Darwin's Four Postulates

Jaime Toussaint

Variations Within Species

According to the law of variations no two individuals are identical. The continuous competition amongst animals has forced them to change according to the conditions in which they are so they can survive successfully.

Darwin states that there are two types of variations:

1. Continuous variations/fluctuations

2. Discontinuous variation

Both variations may be neutral, harmful and useful.

He also stated that living organisms tend to adapt to changing environment due to useful continuous variations.

For example: increased speed, water conservation, height and more.


Those who are born with physical alterations that help them survive, do survive and then pass on those advantageous traits to their offsprings. Therefore animals inherit the DNA of their parents and with this they are able to survive and keep passing on that trait.

Pangenesis: Charles Darwin's hypothetical mechanism for heredity.

This theory of Darwin suggests that the whole of parental organisms participate in heredity. Most of Darwin's model revolved around the idea on inheritance of tiny heredity particles he called gemmules that could be passed down from parent to offspring. The theory implied that an organism's environment could change the gemmules in which ever part of the body and then this modified gemmules would be passed to the offspring.

Survival and reproduction are not random.

Survival and reproduction is not random for animals most posses the needed heritable traits to survive in an environment, this way they can reach an age were they can reproduce and give birth to offsprings.

Natural selection happens when organisms with advantageous traits reproduce more than those who don't! (click to watch video)

Population continues to grow at a fast pace, but food and space remain limited. This causes an everlasting competition between animals that need the same requirements to survive. In this competition, every living species desires to have an advantage other the others. Animals compete in every habitat for basic need of life like food, space, water and more. This is called the struggle for existence and has three types:

Intraspecific: This competition happens amongst animals of the same species. For example, two tigers trying to hunt the same deer.

Interspecific: This competition happens between members of different species. For example: a predator and a prey.

Extraspecific: Also known as Environmental. This competition happens between a living organism and an adverse environmental factor. For example, animals fighting heat, cold, drought, flood and more.

More offsprings are produces that can survive

Generation after generation, more offsprings are conceived that are able to survive in the wilderness and reach their reproductive age. After every generation due to the previously mentioned postulate, some genetic variations are passed down which help deal with the selective pressures such as predation and limited food supply. Among those who get to the age where they can reproduce, some will be able to reproduce more than others. Those who have advantageous heritable traits are going to be able to reproduce more therefore asserting that natural selection and evolution does happen.
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Gayon, Jean. "10 From Darwin to today in evolutionary biology." The Cambridge Companion to Darwin (2003): 240.

Winther, Rasmus G. "Darwin on variation and heredity." Journal of the History of Biology 33.3 (2000): 425-455.