Encyclopedia of Evolution.pdf - Online Reading Center

whether elephants eat any of these fruits. This tells the scientists
little, because elephants are similar to but not the same as mastodons,
and because animals eat not just what their species can eat
but what they have learned to eat from their parents. Even if investigators
could train elephants to eat some of these fruits, this would
not prove that, in the wild, mammoths or mastodons ate them.
Among the examples of North American ghosts of evolution are:
• Bois-d’arcs. These are small trees, noted for their very strong
wood, that have been planted widely across North America as
fencerow windbreaks. Like other trees in the mulberry family, Maclura
pomifera has separate male and female trees; and the females
produce composite fruits that consist of a cluster of berries.
While mulberries are small, sweet, and edible by many species of
birds and mammals (including humans), the fruits of the bois-d’arc
are large (about seven inches, or 15 cm, in diameter), green, hard,
and sticky. (These trees are also called Osage oranges, because
the fruits look a little bit like oranges and grow wild in the tribal
lands of the Osage tribe; and hedge-apples, because the fruits look
a little bit like apples and the trees were planted as hedges.) While
not highly poisonous, the fruits are definitely repulsive and difficult
to digest. There appears to be no animal that eats the fruits.
Squirrels consume the seeds, but this is not dispersal; it kills the
seeds rather than dispersing them. The fruits commonly fall to the
ground and rot; several of the fruit’s dozens of seeds grow, resulting
in a clump of trunks that have grown together. In some cases
the trunks have fused to form what appears to be a single trunk. It
is rare to see a single-trunked bois-d’arc tree in the wild. (Singletrunked
bois-d’arc trees are common in the countryside, but these
were planted in fields that were subsequently abandoned and are
not really wild trees.) Another consequence of the absence of a
disperser is that wild bois-d’arcs are found only in river valleys of
Oklahoma, Texas, Arkansas, and nearby areas. Without dispersers,
the only direction for such large fruits to roll is downhill, into
river valleys. Had they not been planted widely across eastern
North America, they might eventually have become extinct as the
last fruit rolled into the sea.
• Avocados. Wild avocados (Persea americana) are not nearly as
large as the ones in supermarkets, but they are still large enough
that no extant species of animal swallows them whole. Many animals
can nibble at the nutritious fruit, but the seeds are simply
left near the parent in most cases. Which large animal might have
eaten these fruits in the past?
• Honey locusts. Gleditsia triacanthos, a tree in the bean family, produces
very long, spiral seed pods. No native species of mammal
regularly eats these pods, even though they are juicy and sweet.
Once cattle were introduced to North America, many of them
would eat these pods when available. Like bois-d’arcs, honey
locust trees have a recent native range confined largely to river
valleys. Today they have spread again, because they have been
widely planted as urban trees and have escaped again into the
wild. There appears to be a similar lack of dispersal in the mesquite,
a bush in the bean family that also produces sweet pods,
though much smaller than those of honey locust.
Other examples that have been suggested include papayas, mangos,
melons, gourds, and watermelons.
coevolution
The idea that North and South America have many fruits that
are ghosts of evolution is corroborated by African and Asian analogs.
There are numerous fruits in Africa and Asia that are consumed,
and the seeds dispersed, by animals (such as elephants) that
are similar to the ones that became extinct in the Americas. Some
of the ghost fruits are inedible to most animals, but the now-extinct
specialist dispersers may have had, as African and Asian fruit dispersers
do today, intestinal bacteria that break down otherwise
toxic products. Elephants eat clay, which adsorbs toxins from the
digestive system. Might mammoths and mastodons have had similar
adaptations that allowed them to consume fruits such as bois-d’arc?
Fruits are not the only examples of the ghosts of evolution.
Other examples include flowers and stems.
• Flowers. Pawpaws (Asimina triloba), small wild fruit trees of the
eastern deciduous forest of North America, persist primarily by the
spread and resprouting of roots and underground stems. No native
pollinator appears to be reliable. Although there has been no recent
wave of pollinator extinctions as great as that of the Pleistocene
extinction of fruit dispersers, apparently the pollinator of this tree
has become extinct. Recently, native pollinator populations have
precipitously declined, which threatens many native plant species
with ultimate extinction. This may especially be true of large cacti
in the deserts of southwestern North America, which rely on bats to
pollinate their flowers. Thus very soon the bats that pollinate these
cacti might become ghosts of evolution.
• Stems. The bois-d’arc also has large thorns, widely spaced on
the branches, which appear totally ineffective at defending the
leaves against the animals that today eat them, such as deer. Perhaps
the thorns as well as the fruits are the leftover responses
to the now-extinct ghosts of evolution. Wild honey locust trees
have three-pointed thorns (hence the name tri-acanthos) bristling
from their trunks. Hawthorns (genus Crataegus) have thorns even
longer, and more widely spaced, then those of bois-d’arc. Evolutionary
biologists estimate that 10 percent of the woody plant
species in New Zealand have a particularly tangled method of
branching that may have protected them at one time from browsing
by moas, giant birds that were driven into extinction by the
first Maori inhabitants. Other revolutionary biologists, such as C.
J. Howell, disagree.
Thirteen thousand years is not long enough for the trees to
evolve some other mechanism of dispersal. One would expect
that, eventually, the unnecessarily large fruits would no longer be
produced, as mutant forms of the trees produced fruits that could
be easily dispersed by wind or water. This may have happened in
the case of the honey locust. An apparently new species of locust,
the swamp locust Gleditsia aquatica has evolved in southern North
America, perhaps from a Gleditsia triacanthos ancestor. It produces
much smaller pods that float easily in water.
The main point of this essay is that species are “designed”
by evolution not to live in their current environments but in those of
their immediate ancestors—and if that environment changes, there
can be a considerable lag time before evolution designs a response.
The result is, as ecologist Paul Martin wrote, “We live on a continent
of ghosts, their prehistoric presence hinted at by sweet-tasting
(continues)