Encyclopedia of Evolution.pdf - Online Reading Center

neoteny Neoteny is the retention of juvenile characteristics
in adult animals. Neoteny has been an important part of
human evolution.
As juvenile animals grow, they experience both physical
and reproductive maturation. They usually go together:
Individuals develop adult physical characteristics and reproductive
maturity at the same time. But this does not always
occur. In heterochrony (Greek for “different times”), physical
and reproductive maturation occur at different times.
Most examples of heterochrony are of animals that become
reproductively mature while still retaining juvenile characteristics,
a process called paedomorphosis (Greek for “juvenile
form”).
Paedomorphosis may occur in either of two ways. Progenesis
occurs when physical juveniles become reproductively
mature. Reproductive maturity causes physical maturation
to stop; therefore these animals retain juvenile characteristics
until they die. Examples include several kinds of salamanders.
The juvenile phase of the tiger salamander (genus Proteus) is
called the axolotl, which has external gills that allow underwater
existence. Under normal (moist) environmental conditions,
the juvenile phase develops into the adult phase. Under
stressful (dry) conditions, an axolotl can become sexually
neoteny
mature. This premature reproduction produces fewer offspring
than normal adult reproduction, but it has the advantage of
allowing reproduction to occur even when the ponds in which
the salamanders live dry up early in the season. This flexibility
of development allows the individuals of this species to reproduce
well during moist years and reproduce at least a little in
dry years. The mud puppy (genus Necturus) always becomes
reproductively mature in the juvenile phase.
In the late 19th century, zoologist Walter Garstang used
progenesis (although the term had not yet been invented) to
explain the origin of the vertebrates. The invertebrates known
as tunicates (see invertebrates, evolution of) have an
adult phase that attaches to surfaces such as rocks. It siphons
water in through one orifice and out through another and filters
food from the water. It certainly does not resemble a vertebrate.
The juvenile phase of the tunicate looks like a small
headless fish. It has a mouth and gill slits in front, and along
its back, just where human backbones and spinal columns
are found, it has a cartilage rod (a notochord) with the major
nerve running along it. Garstang proposed that the first vertebrates
evolved from animals similar to tunicates that became
reproductively active in the juvenile phase, after which the
former adult phase never recurred.
The baby chimpanzee (left) has a face with features more closely resembling those of humans than does the mother chimpanzee (right). The mother
chimpanzee is using a stick as a tool to collect insects. (Courtesy of Jim Moore/Anthro-Photo)