Encyclopedia of Evolution.pdf - Online Reading Center

example of frequency-dependent selection is that of sex
ratios (see sex, evolution of). If males are rare in a population,
the average male will leave more offspring than the
average female. Conversely, rare females will leave more
offspring, on the average, than common males. Whichever
sex is rare enjoys greater reproductive success. Frequencydependent
selection is considered to be the main process by
which most animal populations maintain an equal mix of
males and females.
Limitations of Natural Selection
Natural selection is not all-powerful. Below are examples of
limitations upon the power of natural selection:
• Physical constraints. Natural selection cannot cause the
evolution of characteristics that violate the laws of physics
and chemistry. For example, natural selection cannot
produce a large animal with skinny legs (see allometry).
Also, the tallest trees in the world are about 350 feet
(about 70 m) tall. Trees much taller than this are not possible,
because there is a maximum height to which wood can
conduct water, and the tallest redwoods may be very close
to this maximum height. Large animals with wheels cannot
evolve, for the wheel could not receive nutrition from the
rest of the body through the axle.
• Phyletic constraints. Once an evolutionary lineage has
adopted a certain adaptation, this adaptation itself puts
constraints on evolutionary change. It is not possible for
an animal to have an exoskeleton and to be large. Once
arthropods evolved exoskeletons (see invertebrates, evolution
of), large size could no longer evolve in their lineage.
• Absence of genetic variation. Natural selection can only
choose from among the genetic characteristics that are
available in the population. These characteristics arise by
chance mutations.
• Linkage of traits. Natural selection may be unable to select
traits separately, if they are linked either on the same chromosome
(as linkage groups; Mendelian genetics) or in
development. In flowers, petal size and stamen size share
developmental events, and natural selection cannot cause
one to become bigger or smaller without the other.
• Plasticity and dispersal. Organisms may respond to changes
in environmental conditions by plasticity or acclimation,
or by dispersal to a new location. These are ways in which
individual organisms can avoid becoming the victims of
natural selection.
• Post-reproductive individuals. Natural selection cannot
eliminate harmful mutations that have their effect when all
or most of the organism’s reproduction has been completed
(see life history, evolution of). This is sometimes called
the “kingpin principle,” based on a story about automobile
manufacturer Henry Ford. He had his engineers study old
Ford cars and find which part of the old cars was in the
best condition. They reported that the kingpins were just
like new. Ford told his factory managers to stop wasting
their time making such good kingpins. There was no need
for kingpins to last forever in a car that did not. Similarly,
natural selection
in organisms, there is no advantage for genes that promote
the health and vigor of post-reproductive individuals.
• The future. Natural selection cannot anticipate the future
and prepare for it.
• Group selection. Natural selection cannot favor characteristics
that benefit the group but reduce the fitness of the
individual that possesses them. Some adaptations result
in harm for the individual (for example, worker bees that
die after stinging a victim, and a mother spider that dies
when her spiderlings eat her) but these are compensated
by an increase in inclusive fitness (see altruism). When
pathogens infect a host, natural selection may favor those
selfish lineages of pathogens that reproduce most rapidly,
even if the host dies as a result. Natural selection may
work against such selfish pathogens if the death of the host
results in the death of the entire pathogen population (see
group selection) or if the illness of the host causes other
hosts to stay away from the sick individual, thus preventing
the dispersal of any pathogens of that population to new
hosts (see evolutionary medicine). But natural selection
cannot result in individuals sacrificing their own fitness
within a population.
Natural Selection Observed in Nature
Below are just a few examples of natural selection that have
been observed in nature.
Stabilizing selection. Evolutionary biologists Arthur Weis
and Warren Abrahamson studied insects that lay their eggs
inside of plant stems. The grub releases a compound that
causes the stem to produce a round swelling called a gall. The
grub can develop, well fed, inside this gall. There are dangers
from predators, however. Gall wasps can recognize galls and
pierce their ovipositors into the gall, laying their eggs on the
grub, and the wasp larvae eat the grub. Birds can also recognize
galls and tear them open, to eat the protein-rich grub.
Gall wasps prefer smaller galls; wasp parasitism therefore
selects for larger galls. Birds prefer larger galls; bird predation
therefore selects for smaller galls. The result is what looks
like stabilizing selection for medium-sized galls but is actually
a compromise of two processes of directional selection.
Directional selection. Soapberry bugs (Jadera haematoloma)
pierce fruits of certain plants with their long beaks.
They are native to the keys of Florida, where they obtained
their food from native balloon vines. After 1925, landscapers
introduced the golden rain tree, an Asian relative of the balloon
vine but with smaller fruits, into central Florida. Soapberry
bugs began to use the new trees as a food source. In
central Florida, natural selection favored bugs with shorter
beaks. Over the next few decades, the central Florida bug
populations evolved shorter beaks, while the bugs in the keys
retained their long beaks. Evolutionary scientists Scott Carroll
and Christin Boyd documented this change by examining
bug collections that had been made before and after the
introduction of golden rain trees to central Florida.
Diversifying selection. In populations of African blackbellied
seedcrackers (a type of bird), individuals have either
large or small beaks. These two beak sizes correspond to two