Natural Selection

What Is Natural Selection?

The theory of natural selection was brought forward by the naturalist Charles Darwin in 1859. Natural selection was an explanation of how evolution was able to occur. His theory was based on the observations that

  • many more offspring are produced than usually survive to reproduce
  • considerable inheritable variations exist among organisms
  • organisms are closely adapted to their environment
  • many different species are very similar
  • a variety of domestic breeds have been developed through careful selective breeding

So Wheres the Proof?

There is an example of natural selection that has great relevance to us today. This is the antibiotic resistance in bacteria. Heres how it is natural selection: The introduction of antibiotics into the environment of bacteria has acted as a selecting factor. Most bacteria have been unable to survive in the presence of antibiotics, but mutation has allowed a small number to be resistant. Another way bacteria are able to survive antibiotics is cell-to-cell contact with others that are already resistant, allowing the exchange of genetic material. Bacteria can also become resistant by the movement of plasmids (circular chromosomes) that contain the information on antibiotic resistance. These resistant bacteria become the "fittest" for the environment as they have the genetic information to live in the presence of the antibiotic. This is what is meant by "resistant strain". Now this new resistant strain has no competition for resources because it is the only strain able to survive in the presence of antibiotics. The abundance of resources means the resistant strain population can reproduce quickly and become common in the entire population of the bacteria species. A specific example of this process is vancomycin-resistant enterococci. Vancomycin is considered the antibiotic last resort when all other antibiotic treatment have failed. This is why it is of great concern to the medical community when there are bacteria that are resistant to it.

What is enterococci?

  • Gram positive bacteria
  • Native to the human intestinal tract
  • Commonly affects immuno-defficient patients
  • Can cause urinary tract infection, bacteraemia, endocarditis, pelvic/abdominal infection and wound infection
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When did the resistant strain spread?

In 1988 there were the first to reports of vancomycin-resistant enterococci (VRE) in England. Shortly after the first isolates of vancomycin-resistant enterococci were reported by scientists and doctors in the United Kingdom and France, similar strains were detected in hospitals located in the eastern half of the United States. The first Australian case was reported in 1994. Since this time, VRE have spread rapidly and are now encountered by hospitals in most parts of the United States.
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How exactly is vancomycin-resistant enterococci resistant?

There are six resistance types, designated VanA though to VanG. Each resistance type has a different tolerance to vancomycin, as well as other antibiotics. Multiple genes are responsible for these different phenotypes. Random mutation and the transfer of antibiotic resistant genes are the two ways VRE is formed. The transfer of antibiotic genes is done by conjugation. Conjugation is the temporary connection of two bacteria to transfer genetic information.
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Enterococci under a microscope

Natural Selection
The video above demonstrates natural selection with enterococci. By the end of this process, a resistant strain is formed.

What is the mechanism for natural selection- therefore evolution?

The answer is genetics. When Charles Darwin proposed his breakthrough theory, he did not know of genetics. However he knew there was something giving specific characteristics from one generation to the next. Genetics is what has allowed there to be variation, and allowed for the mutations and favourable traits to be passed on to the next generation.

What does natural selection mean to us? Heres an example

Natural selection is a process that is fundamental to the success of life on Earth. On the broad scale, it is positive as individuals that can have adaptive traits to ensure more likely survival reproduce and spread the successful trait throughout the population. However there are cases where the adaptation of a species is detrimental to another. The example of this is vancomycin-resistant enterococci. The bacteria is adapting to the treat of antibiotics, allowing new populations to form as resistant strains. To start with the bacteria can cause numerous afflictions to humans, and then a strain is created that can't yet be cured by modern medicine. So natural selection is beneficial to the continued survival of life on Earth, but can have implications between species, like enterococci and humans.



Briefing. (1997). BMJ, 314(7073), pp.3a-3a., (2016). HSC Hub | Changing the way we educate the next generation.. [online] Available at: [Accessed 21 Feb. 2016].

ScienceDaily, (2016). Antibiotic resistance. [online] Available at: [Accessed 21 Feb. 2016].

Wikipedia, (2016). Vancomycin-resistant Enterococcus. [online] Available at: [Accessed 21 Feb. 2016].

YouTube, (2016). Mechanisms of Antibiotic Resistance in Vancomycin-Resistance Enterococci. [online] Available at: [Accessed 21 Feb. 2016].