Encyclopedia of Evolution.pdf - Online Reading Center

sex, evolution of
biologist George C. Williams points out, this would be especially
true under conditions of intense competition—conditions
that nearly every lineage of organisms encounters from
time to time. In fact, in many natural populations, nearly all
of the offspring die. The very few winners are more likely to
be the lucky recipients of superior genes from sexual recombination
than clonal copies of their parents.
The physical environment (for example, climate) is not
the only or even the most important factor in the success of
genetically varied offspring. Evolutionary biologists such as
Robert M. May and William D. Hamilton have proposed that
the diverse offspring produced by sexual reproduction have
an advantage in responding to the biological environment,
in particular to parasites. The genetic diversity of sexual offspring
is minor, from the viewpoint of survival in different
climatic conditions; much of this minor genetic diversity consists
of proteins and other chemicals that confer resistance to
specific diseases. As Hamilton points out, parasites almost
always evolve faster than their hosts. Bacteria can have one
generation every 20 minutes, while it may take 20 years for a
human generation. Evolution proceeds rapidly in most parasites,
a fact that has made medical doctors finally take notice
of the process of evolution (see evolutionary medicine).
An asexually reproducing species is therefore at great risk of
being killed off by parasites that can quickly evolve the perfect
adaptations to infect it. According to this view, dandelions
and pentaploid oxalis are either just lucky (which is why
such examples are rare), or else the parasite populations are
kept under control by climatic conditions. The few asexual
species of animals may persist because they are continually
migrating and parasites do not easily locate them. A species
of snails in New Zealand has both sexual and parthenogenetic
forms. The sexual forms are more common in habitats
where infection by parasitic worms is common. This proposal
overlaps with the red queen hypothesis of evolutionary
biologist Leigh Van Valen.
Genetic diversity in resisting infection must be of crucial
importance in natural populations, for it is of crucial importance
in agricultural populations. Plant breeders continually
search through wild relatives of crop plants for new genes to
introduce either by crossbreeding or by genetic engineering.
Some of these genes are for faster growth or improved yield,
but often the genes that the plant breeders are looking for
are genes that confer resistance against viruses, bacteria, and
fungi. When plant breeders have ignored this genetic diversity,
fungus blights have broken out and killed vast acreages
of crops. This occurred in the early 1970s when Southern
Corn Leaf Blight killed many corn plants that all shared the
same ineffective resistance genes. A similar argument applies
to the breeding of livestock, though livestock breeders usually
cross different lines of livestock rather than seeking genes
from wild populations. Plant and animal breeders have discovered,
sometimes through multimillion-dollar mistakes,
that genetic diversity in crop populations is essential to keep
diseases under control; it seems certain, therefore, that wild
populations need genetic diversity, therefore sex, for exactly
the same reason.
Elimination and avoidance of bad mutations. August
Weismann, a cell biologist of the late 19th century, proposed
that sexual reproduction not only allows good combinations
of genes to succeed but allows bad mutations to be partially
eliminated. This idea was expanded by geneticists R. A.
Fisher (see Fisher, R. A.) and Hermann Muller, and evolutionary
biologist William D. Hamilton. Most mutations are
harmful. Many of them are recessive, which means that they
can be hidden by the functional dominant alleles. In a diploid
individual, a pair of genes may consist of one good allele and
one bad one; if the bad genes are recessive, the good allele
will hide the bad one. This process could go on and on, causing
a buildup of bad alleles. However, in sexual recombination,
the pairing of good and bad alleles is broken. Some of
the offspring (the heterozygotes) will receive one good and
one bad allele; some (the homozygous dominant individuals)
will receive two good ones; some (the homozygous recessive
individuals) will receive two bad ones. The offspring with two
bad alleles will probably die. The death of the homozygous
recessive offspring partially cleans out the bad genes from the
population. The process is never complete, because bad genes
can always hide in the heterozygotes, but, according to this
theory, it is better than nothing, which is what asexual propagation
would do (see population genetics).
Sexual reproduction not only allows bad mutations to be
eliminated but also allows them to be avoided before being
Each point on this graph represents a human population. Childhood
mortality rates vary greatly among populations. The horizontal axis is the
mortality rate for children whose parents are unrelated. The vertical axis
is the mortality rate for children whose parents are first cousins, in the
same population. The line represents equal mortality rates for children of
unrelated and first-cousin marriages. The points show a predominantly
greater mortality rate for first-cousin marriages within these populations.
(Adapted from Bittles and Neel)