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
Asexual reproduction takes a variety of forms (see Figure 1). The simplest onecelled 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 (offspring) 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
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 (see Figure 2), 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—crossingover 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, brown-eyed 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.
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.
See Also
Egg; Embryonic Development; Fertilization
Bibliography
Conn, David Bruce. Atlas of Invertebrate Reproduction and Development. New York: John
Wiley and Sons, 2000.
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.
Figure 1: Some Methods of Asexual Reproduction
1. binary fission -- involves an equal division of both
the organism cytoplasm and nucleus to form two
identical organisms
-- the diagram of the protist at the right is example of
this
2. sporulation (spore formation) -- is reproduction
involving specialized single cells coming from one
parent
-- the diagram of mold spores being formed at the
right is an example of this
3. budding -- involves one parent dividing its nucleus
(genetic material) equally, but cytoplasm unequally
-- the diagram of a yeast and the hydra at the right
are examples of this
4. Fragmentation (regeneration) – a small piece of
the organism will grow into a whole new organism
-- the diagram of planarian (flat worm) being formed
at the right is an example of this
--Starfish also have the ability to reproduce this way
Figure 2: Meiosis Overview