Encyclopedia of Evolution.pdf - Online Reading Center

0 coevolution
What Are the “Ghosts of Evolution”?
Many evolutionary adaptations can only be understood as evolution
of one species in response to another species, a process known
as coevolution. Coevolution can modify general interactions such
as herbivory or predation, or it can result in very close symbiotic
relationships between two species. In coevolution, the evolution
of each species is influenced not by the mere presence, but by the
evolution, of the other species. To understand an adaptation that
results from coevolution between two partners, one has to at least
know the identity of the other partner. But what happens if one of
the partners has become extinct? The other species may continue
manifesting its adaptations, perhaps for thousands of years, even
though the adaptations are now meaningless. The adaptations of
the surviving species now become puzzling, because the other species
has become one of the “ghosts of evolution.”
In order for symbiotic adaptations to continue being
expressed, even when the other partner is a ghost, the adaptations
must not be detrimental to the organism, otherwise its cost would
be so great that natural selection would get rid of the adaptations
or the species that has them. Furthermore, the extinction of
one species should have been relatively recent, otherwise natural
selection, operating over long periods of time, would presumably
eliminate the adaptations.
One of the types of symbiotic interaction is parasitism, in
which the parasite benefits at the expense of the host. Coevolution
favors hosts that resist parasites. If the parasite becomes extinct,
the host may continue defending against it. Some observers maintain
that some human blood proteins are examples of defenses
against bacterial parasites that are now rare. A mutated form of the
CCR5 protein, which is on the surfaces of some human white blood
cells, may have conferred resistance to bubonic and pneumonic
plague, which would explain why it is most prevalent (even though
it is still less common than the normal CCR5 protein) in northern
European countries. Calculations of the rate of evolution of this
mutant protein suggest that it originated at about the time of the
Black Death of 1347–50, and it may help to explain why subsequent
outbreaks of the plague were less severe than the Black Death. The
plague bacillus, Yersinia pestis, is not actually a ghost; it still exists.
However, it is sufficiently rare—mainly because it is spread by rat
fleas, and public health measures now keep rats and humans from
as close contact as occurred in the Middle Ages—that it is almost
a ghost. The CCR5 protein has recently become a subject of intense
interest, as it appears to be one of the proteins that HIV uses to
gain entry into certain white blood cells (see AIDS, evolution of). It
has also been suggested that the mutant form of the cell membrane
chloride transport protein, a mutation that causes cystic fibrosis,
was once favored by natural selection because it conferred resistance
to diseases. This would explain why the mutation is so common:
one in 25 Americans of European descent carry the mutation.
Since the adaptations carry little cost, and the parasites have only
recently become uncommon, the adaptations persist.
Another type of symbiotic interaction is mutualism, in which
both species benefit. One major category of examples of ghosts of
evolution is certain types of fruits. The function of a fruit is to get
the seeds within it dispersed to a new location. Some fruits have
parachute-like structures or wings that allow the wind to disperse
the seeds to new locations. Other fruits use animal dispersers.
Spiny fruits cling to the fur of mammals. These fruits have probably
not coevolved with specific mammalian species; any furry mammal
can carry a cocklebur fruit and scatter its seeds. But coevolution
is likely to occur between animals and plants with fruits that are
soft, sweet, fragrant, and colorful. All four of these adaptations are
costly to the plant that produces them, and specific to a relatively
small group of animals that eat the fruits. A fruit may appeal to one
kind of animal, but its particular characteristics, especially flavor,
may be uninteresting or even disgusting to other animals. Therefore
if a species of animal that eats fruits becomes extinct, the seeds in
those particular fruits may no longer be dispersed to new locations.
If this should happen, the species of plant will not necessarily
become extinct, although it will probably suffer a reduced population
because fewer seeds are dispersed to suitable new locations.
The fruits will simply fall to the ground near the parent and grow
there. Some seeds will not germinate right away unless they have
passed through an animal intestine; however, these seeds often
germinate eventually even without this treatment. The result will
be clumps of unhealthy, competing plants, but at least they will not
immediately die. Another species of animal may become attracted
to the fruit, but they are unlikely to be as effective as the original
species of animal with which the plant species coevolved. Consider
an example, in which a large animal consumes the fruits of
a species of tree. The animal chews and digests the fruits but does
not chew the seeds. The seeds, with hard coats, pass through the
digestive tract intact and can germinate. This animal is an effective
dispersal agent. If this large animal species becomes extinct,
a smaller animal species may consume the fruits. However, the
smaller animal may not swallow the whole fruit and may either pick
out the seeds or actually crush and eat them. In either case, the
smaller animal is not acting as an effective dispersal agent, the
way the large animal did. In some extreme cases, the fruits pile up
on the ground and rot.
North America has an impressive number of plant species
that produce fruits that seem to have no animals that disperse
them, often because they are too large for any extant animal species
to eat. Their dispersers would appear to be ghosts of evolution.
North America had many large mammal species, until the end
of the last Ice Age, when two-thirds of the genera of large animals
became extinct. This included mastodons, mammoths, horses, and
giant sloths. It is unclear to what extent this was caused by the climate
changes that were occurring at that time, or by overhunting
by the newly arrived humans (see pleistocene extinction). These
animals are probably the ghosts. South America also suffered a
wave of extinctions, about the same time as North America. North
America has many more ghosts of evolution than Eurasia, and
it also suffered a far larger number of large mammal Pleistocene
extinctions than Eurasia. Ecologists Daniel Janzen and Paul Martin
first suggested that this phenomenon is widespread in North and
South America.
Scientists cannot identify ghost dispersers with certainty,
because nobody knows whether any of these animals actually
would have eaten the fruits. If scientists hypothesize that mastodons
dispersed certain fruits, the best that they can do is to see