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Phenotype
Variation - The Observable Fuel for Evolution and the Cambrian
Explosion
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Genetics
is a science of difference in genes, whereas phenotype is the
resulting variation we can
among organisms. Deriving
from
Greek
phainein,
'to show' + typos, 'type', phenotype is the sum total of an
organism's characteristics or traits, such as its morphology,
development, biochemical or physiological properties, phenology,
behavior, and products of behavior (mating) that we can observe,
which are a collection of traits. The phenotype determines
an organism's physical attributes that, in turn, directly determines
ability
to survive
and reproduce.
A descendant’s inheritance of physical properties occurs
as a secondary consequence of inherited genes. Genes contribute
to a trait, sometimes many for a given trait, and the phenotype
is the observable expression of the genes. This could be a
camouflage color pattern, ability to flee quickly, ability
to ambush or chase down prey, high fecundity, or production
of a toxin to counter predators. Traits that favor survival
and reproduction will enable the favored individual to pass
them on to descendants, where natural selection cause the trait
to be retained within a population’s gene pool. Without
phenotypic variation, there would be no evolution by natural
selection.
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Darwin
on Observable Variation
The
Origin of Species - Charles Darwin - Chapter 5 - Laws of
Variation
When a variation is of the slightest use to a
being, we cannot tell how much of it to attribute to the accumulative
action of natural selection, and how much to the conditions
of life. Thus, it is well known to furriers that animals of
the same species have thicker and better fur the more severe
the climate is under which they have lived; but who can tell
how much of this difference may be due to the warmest-clad
individuals having been favoured and preserved during many
generations, and how much to the direct action of the severe
climate? for it would appear that climate has some direct action
on the hair of our domestic quadrupeds.
The
Origin of Species - Charles Darwin - Chapter 14 - Recapitulation
and Conclusion
Now
let us turn to the other side of the argument. Under domestication
we see much variability. This seems to be mainly due to the
reproductive system being eminently susceptible to changes
in the conditions of life so that this system, when not rendered
impotent, fails to reproduce offspring exactly like the parent-form.
Variability is governed by many complex laws, -- by correlation
of growth, by use and disuse, and by the direct action of
the physical conditions of life. There is much difficulty
in ascertaining how much modification our domestic productions
have undergone; but we may safely infer that the amount has
been large, and that modifications can be inherited for long
periods ....
....
Many other facts are, as it seems to me, explicable on this
theory. How strange it is that a bird, under the form of
woodpecker,
should have been created to prey on insects on the ground;
that upland geese, which never or rarely swim, should have
been created with webbed feet; that a thrush should have
been created to dive and feed on sub-aquatic insects; and
that a petrel should have been created with habits and
structure fitting it for the life of an auk or grebe! and
so on in
endless other cases. But on the view of each species constantly
trying to increase in number, with natural selection always
ready to adapt the slowly varying descendants of each to
any unoccupied or ill-occupied place in nature, these facts
cease to be strange, or perhaps might even have been anticipated....
....
Variability from the indirect and direct action of the external
conditions
of life, and from use and disuse; a Ratio of Increase so
high as to lead to a Struggle for Life, and as a consequence
to Natural Selection, entailing Divergence of Character and
the Extinction of less-improved forms. Thus, from the war
of nature, from famine and death, the most exalted object
which we are capable of conceiving, namely, the production
of the higher animals, directly follows. There is grandeur
in this view of life, with its several powers, having been
originally breathed into a few forms or into one; and that,
whilst this planet has gone cycling on according to the fixed
law of gravity, from so simple a beginning endless forms
most beautiful and most wonderful have been, and are being,
evolved.
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Examples
of observable variation - the art of Ernst Heinrich Philipp
August Haeckel
(February 16, 1834 – August 9, 1919). Among many accomplishments,
Haeckel promoted Charles Darwin's work in Germany
and
developed
the
controversial recapitulation theory (i.e., ontogeny repeats
phylogeny), hypothesizing that a organism's biological
development, or ontogeny, parallels and summarizes its species'
evolutionary development, or phylogeny.
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Batrachia
is a clade of amphibians that includes frogs and salamanders: Class:
Amphibia,
Clade: Batrachomorpha,
Subclass: Lissamphibia,
Superorder: Batrachia |
Fish
of genus Ostracion, a member of the boxfish family (Ostraciidae)
- Class:
Actinopterygii
Order: Tetraodontiformes |
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