Biology Reference
In-Depth Information
under the influence of changed conditions are not genes but the chemistry of the
organism and the patterns of expression of genes.
Living organisms have evolved various morphological, physiological, behavioral,
and life history adaptations.
Behaviors can be one or a number of sequential actions that the animal performs
in response to an external or internal stimulus and in some cases even in the absence
of a stimulus. Animal behaviors may be grouped into two main classes: innate
and learned behaviors. Innate behaviors are inherited traits that organisms display
without previous experience (see page 228, “Epigenetics of Behavior and Social
Attachment in Animals”).
Learned behaviors are the most plastic and rapidly changing of the phenotypic
adaptations. Behavioral plasticity is especially the characteristic of metazoans, but
plants and unicellulars also display surprising behavioral plasticity and flexibility
in response to environmental conditions or stimuli. Behavior is often influenced by
learning, especially in metazoans. The mechanics of animal behavior are similar in
both innate and learned behaviors because both rely on neural circuits in the cen-
tral nervous system (CNS), which generate and control the behavior ( Baker et al.,
2001; Gould, 1982 ). This is also why the innate behaviors are often influenced by
learning and learned behaviors are influenced by innate behaviors. A clear example
of learning influencing an innate behavior is the imprinting in goslings that, in the
absence of the mother goose, learn to follow a moving object as they would follow
their mother.
In an early evolutionary stage, behavioral adaptation relied heavily on the evolu-
tion of innate standard reactions or instincts rather than learning. This is the case
with unicellulars and plants. However, there is evidence that unicellulars can learn
from previous experience and conditions and can modify their behavior in response
to later exposure to the same conditions. Learning implies remembering previous
experiences.
The differentiation of the neuron and the evolution of the nervous net/CNS in
metazoans during the Cambrian explosion provided metazoans with unprecedented
behavioral plasticity, based on learning, which determined to a greater extent their
evolutionary success. In mammals and especially in humans, the share of the learning
in the overall behavioral inventory continued to increase in the course of vertebrate
evolution. While it is known that this experience-induced memory in metazoans is
stored in the nervous net/CNS, there is only speculation on the location or possible
carriers of that memory in unicellulars and plants (see page 22, “The Control System
in Unicellulars” and page 42, “The Plant Bauplan and Control System”).
This section explores intragenerational adaptations and only tangentially con-
sider the accompanying stress conditions, which arise in response to external/internal
stimuli due to sudden changes in the environment. Intragenerational adaptations in
metazoans are discrete changes in morphology, physiology, behavior, life history, or
psychology arising in response to environmental stimuli, but they are not inherited
in the offspring in the absence of the stimuli that first induced their emergence. The
inherited adaptations will be discussed in the last chapter of this topic.
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