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missing links
Batesian mimicry. Named after a 19th-century British
naturalist who first observed this phenomenon in the Amazon
rain forest (see Bates, Henry Walter), this form of mimicry
occurs when nonpoisonous prey (mimics) evolve a resemblance
to poisonous animals (models). The mimic therefore
benefits from the protection afforded by the poisonous models,
without themselves having to expend resources to make
themselves poisonous. Examples include stingless flies that
resemble bees and wasps, and the viceroy butterfly (Limenitis
archippus) that resembles the poisonous monarch butterfly
(Danaus plexippus). If the mimic becomes too common, relative
to the model, the system becomes unstable, because when
the predators occasionally eat the mimic it will not suffer the
consequences that originally selected for the ability to recognize
the model.
Mimicry provides numerous excellent examples of natural
selection. Batesian mimicry formed an important piece of
evidence that Charles Darwin cited in his original presentation
of evolutionary theory (see Darwin, Charles; origin
of species [book]).
Further Reading
Kaiser, Roman. “Plants and fungi use scents to mimic each other.”
Science 311 (2006): 806–807.
Ruxton, Graeme D., Thomas N. Sherratt, and Michael P. Speed.
Avoiding Attack: The Evolutionary Ecology of Crypsis, Warning
Signals and Mimicry. New York: Oxford University Press, 2005.
missing links “Missing links” refers to fossil evidence of
evolutionary transitions that has not been found. The term
makes little sense today, as practically all “missing links”
have been found. This is particularly amazing when one considers
the low probability that any individual, or even any
species, might be preserved (see fossils and fossilization).
Fossils have been found that confirm most of the transitional
states between major (and minor) groups of organisms.
For example:
• The transition of reptiles to birds required the evolution
of feathers, the loss of tail vertebrae, the loss of teeth, and
the development of the furcula or breastbone, among other
things. A missing link might have feathers but have reptilian
skeletal features such as tail vertebrae and teeth. Numerous
fossils of feathered dinosaurs have been found, as well as
primitive birds that had teeth (see birds, evolution of).
The most famous of these fossils is arcHaeopteryx.
• The transition of reptiles to mammals required numerous
changes, some of which can be preserved in fossils. One
of these is the transition of some reptilian jawbones into
mammalian ear bones, and the relocation of the jaw joint.
A missing link might have intermediate jawbones and two
jaw joints, one reptilian and one mammalian. A series of
such fossils, the therapsids or mammal-like reptiles, has
been found that displays these features, including some
with two jaw joints (see mammals, evolution of).
• The transition of terrestrial mammals to whales required,
among other things, the loss of hind legs, the transition of
front limbs from legs into paddles, and the migration of the
nostrils to the top of the head. A missing link might have
small limbs and a skull intermediate between that of terrestrial
mammals and modern whales. A series of such fossils
has been found (see whales, evolution of).
It is not always certain that the transitional fossil or
fossils are actually the direct ancestors of the modern forms.
Archaeopteryx is a good example. There were other birds
that had more modern characteristics and that lived soon
after Archaeopteryx. Therefore Archaeopteryx itself may
not have been an ancestor of modern birds. It was probably
a cousin, not an ancestor. Archaeopteryx and the true ancestor
of modern birds descended from a common ancestor,
and Archaeopteryx had evolved less than the ancestor of
modern birds at the time that Archaeopteryx lived. Archaeopteryx
is very clear confirmation of a transitional state
between Mesozoic dinosaurs and modern birds. Feathered
dinosaurs lived at the very time that one would expect them
to have lived, if birds evolved from dinosaurs, but scientists
are not sure which feathered dinosaur might have been the
true ancestor.
In some cases, the evolution of a complex adaptation
might appear improbable or even unbelievable. This is one
of the major claims of intelligent design. In such cases,
the discovery of a fossil “missing link,” or the living descendant
of a “missing link,” can demonstrate that the complex
adaptation did, in fact, evolve, even if an observer could
not have imagined how. One example of many is the complex
set of adaptations displayed by desert cacti such as the
saguaro of the American southwest. Such cacti can survive
and flourish in desert areas because they carry out photosynthesis
and store water in thick green stems rather than
leaves. In order to do this, cacti underwent several evolutionary
modifications. Their leaves were reduced to small and
temporary structures that did not carry out photosynthesis;
stomata, the pores normally found on leaves, developed
on stems instead; bark development was suppressed on the
stems, allowing them to be soft and green rather than hard
and brown; instead of many small branches, they developed
large, thick branches that stored water; and they developed a
modified form of photosynthesis known as CAM (see photosynthesis,
evolution of). So complex are these adaptations
that some observers are tempted to attribute them
to special creation. But, in fact, there are several species of
plants of the genus Pereskia which are almost perfect intermediates
between desert cacti and their ancestors. They live
in dry mountainous areas of Central and South America.
Pereskia plants have leaves, even though they are succulent
leaves; some have stem stomata, some do not; some have
delayed bark formation, some do not. All of them are bushes
or trees, rather than having succulent branches. Even their
flowers are intermediate between those of desert cacti and
their ancestors. Many have succulent leaves, and some of
them can use CAM under drought conditions. They closely
resemble the “missing links” that must have existed around
30 million years ago.
Throughout this encyclopedia, transitional forms are discussed.
This next section focuses on “missing links” that seem