Encyclopedia of Evolution.pdf - Online Reading Center

sexual selection
through this tube to complete the act of fertilization. The
structure through which the pollen tube must grow, the
style, can be said to exist for the sole purpose of “weeding
out” inferior pollen grains. Botanist Allison Snow has
found that, in the bush Hibiscus moscheutos, pollen grains
from some plants grow tubes faster than pollen grains
from other plants. Since flowers often receive more pollen
than they need, there is often a race between the different
pollen tubes to be the first ones to arrive at the immature
seeds—which may constitute sexual selection among the
pollen tubes.
• In terms of differences among pollen grains in the rate
of growth, the style functions just as the food source
through which the tube grows, but a pistil is more than
just a food source for the pollen tube. The pistil itself may
chemically encourage the growth of some pollen tubes
and discourage others. This is difficult to demonstrate. It
has been shown that there is greater variation in pollen
tube growth rate in the pistils of healthy plants than of
plants that are experiencing stresses such as drought. This
implies but does not prove that a healthy plant chemically
distinguishes among pollen tubes. The haploid cells surrounding
the egg cell, in the immature seed, have genes
that control the release of sperm from the pollen tubes.
This implies the possibility of sexual selection in which
the female cells are in control of which sperm are allowed
to fertilize the immature seeds.
• Botanist Diane Marshall found that radish plants may provide
more food to the developing fruits that contain seeds
that are more genetically variable (the result of a greater
diversity of pollen donors) than to fruits that contain
genetically more uniform seeds. Botanists K. B. Searcy and
M. R. McNair found that monkeyflower plants (Mimulus
guttatus) that grow in soil that contains mildly toxic levels
of copper selectively abort seeds that had been fertilized by
pollen from donors that were not resistant to copper. These
may be examples of sexual selection in which the maternal
plant selects among pollen sources.
• The endosperm is the food supply for the embryo inside the
seed. It is triploid, having formed from two sets of chromosomes
from the plant that produces it but only one set
from the pollen. Several researchers, such as botanist Mark
Westoby, have interpreted this as sexual selection in which
the maternal plant gains more control than the paternal
plant in controlling the growth of the embryo.
Therefore while natural selection explains the diversity
of adaptations that allow organisms to survive and reproduce
in different environments, and in response to other species, it
is sexual selection that primarily explains much of the seemingly
arbitrary characteristics of animals, and perhaps many
characteristics of plants as well. Natural selection has made
the world of organisms efficient; sexual selection has made it
wildly beautiful.
Further Reading
Albert, Arianne Y. K., and Sarah P. Otto. “Sexual selection can resolve
sex-linked sexual antagonism.” Science 310 (2005): 119–121.
Andersson, Malte. “Evolution of classical polyandry: Three steps to
female emancipation.” Ethology 111 (2005): 1–23.
Bernasconi, G., et al. “Evolutionary ecology of the prezygotic stage.”
Science 303 (2004): 971–974.
Cuervo, J. J., and R. M. De Ayala. “Experimental tail shortening in
barn swallows (Hirundo rustica) affects haematocrit.” Functional
Ecology 19 (2005): 828–835.
Darwin, Charles. The Descent of Man, and Selection in Relation to Sex.
London: John Murray, 1871. Reprinted with introduction by James
Moore and Adrian Desmond, New York: Penguin Classics, 2004.
Dean, Rebecca, Michael B. Bonsall, and Tommaso Pizzari. “Aging
and sexual conflict.” Science 316 (2007): 383–384.
DelBarco-Trillo, Javier, and Michael H. Ferkin. “Male mammals
respond to a risk of sperm competition conveyed by odours of
conspecific males.” Nature 431 (2004): 446–449.
Diamond, Jared. Why Is Sex Fun? New York: Basic Books, 1997.
Haig, David, and Mark Westoby. “Parent-specific gene expression and
the triploid endosperm.” American Naturalist 134 (1989): 147–155.
———. “Seed size, pollination costs and angiosperm success.” Evolutionary
Ecology 5 (1991): 231–247.
Hamilton, William D., and Marlene Zuk. “Heritable true fitness and
bright birds: A role for parasites?” Science 218 (1982): 384–387.
Heinsohn, Robert, Sarah Legge, and John A. Endler. “Extreme
reversed sexual dichromatism in a bird without sex role reversal.”
Science 309 (2005): 617–619.
Judson, Olivia. Dr. Tatiana’s Sex Advice to All Creation. New York:
Henry Holt, 2002.
Marshall, Diane L., and M. W. Folsom. “Mate choice in plants: An
anatomical to population perspective.” Annual Review of Ecology
and Systematics 22 (1991): 37–63.
———, and Pamela K. Diggle. “Mechanisms of differential pollen
donor performance in the wild radish, Raphanus sativus (Brassicaceae).”
American Journal of Botany 88 (2001): 242–257.
Miller, Geoffrey. The Mating Mind: How Sexual Choice Shaped the
Evolution of Human Nature. New York: Doubleday, 2000.
Miller, Paige M., et al. “Sexual conflict via maternal-effect genes in
ZW species.” Science 312 (2006): 73.
Pomiankowski, A., and Y. Iwasa. “Runaway ornament diversity
caused by Fisherian sexual selection.” Proceedings of the National
Academy of Sciences USA 95 (1998): 5,106–5,111.
Searcy, K. B., and M. R. McNair. “Developmental selection in
response to environmental conditions of the maternal parent in
Mimulus guttatus.” Evolution 47 (1993): 13–24.
Snow, Allison A. “Postpollination selection and male fitness in
plants.” American Naturalist 144 (1994): 569–583.
———, and Timothy Spira. “Pollen-tube competition and male fitness
in Hibiscus moscheutos.” Evolution 60 (1996): 1,866–1,870.
Trivers, Robert. “Parental investment and sexual selection.” Pages
136–179 in Campbell, B., ed., Sexual Selection and the Descent
of Man. Chicago: Aldine, 1972.
Van Oort, H., and R. D. Dawson. “Carotenoid ornamentation of
adult male common redpolls predicts probability of dying in a salmonellosis
outbreak.” Functional Ecology 19 (2005): 822–827.
West, Peyton M. “The lion’s mane.” American Scientist 93 (2005):
226–235.
Willson, Mary F., and Nancy Burley. Mate Choice in Plants: Tactics,
Mechanisms, and Consequences. Princeton, N.J.: Princeton University
Press, 1983.