Darwin's Four Postulates
Jesús Guerra Ocampo
Darwin's first Tree of Life sketch from one of his notebooks. Here he speculates about the connection bewteen the different species.
On the Origin of Species
The title page for the first edition of Darwin'd book. He later dropped the "On" from the title. The first edition sold out within a day.
The individual organisms that make up a population vary in the traits they posess.
All of the members of a species differ from one another. Despite their obvious similarities, there are stark differences, both physical and behavioral between different of organisms of the same species. These can be caused by the inherent nature of the organisms, or by outside factors, such as malnutrition or injury. The first are hereditary, while the latter are not, as Lamarck proposed. The traits acquired by an organism during its lifetime, which may set it apart from its peers are not passed down to its offspring. Determining which characteristics are cause by nature and nurture has been a constant struggle for biologists, especially in Darwin's time, when the mechanism for heredity had not yet been discovered.
Some trait differences are passed on to offspring, meaning that they are heritable.
The variability of organisms, which constitutes the first postulate, is complemented by the transmission of these variations. Although Darwin had no knowledge of the existence of DNA and RNA he correctly saw that traits were passed down from generation to generation, chosen either by the environment (natural selection) or by humans (artificial selection) as he identified was the case with the different breeds of dogs and other domesticated animals. This, he proposed, was the basis for evolution, since organisms with more favorable traits were more likely to survive, reproduce and pass those traits to their offspring.
Some individuals produce more offspring than others, and only a subset of the offspring that are produced survive long enough to reproduce.
Despite, or maybe because of, the variability of organisms, it is common for species to produce more offspring than is likely to survive. This is not a waste of energy or resources, but rather a fail-safe mechanism to ensure that at least some of the new organisms manage to live long enough to reproduce. It is of course tied to the causes of mortality, which kill off many of the members of a species before they are of reproductive age. This overproduction, combined with variability, makes for differential survival, that is to say, individuals with more favorable traits will prevail over their peers and competitors.
4. Differential Survival
Individuals with certain traits are more likely to produce the greatest number of offspring.
The lack of resources and the presence of predators make for a fierce competition for organisms to survive. Variation among individuals dictates that some will be better suited to face the challenges life presents and pass those traits to their own offspring. Mortality is the rule, survival is the exception. To quote Darwin:
"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.”
Darwin, C. (2004). The Origin of Species. New York: Barnes & Noble
Black, M. Taylor, E., Monroe, J., Allison, L., Podgorski, G., Quillin, A., (2014). Biological Science. Pearson Education, Inc.
Sarukhán, J. (2009). Las musas de Darwin. México: FCE