Genetic Variation

Phenotype Variation - The Observable Fuel for Evolution and the Cambrian Explosion

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.

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 whichHaeckel art of Batrachia, a clade of amphibians that includes frogs and salamanders 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

DarwinNow 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.

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.

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