Living and Adapting to Its Own Habitat - Building the Most Complex Structure on Earth - page 193

and a dog has a sharper sense of smell. Evolutionarily, it is not so unfortunate that
that they did not share each other's best senses, because what they have are the best
adaptations for their respective environments.
Because living organisms in nature do not always succeed in evolving adaptations
that help their survival, biologists have concluded that there are constraints on the
adaptive capability of living organisms.
From the early stages of individual development, cell differentiation is determined
by the epigenetic information parentally provided to gametes in the form of cyto-
plasmic factors, centrioles, cytoskeletal elements, and so on. This epigenetic infor-
mation is, to a certain degree, conservative and is the cause of the developmental
constraints on early development. For reasons not yet clearly understood, there is a
stage in metazoan development, the phylotypic stage (see p. 144, Chapter 3, section
“The Phylotypic Stage”), that has been refractory to any change over the last 500
million years since the Cambrian period. The post-phylotypic stage that follows it is
highly prone to adaptive morphological changes, as it may be reasonably concluded
from the breathtaking diversity of extant and extinct of forms in the animal kingdom.
Constraints to adaptive morphological changes, however, also exist in the post-
phylotypic stage. These constraints have determined the limits of morphological
innovations and “imperfections” that are often observed in the metazoan adaptations.
In the course of evolution, living organisms came up with fascinating phenotypic
adaptations in the form of discrete changes in the physiology, morphology, behav-
ior, and life history that help them to sustain, maintain, or increase their fitness in
the changing environment. Phenotypic adaptation in its broadest sense includes a
number of biological phenomena such as the reaction norm and all the forms of the
intragenerational and transgenerational plasticity that will be discussed in separate
sections at the end of this chapter.
Most physiological adaptations take place at the molecular level and, conven-
tionally, they are considered related to the occurrence of beneficial spontaneous
mutations that lead to the evolution of new genes or improve the products of exist-
ing genes. But such favorable mutations are very rare and, as Dobzhansky famously
remarked, the chances of finding a favorable mutation among spontaneous mutations
are like finding a needle in a haystack. And it is well known why present products
of genes, proteins, and RNAs are selected positively for improved biological func-
tions during hundreds of millions to ~3 billion years; hence, any spontaneous change
has a much higher probability of deteriorating rather than improving their functions.
As rare as they are, beneficial mutations have only a slim chance of preservation in
populations of dioecious species.
Living organisms have an inherent ability to adapt to various degrees of change in
the environment, known as phenotypic plasticity, which implies phenotypic changes
that arise in response to the changed conditions in the environment. These changes
unfold as a series of continuous quantitative changes (norm of reaction) or as dis-
crete qualitative changes. The latter implies the development of new traits, and hence
will be termed as developmental plasticity, which will be dealt with later in this
chapter. Both are epigenetically rather than genetically determined; what changes