Darwin's Postulates

by Regina Zavala

Intro Video

12.2.2 Darwin's Principles of Natural Selection

1. Variation Among Individuals Within Species

Darwin's first postulate ties back to the units we saw previously regarding phenotypes, genotypes, and Punnet Squares. In this first "rule" Darwin claims that there will be variation even within a single species. Phenotypes need to change in order for the process of natural selection to take place. Without any choices, nature would not have anything to choose from or any distinguishing characteristics that make a certain organism unique and more adapted to selective pressures that might be inserted in said environment. In the examples below it is evident that although humans, finches, and rabbits all belong to a certain species, they are all unique as they each have inherited different properties.

2. Heritable Variation

The second postulate is directly related to the first. Heritable variation basically implies that a significant proportion of the variation within species exists because every offspring inherits genetic information from its parents, eventually forming a genotype with it. Inheritance is the passing of genetic data from parents to their offspring. In a similar manner, inherited traits are those we can attribute to our parents. These specific traits we get from each one of our parents are responsible for enriching the genetic pool within a species, making genotypes and phenotypes more diverse.
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Punnet Squares

The image displayed above is a perfect example of how inheritance variation occurs. Each of the four offsprings represented gets one trait from each of its parents, it could be either recessive or dominant. These combinations of "B" and "b" are what determine eye color in this case. For these specific offsprings, the results were all hybrid, or the combination "Bb."

3. Differential Survival

Another sample paragraph

Differential Survival talks about how populations usually increase proportional to the quantity of food there that is available to them. However, some of the population cannot help but being wiped away by other natural factors that could be referred to as "selective pressures." The deciding factor when it comes to getting wiped out or not, is the organism's ability to adapt to said pressure. Some organisms will have evolved by then and their more favorable qualities or "advantageous traits" will give them an edge over the "regular" population that will ultimately help them survive when disaster strikes.

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4. Survival of the Fittest (Extinction)

Another sample paragraph

As mentioned previously, some species will not make it on Earth. In other words, they'll become extinct. The lack of the advantageous traits that the species didn't inherit from its parents makes it "weaker" in the face of natural selection. The species is not fit enough to survive, therefore it becomes extinct. Sometimes the selective pressures become overwhelming for the species and regardless of how willing it is to fight these pressures, it won't have the ability to keep defeating them forever. Meanwhile, another species that has evolved through the generations has already developed a shield against that pressure. That's the species that will thrive.