Encyclopedia of Evolution.pdf - Online Reading Center

0 sex, evolution of
tures; the microspores grow into multicellular haploid male
structures. The multicellular haploid structures produce
gametes: Females produce egg cells or nuclei, males produce
sperm cells or nuclei. In mosses and ferns, the multicellular
haploid structures grow on moist soil; the male structures
release sperm that swim through a film of water to the egg
that remains in a female structure. In seed plants (such as
pine trees and flowering plants), the multicellular haploid
structures do not grow on the ground. Instead, the female
structures remain protected and fed inside of the immature
seed, where they produce egg nuclei. The male haploid structures
are pollen grains, which are carried through the air (by
wind or by animals) to the female structures. The pollen grain
grows a tube, through which its sperm nucleus travels toward
the egg nucleus (see angiosperms, evolution of; gymnosperms,
evolution of).
In animals, the sexual life cycle is simpler. Meiosis
directly produces gametes. In animals with external fertilization,
the male animal releases sperm into the environment,
where they swim toward the eggs, which the female has also
laid in the environment. The environment must be moist for
this to succeed. In animals with internal fertilization, the male
releases sperm into the body of the female. The sperm must
still swim to the egg, but they do so in the protected internal
environment of the female’s body.
Sex determination (the determination of whether an
individual is male or female or some combination of the two)
can be environmental or genetic. With environmental sex
determination, for example in reptiles, environmental conditions
such as temperature determine the sex of the individual.
There is no consistent pattern among reptile species as
to which incubation temperature causes the differentiation of
which gender.
With genetic sex determination, the genes of the individual
determine the sex into which the embryo develops.
In many animals, gender is determined not just by genes but
by sex chromosomes. In humans, females have two X chromosomes
while males have an X and a Y; females are the
homogametic sex in humans. In birds, butterflies, snakes,
fishes, and some plants, males have two Z chromosomes
while females have a Z and a W, making females the heterogametic
sex.
There is no universally accepted explanation as to why
there should be separate sex chromosomes. Sexual differentiation
of chromosomes, as with X and Y, effectively prevents
crossing over (exchange of chunks during meiosis).
This allows the two chromosomes to specialize on genes
specific to each sex. The fitness interests of the two sexes
are not the same. For example, a gene that controls calcium
metabolism might be used for antlers by males and for milk
by females.
In humans, not all male genes are on the Y chromosome.
Most genes that determine male sexual characteristics are on
other chromosomes. The Y chromosome does carry the gene
that switches on the other male genes. Conversely, there is a
female-determining gene on the X chromosome. Sex determination
can, however, be complex:
• Male-determining genes on the Y chromosome switch on
the production of juvenile and adult forms of testosterone.
Therefore the presence of a Y chromosome usually results
in maleness. In order for testosterone to produce male
characteristics, however, there must be a testosterone receptor
protein. Some individuals have a defective testosterone
receptor, therefore the testosterone has no effect on their
physical characteristics. These XY individuals develop into
females. Normal XX females have and use a little bit of
testosterone, but the XY individuals with defective receptors
do not, and they develop into what have been called
super-females.
• Some XY individuals cannot produce the juvenile form
of testosterone, yet can produce and respond to the adult
form. These individuals grow up as little girls, then at
puberty they turn into boys (see population genetics).
• XO individuals (with one X chromosome and no Y; Turner’s
syndrome) are female but sterile, and while often of
normal intelligence they have certain impairments of social
understanding.
• In most XY individuals, the male-determining factor overpowers
female characteristics. But if the female determining
factor is doubled on the X chromosome, an XY individual
(genetically male) develops as a female.
As evolutionary biologist Ashley Montagu said, Y is
not equal to X. The X chromosome carries many traits,
essential to males as well as females, while the Y chromosome
is very small and consists mostly of noncoding
DNA. Why is the Y small? The explanation that is usually
presented is a conflict of interest between male and female
genes (see selfish genetic elements). Suppose a mutation
occurred on the X chromosome that damaged the Y chromosome.
This would not be good for males, but since the
X chromosome spends two-thirds of its time in females, it
may be selected anyway. This would lead to males becoming
rare, an occurrence that seems to have happened in
some species of insects. In response, the Y could lose genes,
evolving to be a smaller target for destructive genes produced
by the X chromosome.
Evolutionary Reasons for Sex
Scientists have long puzzled over the evolution of sex. Why do
organisms go through all the trouble of producing eggs and
sperm, when they could just produce copies of themselves?
Some plants produce new plants from specialized structures
such as runners, rhizomes, bulbs, or corms. Such
asexual propagation is a common way for humans to make
cuttings from plants. Individuals from asexual propagation
are much more likely to survive than seeds. A runner
from a plant, for example, is much larger than a seed, and
thus more likely to survive; moreover, the runner can remain
attached to the parent plant and be fed by it until it is large
enough that its survival is virtually assured. Oxalis pes-caprae
is a little yellow wildflower that grows in the springtime
in the mild environments of the Mediterranean, Australia,
and California. It is pentaploid, which means that its chro-