Encyclopedia of Evolution.pdf - Online Reading Center

eliminated—that is, if sexual recombination occurs between
individuals that are not genetically similar to one another.
Consanguineous matings (between close relatives, who
may share many of the same bad mutations) may produce
a relatively large number of offspring that are homozygous
for those mutations, a result frequently called inbreeding
depression. In humans it is well known that incest leads to
a high incidence of deleterious mutations (see figure on page
372). This would seem to be the case for all species, but it
apparently is not. Some lineages of plants exhibit inbreeding
depression, and some do not. Inbreeding depression in
plants is most noticeable when individuals experience stressful
conditions or competition. Depression in plants may show
up more clearly in the second generation of inbreeding; the
effects of inbreeding may have been hidden during the first
year by maternal resources provided in the seeds. Sibling and
other consanguineous matings occur routinely in some invertebrates.
For example, there is a wasp (Scleroderma immigrans)
in which the females not only mate with their own
sons but their own grandsons. Their populations appear to
have very few deleterious mutations. In the case of the wasps,
this may be because the males are haploid, under which condition
all deleterious genes are expressed and eliminated (see
reproductive systems).
It is possible that sexual reproduction has not been
selected at all. Paleontologist Niles Eldredge explains that
sexual species tend to produce new species more quickly than
do asexual species, a process called species sorting. A lineage
that produces many new lineages will persist over time, just
by chance, more than a lineage that produces few or no new
lineages. Random events of extinction can readily eliminate
asexual lineages, which speciate little, but not sexual lineages.
To Eldredge, then, the evolution of sex may represent species
sorting rather than natural selection.
Several plausible reasons for the evolutionary advantage
of sexual reproduction have been proposed. The major difficulty,
according to some evolutionary biologists, is not to
explain why sexual reproduction exists but rather to explain
why it is obligate in so many species.
The evolution of sex provides the perfect illustration
that natural selection favors individuals, rather than the
whole species. Since male animals (and the male function of
plants) can produce many more offspring than females, it
would seem that a population would not require very many
males. Indeed, in agricultural settings, farmers and ranchers
keep only a few choice males as sources of pollen (in
trees such as date palms) or sperm. The sex ratio of agricultural
populations is therefore heavily weighted toward
the females. However, in wild populations, the sex ratio is
usually close to 1:1. Suppose that one begins with a wild
population that consists, like an agricultural population,
of many females and a few males. The few males would
leave many offspring. Any mutation that caused a female
to produce more male offspring would therefore be favored
by natural selection. This process of frequency-dependent
selection would continue until males constituted half of the
population. Departures from a 1:1 sex ratio can occur as a
sexual selection
result of cytoplasmic sterility factors (see selfish genetic
elements).
There is little evidence regarding the origin of the process
of meiosis, which produces sex cells, or of fertilization, which
brings sex cells together. Lynn Margulis and Dorion Sagan
point out that sexual fusion is not very different from other
kinds of cellular fusion that merge the genetic components of
the two cells (see symbiogenesis).
Natural selection favors the sexual production of diverse
offspring, mainly because it cleanses bad mutations from the
population and produces a great diversity of genes that resist
parasites; these benefits accrue to individuals, not just to
populations.
Further Reading
Bell, Graham. The Masterpiece of Nature: The Evolution and Genetics
of Sexuality. Berkeley: University of California Press, 1982.
Bittles, A. H., and J. V. Neel. “The costs of human inbreeding and
their implications for variations at the DNA level.” Nature Genetics
8 (1994): 117–121.
Cosmides, Leda M., and John Tooby. “Cytoplasmic inheritance and
intragenomic conflict.” Journal of Theoretical Biology 89 (1981):
83–129.
Hamilton, William D., R. Axelrod, and R. Tanese. “Sexual reproduction
as an adaptation to resist parasites.” Proceedings of the
National Academy of Sciences USA 87 (1990): 3,566–3,573.
———. Narrow Roads of Gene Land, volume 2: Evolution of Sex.
New York: Oxford University Press, 2002.
Hurst, Laurence D. “Selfish genetic elements and their role in evolution:
the evolution of sex and some of what that entails.” Philosophical
Transactions of the Royal Society of London B 349
(1995): 321–332.
———. “Why are there only two sexes?” Proceedings of the Royal
Society of London B 263 (1996): 415–422.
Lively, Curtis M. “Evidence from a New Zealand snail for the maintenance
of sex by parasitism.” Nature 328 (1987): 519–521.
Margulis, Lynn, and Dorion Sagan. Mystery Dance: On the Evolution
of Human Sexuality. New York: Simon and Schuster, 1991.
Otto, Sarah P., and Scott L. Nuismer. “Species interactions and the
evolution of sex.” Science 304 (2004): 1,018–1,020.
Paland, Susanne, and Michael Lynch. “Transitions to asexuality result
in excess amino acid substitutions.” Science 311 (2006): 990–992.
Ridley, Matt. The Red Queen: Sex and the Evolution of Human
Nature. New York: HarperCollins, 1993.
Takebayashi, Naoki, and Peter L. Morrell. “Is self-fertilization an
evolutionary dead end? Revisiting an old hypothesis with genetic
theories and a macrorevolutionary approach.” American Journal
of Botany 88 (2001): 1,143–1,150.
West, Stuart A., Curtis M. Lively, and Andrew F. Read. “A pluralist
approach to the evolution of sex and recombination.” Journal of
Evolutionary Biology 12 (1999): 1,003–1,012.
Williams, George C. Sex and Evolution. Princeton, N.J.: Princeton
University Press, 1975.
sexual selection Sexual selection is the selection of characters
that enhance reproductive success rather than (even at
the expense of) survival. The individuals in which the characteristic
is well developed will reproduce more often, and leave