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When we think of reproduction, we think of sex, but many species can reproduce asexually—the offspring are exact genetic copies of the parents. Most of the animal species that reproduce asexually are invertebrates living in rather constant environments. However, examples of asexual reproduction do occur in vertebrates. Reproduction by female sharks living in aquaria with no males have been reported. The natural habitat of these fish is the open ocean where potential mates might not be readily available. Asexual reproduction is efficient because no time or energy is wasted on mating and every member of the population can convert resources into offspring.

You can demonstrate to yourself the efficiency of asexual reproduction with a thought exercise. Start with two females, one reproducing sexually and one asexually. Assume that each can have two offspring, but in the case of the sexually reproducing female, one will be a male. Also, assume that in each subsequent generation all of the females (mothers and daughters) will have two offspring each. In the first generation, the asexually reproducing female will have two daughters who can then each have two female offspring. Thus in the next generation there will be three females having two offspring each. The sexually reproducing female will give rise to only one female, so in the next generation there will be two females reproducing having two offspring each—only one of which is female. Run this thought exercise through four or five cycles and compare the numbers of offspring produced.