Encyclopedia of Evolution.pdf - Online Reading Center

eye similar to that of vertebrates. Some shrimp have both
compound and lens-type eyes.
• Eyes have evolved in three lineages of gastropod mollusks
(snails and their relatives), separately from the eyes of
cephalopods such as squids and octopus.
• Some marine worms have also evolved eyes.
• One group of coelenterates has evolved eyes.
• Vertebrates have also evolved eyes.
Each of these groups has eyes that receives visual information,
but in a different way. For example, the eyes of the
squid are nearly as complex as vertebrate eyes. In vertebrates,
the nerve cells are in front of the retina, while in the squid
they are behind the retina. Humans, other apes, and Old
World monkeys have three-color vision; this type of vision
has evolved independently in howler monkeys of the New
World. All of the above examples are true eyes, with lenses
and retinas, as opposed to simple eyespots that detect the
presence of light.
Animal societies. Social evolution has converged in different
lineages of animals.
• The “army ant” lifestyle may have evolved separately in
Africa and South America, including the behavior of forming
a living nest out of their own bodies that has a limited
ability to regulate the internal temperature. In turn, seven
different lineages of staphylinid beetles have evolved into
army ant mimics, which can live among the ants without
being recognized as intruders.
• The reproductive system in which one dominant female
reproduces and the other females do not, is well known in
the insects (ants, bees, and wasps). Three different lineages
of coral reef shrimp have evolved a similar system. It is
also found in one group of mammals, the naked mole rats.
The insect and mammal lineages achieved this system differently,
however. In ants, bees, and wasps, the males have
only half the chromosomes of the females, and the difference
between queens and workers results from being fed
different foods as they developed. In mole rats, the males
have the same number of chromosomes as females, and the
dominant female inhibits reproduction in the other females
by behavioral means.
Animals finding their way in the dark. Different animal
lineages have converged on alternatives to vision.
• Echolocation has evolved several times independently. Bats
emit sound waves and use the reflection of those sounds
with which they may create mental maps of their surroundings,
including the locations of prey insects. Some kinds of
fishes, and many cetaceans such as dolphins, use a similar
system in underwater locations that are too murky for
vision to be of much use. Oilbirds in South America, and
swiftlets in Asia, use echolocation also. In turn, some of the
prey animals sought by these echolocators have evolved a
defense mechanism: the production of rapid clicks to confuse
the predator. This defense has evolved separately in the
moths hunted by bats, and the shad hunted by cetaceans.
• Electrolocation has also evolved independently, at least six
times in different lineages of fishes. These animals use elec-
Convergences in Marsupial
and Placental Mammals
convergence
Marsupial Placental
Adaptation example example
Burrowing form Marsupial mole Mole
with poor eyesight
Burrowing form Wombat Badger
with good eyesight
Long snout, eating ants Numbat Anteater
Small, eating seeds Marsupial mouse Mouse
Climbs trees Cuscus Lemur
Glides from trees Flying phalanger Flying squirrel
Small, hopping Bandicoot Rabbit
Large herbivore Kangaroo Deer
Feline form Tasmanian cat Bobcat
Canine form Tasmanian wolf Wolf
tric currents generated by muscles to create a mental map
of their surroundings, but the electric currents are different
and come from different sets of muscles in the six lineages.
They also use electrical signals for communication. Electrolocation
can involve an increase in brain size, just as did
the development of vision; in electrolocating fishes, it is the
cerebellum that has increased in size, not the cerebrum as
in vertebrates that rely on vision.
Animal body form and physiological adaptations. Different
lineages of animals have converged upon similar body
forms.
• Marsupial mammals in Australia and placental mammals
in the northern continents (see mammals, evolution
of) have undergone separate adaptive radiation but
have converged upon some striking similarities of form
(see table). Some of the convergences produce forms that
are scarcely distinguishable (as in placental and marsupial
mice), while others are convergences of function more than
of appearance (as in deer and kangaroos).
• A similar convergence has occurred in the placental mammals
that evolved in Africa and the placental mammals that
evolved in the northern continents (North America and
Eurasia) that separated near the beginning of the Cenozoic
era. This convergence has occurred in separate lineages
of grazing mammals, of otters, of insectivores, of burrowers,
and of anteaters.
• Flight has evolved separately in insects, reptiles, birds,
and mammals (see invertebrates, evolution of; reptiles,
evolution of; birds, evolution of; mammals,
evolution of). Insect wings are outgrowths of the external
skeleton. The flying reptiles such as the pterosaurs of
the Mesozoic era had wings supported by one long finger.
Birds have wings without fingers. Bats have wings
with several fingers (see figure on page 92). The ability to
glide evolved separately in the now-extinct kuelineosaurs,
in which extended ribs produced a gliding surface; flying