Darwin's Four Postulates
Variations Within Species
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.
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
Ao, Ping. "Laws in Darwinian evolutionary theory." Physics of life Reviews 2.2 (2005): 117-156.
Black, Michael, Emily Taylor, Jon Monroe, Lizabeth Allison, Greg Podgorski, Kim Quillin, and Scott Freeman. Biological Science. 5th ed. Pearson. Print.
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.