Darwin's Four Postulates

The Origin of Species

Postulate #1

In All Species There Is Heritable Variation

Darwin argued that every single thing in the planet derived its genotype from that of those who procreated it. Then why is no offspring an exact clone of the parents? While the offspring may possess different genes for different traits, only some of these are expressed, depending on whether they are dominant or recessive. The large variety of genes present in a population that is not isolated makes it impossible for a child to be exactly the same as the parents.
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Postulate #2

More Progeny Are Produced Than Can Survive

This postulate is directly related to Darwin's idea of Natural Selection, the means through which species evolved. Certainly, every generation of a specific species will contain more than one offspring; in some cases, they will contain a myriad of them, each with a different traits. Nevertheless, not all of them will possess the traits that are necessary for surviving the conditions that are present in the environment to which they are introduced. These conditions may range from the harshness of climatic conditions, to overall accessibility to food sources, to deadly predators, to epidemic viruses. Only the individuals that possess traits that enable them to cope with these conditions will grow and procreate.
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Postulate #3

Survival And Reproduction of Individuals Is Not Random

This postulate extends the idea expressed in the previous one. Darwin argued that species do not survive and, furthermore, procreate arbitrarily - their survival is based on their possession of traits that enable them to cope with the conditions of the environment in which they are found. Naturally, because the "fittest" survive, they will eventually mate and produce offspring that possess the genes and express some of the traits that allowed for the survival of the parental generation.
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Postulate #4

More Fit Phenotypes Are Present In Subsequent Generations

Every species that exists today is an example of this final postulate. Had we not acquired distinct traits that made us more capable of dwelling in this planet, we would not exist in our present state. Human beings are fascinating in the sense that they developed organs that allowed them to manipulate the world almost to their liking, such as brains and hands with opposable thumbs. However, we are not the only example of "fit" phenotypes. Throughout the rest of the animal kingdom, we find a plethora of species whose phenotypes reflect fascinating features and mechanisms that enable them to fight threats, such as camouflage and body poison. But even in the simplest things, from beak shape to neck size, the traces of evolution and natural selection are evident in all living things. We are the product of an incredible process that has been taking place for millions of years - and that will surely keep on going for eternity.
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Sources

Buffalo State University. (n.d.). Chapter 3: Darwinian Natural Selection. Retrieved April 7, 2015, from:

http://faculty.buffalostate.edu/penaloj/bio405/outline3.html


Evansville University. (n.d.). Natural Selection. Retrieved April 7, 2015, from:

http://faculty.evansville.edu/de3/b32003/lecture%20summaries/selection.pdf


University of California Berkeley. (n.d.). Introduction to Ecological Genetics. Retrieved April 7, 2015, from:

http://ib.berkeley.edu/courses/ib162/Week1.htm


University of Georgia. (n.d.). Natural Selection. Retrieved April 7, 2015, from:

http://wallace.genetics.uga.edu/groups/evol3000/wiki/d0776/Natural_Selection.html


University of Michigan. (2010, October 10). Evolution and Natural Selection. Retrieved April 7, 2015, from:

http://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selection.html