Encyclopedia of Evolution.pdf - Online Reading Center

e able to do the same thing. Second, as indicated above, conflicts
of interest do frequently occur among the genes of an
organism. Natural selection eliminates the individuals whose
genes are in too great a conflict.
Genes and other replicative elements of the DNA often
operate in a manner that human observers call selfish.
Natural selection favors the ability of other genes to suppress
the selfishness of these elements. Evolutionary biologist
Egbert Leigh has called this a “parliament of genes,”
in which selfish interests of different constituencies balance
one another, allowing the work of the cell usually to get
done.
Further Reading
Burt, Austin, and Robert Trivers. Genes in Conflict: The Biology of
Selfish Genetic Elements. Cambridge, Mass.: Harvard University
Press, 2006.
Cavalier-Smith, Thomas. “Economy, speed and size matter: Evolutionary
forces driving nuclear genome miniaturization and expansion.”
Annals of Botany 95 (2005): 147–175.
Charlesworth, Deborah, and Valérie Laporte. “The male-sterility
polymorphism of Silene vulgaris: Analysis of genetic data from
two populations and comparison with Thymus vulgaris.” Genetics
150 (1998): 1,267–1,282.
Dawkins, Richard. The Selfish Gene. New York: Oxford University
Press, 1976.
Doolittle, W. Ford, and C. Sapienza. “Selfish genes, the phenotype
paradigm and genome evolution.” Nature 284 (1980): 601–603.
Fishman, Lila, and John H. Willis. “A novel meiotic drive locus
almost completely distorts segregation in Mimulus (monkeyflower)
hybrids.” Genetics 169 (2005): 347–353.
Frank, S. A. “Sex allocation theory for birds and mammals.” Annual
Review of Ecology and Systematics 21 (1990): 13–55.
Haig, David. Genomic Imprinting and Kinship. New Brunswick,
N.J.: Rutgers University Press, 2002.
Herbst, E. W., et al. “Cytological identification of two X-chromosome
types in the wood lemming (Myopus schistocolor).” Chromosoma
69 (1978): 185–191.
Jones, Steve. Y: The Descent of Men. New York: Houghton Mifflin,
2003.
Leigh, Egbert G. J. “How does selection reconcile individual advantage
with the good of the group?” Proceedings of the National
Academy of Sciences USA 74 (1977): 4,542–4,546.
Pardo-Manuel de Villena, F., and C. Sapienza. “Nonrandom segregation
during meiosis: The unfairness of females.” Mammalian
Genome 12 (2001): 331–339.
Sykes, Bryan. Adam’s Curse: A Future without Men. New York: Norton,
2004.
sex, evolution of Sex is the major biological process
by which genes are recombined among members of the
same species. Almost all species have some form of genetic
recombination.
Sexual Recombination in Different Life-forms
Even bacteria have genetic recombination. Within many
bacterial species, one cell grows a tube toward another cell,
through which a small circle of DNA, called a plasmid,
sex, evolution of
travels from one cell to the other. Alternatively, under certain
conditions bacteria can absorb plasmids that have been
released into their environments. Bacteria of different species,
even different genera, can exchange plasmids. This is the
main way in which resistance to antibiotics can spread from
one species of bacteria to another; resistance to the antibiotic
vancomycin spread from relatively harmless intestinal bacteria
to harmful staph bacteria in this way (see resistance,
evolution of).
Many biologists define species in terms of the ability to
exchange genetic information (see isolating mechanisms;
speciation). If this concept is applied literally, then there
may be, as one biologist has claimed (see Margulis, Lynn),
only one gigantic worldwide species of bacteria. The biological
species concept is also difficult to apply to many plants, in
which cross-pollination can occur among species in a genus,
or even between genera. Whether complete or partial, genetic
isolation is necessary for species to diverge.
Among eukaryotes (see eukaryotes, evolution of)
sexual reproduction occurs by the alternation of meiosis
and fertilization. In most eukaryotic cells, chromosomes
occur in pairs; these cells are called diploid. Under certain
conditions, some of these diploid cells undergo a special
kind of cell division known as meiosis, in which the chromosome
pairs are separated, producing cells that are haploid
instead of diploid. Haploid cells, therefore, have only
half the number of chromosomes that diploid cells have.
In humans, for example, diploid cells (which are nearly all
of the cells of the body) have 46 chromosomes, consisting
of 23 pairs; the haploid cells (ova and sperm cells) have
only 23 chromosomes, with no pairs. The haploid cells of
eukaryotes either fuse together or else they grow into structures
that produce other haploid cells that fuse together
(see below). The haploid cells that fuse (eggs and sperm)
are called gametes. When gametes unite, fertilization has
occurred (see Mendelian genetics). In eukaryotic species,
meiosis alternates with fertilization, resulting in an alternation
between haploid and diploid. Each such cycle is called
a generation.
In fungi, many protists, and some plants, the gametes are
the same size (the species are isogamous), therefore neither
gamete may be called male or female. However, in plants and
animals, meiosis produces both large and small reproductive
cells. The large reproductive cells are considered female, and
the small ones are considered male. The most likely advantage
for the evolution of male vs. female reproductive cells is
specialization. Isogamous reproductive cells are not particularly
good at moving or at nourishing the embryo that develops
from them. In contrast, small male reproductive cells can
move efficiently. Because they are small they can be numerous,
therefore some of these male reproductive cells can reach
their target. Large female reproductive cells can efficiently
nourish the embryo.
In plants, meiosis produces spores. In most plants, large
female spores are called megaspores (from the Greek for
large) while small male spores are called microspores. The
megaspores grow into multicellular haploid female struc