Reproduction, Asexual and Sexual
Reproduction, Asexual and Sexual
Organisms must reproduce and, in the context of evolution, must choose among different methods to do so. There are two major strategies for reproduction—sexual and asexual. Each tactic has its own advantages and disadvantages, and each is appropriate for certain situations. Vertebrates, such as humans, are almost exclusively sexual in their reproduction, many simpler animals are asexual. To decide which reproductive strategy may prove advantageous in a given set of circumstances, it is important to understand how they differ.
Asexual reproduction takes a variety of forms. The simplest one-celled organisms may reproduce by binary fission, in which the cells simply divide in half. This form of reproduction creates a clone of the parent, and has the benefit of usually being very quick and energy efficient. For example, bacteria that reproduce by binary fission can give rise to progeny every few hours.
Some organisms, such as Cryptosporidium parvum, a sporozoan that causes traveler's diarrhea, may utilize multiple fission, in which they split into more than one offspring simultaneously. In multicellular organisms, a similar tactic is called fragmentation. In this process, small pieces break off and grow into new organisms. Still other organisms reproduce by budding , in which a smaller copy of the parent grows on the body and eventually splits off to begin life on its own.
All these variations of asexual reproduction have one thing in common, the offspring is a direct clone of the parent. The purpose of reproduction is to propagate one's own genes. Evolutionarily, asexual reproduction is a good deal for the parent. It is quick, simple, and the genes of the parent will not be diluted by those of another individual. In addition, an organism that reproduces asexually can reproduce about twice as fast as one that reproduces sexually. This has shown to be true with the whiptail lizard of the southwestern United States, which can reproduce both sexually and asexually under different conditions.
Sexual reproduction is more much complex than asexual reproduction. It requires the production of sex cells, or gametes , which have half the number of chromosomes of all other cells in the organism. When the organism needs to make sex cells, it undergoes meiosis, which produces haploid cells (one copy of the genome ) from diploid cells (two copies of the genome). A key aspect of meiosis is that the two copies of a single chromosome can cross over to create a completely new chromosome that contains a new combination of genes. The net effect of crossing-over is that genes on a specific chromosome can change position from one chromosome to the next. This means that genes from both parents may end up next to each other on the same chromosome. Where genes are concerned, switching from chromosome to chromosome is a good way to ensure they will keep active in a given population.
Once the gametes are made in the male and female, they must meet with one another to form offspring. The sperm from the male provides one copy of a genome. The egg from the female provides another copy of a different genome. Thus, the offspring of sexually reproducing organisms has more than one opportunity to switch genes around—crossing-over and the union of the two parents.
Comparing Sexual and Asexual Reproduction
However, note how much energy sexual reproduction takes. The sex cells must be made, and as each parent contributes only half the genome, it propagates only half as many genes from each offspring as does an asexually reproducing organism. Recall that an organism is most interested in propagating its genes; indeed, that is the whole point of reproduction. To reproduce sexually is to reduce the amount of genetic material one reproduces by half, and this reduction does not even take into account the effort sexually reproducing organisms must make to find mates, then impress, select, or defend them. Nevertheless, nearly all higher animals reproduce sexually. Why? The answer to this question is far from settled, but biologists have a few good clues.
The most important thing about sexual reproduction is its ability to switch around successful genes. If it is beneficial to an organism's survival to be both tall and have blue eyes, a short, blue-eyed parent and a tall, browneyed parent can get together and stand a good chance of producing off-spring with both characteristics. If they reproduced asexually, a short, blue-eyed parent would have to wait around for a height-inducing genetic mutation to change height and eye color. And because mutations, which are basically genetic mistakes, tend to cause bad effects, the mutation rate in most organisms is exceedingly slow. While it would take only a generation for sexually-reproducing parents to beget tall offspring with blue eyes, it might take an asexually-reproducing parent hundreds or thousands of generations!
Asexually reproducing organisms do not readily share genetic material, but they do reproduce much faster. And because asexually reproducing organisms reproduce faster, they do exceptionally well in situations where they have no competition. With sexually-reproducing competition nearby, however, the asexual organisms will quickly be outadapted and outevolved by their neighbors, even though the asexual organisms may have superior numbers due to fast reproduction. Many biologists think that intense competition gives rise to sexual reproduction, because the competition requires rapid innovation and distribution of the most successful genes.
Although these arguments for the existence of sexual reproduction might seem evolutionarily sound, the alleged advantages of sexual reproduction over asexual reproduction are still quite controversial among biologists. Some biologists think that only replicating half of your genes in exchange for sexual reproduction is not an even trade. Others suggest that dilution of groups of genes does not matter. Furthermore, a sexually reproducing organism must expend a great amount of effort to find a mate, in both behavior and new body structures and appendages. Biologists believe that sexual selection drives gender size and appearance, plumage, behavior, and many other energetically expensive strategies.
Can it be possible that sexual selection, with all its demands, is worth the moderate amount of recombination that results from sexual reproduction? If not, why do all vertebrates, many invertebrates, and most plants sexually reproduce? Many prominent biologists have considered these questions, such Richard Dawkins, J. Maynard Smith, G. C. Williams, and others.
It seems likely that the ability to swap around already successful genes, rather than being forced to sit around and waiting for mutations, is a more successful strategy for complex organisms. And less complex organisms can get by without the larger energy and resource investment that sexual reproduction demands.
see also Egg; Embryonic Development; Fertilization; Reproductive System.
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Curtis, Helena, and N. Sue Barnes. Biology, 5th ed. New York: Worth Publishing, 1989.
Hayssen, Virginia, Ari Van Tienhoven, Ans Van Tienhoven, and Sydney Arthur Asdell. Asdell's Patterns of Mammalian Reproduction: A Compendium of Species-Specific Data. Ithaca, NY: Comstock Publication Associates, 1993.
Norris, David O., and Richard E. Jones, eds. Hormones and Reproduction in Fishes, Amphibians, and Reptiles. New York: Plenum Press, 1987.
Purves, William K., and Gordon H. Orians. Life: The Science of Biology. Sunderland, MA: Sinauer Associates Inc., 1987.
There are a few species of vertebrates that reproduce asexually. The whiptail lizard, which lives in the desert grasslands of the southwestern United States, may reproduce sexually or asexually. Do asexually-reproducing lizards show less genetic variability than sexually-reproducing ones? They do, just like the theory says they should.