Encyclopedia of Evolution.pdf - Online Reading Center

selfish genetic elements
Sex chromosomes. In many species, the gender of an
individual is determined by sex chromosomes. In humans,
females have two X chromosomes, while males have an X
and a Y chromosome, although in other species the pattern
can be quite different (see sex, evolution of). The chromosome
associated with males (in humans, the Y) is usually
much smaller than the chromosome associated with
females (in humans, the X). Consider what would happen
if the X chromosome encodes a gene that produces a chemical
that destroys the Y chromosome. An X chromosome
that destroyed Y chromosomes would cause the reproductive
cells to more commonly carry the X than the Y chromosome—resulting
in a disproportionately large number of
female offspring. From the viewpoint of the X chromosome,
this could be advantageous. Since X chromosomes are inside
of female cells two-thirds of the time and male cells only
one-third of the time, an X chromosome that killed Y chromosomes
might be passed on more efficiently into the next
generation than an X chromosome that did not do so. The
result of the spread of the dangerous X chromosome would
be a population that consisted mostly of females, with few
males.
What process might be able to stop the spread of the
dangerous X? Once again, it is possible that silencing factors
in the nucleus might be involved. Another process,
however, has apparently been the reduction in size of the
Y chromosome. Many of the genes in the Y chromosome
have migrated to other chromosomes, with the result that
the Y chromosome (in humans) now has very few genes.
The Y chromosome, being smaller, has fewer sites upon
which chemicals encoded by genes in the X chromosome can
bind—that is, the Y chromosome has evolved to become a
smaller target.
In wood lemmings, some X chromosomes (denoted X*)
are dominant feminizers: X*Y are female, not male. When
an X*Y female mates with a normal XY male, one-fourth of
the offspring will be YY and will never develop. As a result,
two-thirds of the surviving offspring will carry the X* chromosome,
not the expected one-half. This allows the X*
chromosome to spread through the rodent population. Once
again, the frequency-dependent reproductive advantage of
rare males counteracts this trend.
Meiotic drive. Meiotic drive can occur in two ways.
First, it may occur when a chromosome that originated from
either the mother or the father ends up in more than half of
the gametes produced by the offspring. Second, it may occur
when gametes with a chromosome either from the mother or
the father are more successful at fertilization.
The offspring of the F 1 generation receive alleles from
both the mother and the father. The pattern expected from
Mendelian genetics is that half of the gametes produced by
meiosis in these F 1 individuals will carry the paternal allele,
and half will carry the maternal allele. This produces the
familiar 3:1 ratio in the F 2 generation. In contrast, meiotic
drive can cause the paternal allele or the maternal allele to be
present in more than half of the gametes.
One way this can happen is through what have been
called “selfish centromeres.” During meiosis, protein
strands pull the chromosomes apart so that each resulting
cell receives a full set of chromosomes. Centromeres are
segments of DNA in a chromosome to which these protein
strands attach. A larger centromere might be able to link
with more strands, allowing its chromosome to be preferentially
pulled to one of the resulting cells. There is no
advantage associated with this in the production of sperm
or pollen, since spermatocytes in animals and microsporocytes
in higher plants typically produce four sperm or
pollen grains. That is, both the maternal and paternal chromosomes
are assured of ending up in a male sex cell. In the
production of eggs or ovules, however, a conflict of interest
can arise that creates a situation that may favor selfish
centromeres. During meiosis of oocytes in animals and
megasporocytes in higher plants, only one egg or ovule is
produced; the other two or three cells (polar cells) disintegrate.
The chromosome with a bigger centromere (a selfish
centromere) may be more likely to end up in the egg
or ovule rather than being lost in a polar cell. Research
by evolutionary biologist Lila Fishman has shown that, in
hybridization between two species of monkeyflower (Mimulus
guttatus and M. nasutus), the M. guttatus chromosomes
get passed on in the ovules of hybrids much more
effectively than do the M. nasutus chromosomes, perhaps
because the M. guttatus chromosomes have better centromeres.
About 10 percent of species have “B chromosomes,”
which end up in reproductive cells more often than other
chromosomes.
Meiotic drive may also occur if the sperm or pollen with
a chromosome from one of the original parents are more successful
at fertilizing eggs or ovules than are the sperm or pollen
that contain the chromosome from the other parent. This
amounts to competition between sperm as they swim or pollen
tubes as they grow through female cone tissue or the style
of a flower. This is not likely to occur with eggs or ovules,
since eggs and ovules are largely stationary. Paternal or
maternal alleles may also promote the death of zygotes that
contain the other allele.
Gene-centered view of selection. Evolutionary biologist
Richard Dawkins (see Dawkins, Richard) explains natural
selection in terms of selfish genes. Genes are using cells as
their vehicles for reproduction, and natural selection favors
the most efficiently selfish genes.
This viewpoint has been much criticized. Critics have
pointed out that in very few cases can genes express themselves
individually in the phenotype of the organism. Genes
act in complex developmental pathways. When natural selection
acts upon individuals (causing some to reproduce more,
some less, some not at all) it has at best an extremely indirect
effect on favoring one gene over another.
At least two responses to these criticisms are possible.
First, one of the ways in which selfish genes can get themselves
most effectively passed on into the next generation is
by cooperating with other genes. On the level of the individual,
this happens frequently when the coevolution of
species produces symbiosis, and when it progresses as far as
symbiogenesis. If individuals and species can cooperate with
one another even though they are selfish, then genes should