houseplant, and a sea star regrows a whole body from a severed arm. Which types of asexual reproduction are these?

The cutting is vegetative propagation, a new plant growing from a stem. The sea star is fragmentation, a piece of the body regrowing into a complete individual.

The cost of sex

Sexual reproduction is expensive by comparison:

• Two kinds of gametes, sperm and eggs, must be made, often in huge numbers.
• Special body parts are usually needed to make gametes and offspring.
• In animals, the two sexes must find each other, and courtship takes energy.

Yet the vast majority of eukaryotic species reproduce sexually. Biologists infer that sex must carry an advantage strong enough for natural selection to favor it. That advantage is genetic variety in the offspring, and it starts with meiosis.

Check yourselfWhy might a species living in a very stable environment rely mostly on asexual reproduction?

When conditions don’t change, offspring identical to a successful parent are likely to do well too, so there’s little benefit from variety. Asexual reproduction also saves the energy of finding mates and making gametes.