5.2 Natural selection

Use theories to explain natural phenomena—the theory of evolution by natural selection can explain the development of antibiotic resistance in bacteria. (2.1)

Evolution occurs at both the micro and macro levels.  Macroevolution is the eye-catching form, where we see species changing into dramatically new ones. This process though takes time and is not directly observable. 

Microevolution, while less “glamorous” is no less interesting. Indeed, it has applications that are amongst the most serious concerns in health, medicine and agriculture. This is the ability of populations of bacteria, protists, fungi, insects or plants to evolve resistance to antibiotics, drugs, pesticides and other chemicals used to control them. 

The resistance of bacteria to antibiotics has occurred at an incredible rate, as the image below shows:

Image from CDC.gov

What is particularly concerning about this is shown in the following graph – the number of antibiotics being developed approved continues to decline, which leaves fewer options for treatment.

Understanding the process of evolution is critical to estimating the number and type of new drugs that are needed to combat them.  It is thus necessary to understand that antibiotic use represents a very strong selection pressure. Given the reproductive potential of bacteria (more offspring are born than can survive) and the variation that is possible (through both mutation and horizontal gene transfer) it should therefore come as no surprise that populations rapidly evolve resistance.  Evolution and natural selection are thus not the dated musings of a 19th-century naturalist, but of critical importance to health problems of the 21st-century: in the US alone, over 2 million illnesses and 23,000 deaths per year are directly attributed to evolved resistance.

From an assessment perspective, antibiotic resistance in bacteria is a great example to use when responding to and extended response question on evolution/natural selection.


“About Antimicrobial Resistance | Antibiotic/Antimicrobial Resistance | CDC “. Centers for Disease Control and Prevention. Cdc.gov., 2016. Web. 11 Dec. 2016.

“Microevolution”. Understanding Evolution. University of California Museum of Paleontology.Evolution.berkeley.edu. 2016. Web. 11 Dec. 2016.


5.4 Cladistics

Falsification of theories with one theory being superseded by another—plant families have been reclassified as a result of evidence from cladistics.

The use of DNA sequences to classify organisms has been an important breakthrough in classification. Previously, species were classified primarily on morphology (physical characteristics), which works some of the time but is less useful in other situations. Thus seemingly unrelated organisms have been grouped more closely together and those that were thought to be very closely related have been found to be more distant.  Carl Woese used gene sequencing to not only overturn the existing dogma of the 5-Kingdom system, but to also propose that Archaea are more closely related to humans (eukaryotes) than to other prokaryotic bacteria. This was a major paradigm shift  in microbiology and has since been recognised as “…one of the 20th century’s landmark achievements in biology…” by Dr. Nigel Goldenfeld (“Carl Woese | Carl R. Woese Institute For Genomic Biology”).

In another example, the Figwort family of flowering plants, underwent a dramatic recent reclassification.   The figworts were a large family classified under the family Scrophulariaceae and included the popular snapdragons and foxgloves. Using three genes found in the chloroplast, researchers were able to determine that there were significant differences in lineage and so an entire family had to be reclassified into six families. See Olmstead et al. (2001), full text available online, for the full scientific story.

Cladogram representing changes to the classification of the snapdragons. Olmstead et al. (2001).


n.a. ” Carl Woese | Carl R. Woese Institute For Genomic Biology”. Igb.illinois.edu. University of Illinois, 2016. Web. 25 Apr. 2016.

Essig, F. “Whatever Became of the Snapdragon Family?”. BotanyProfessor. Botanyprofessor.blogspot.  April 5, 2012. Web. April 25, 2016.

Olmstead, Richard G., Claude W. dePamphilis, Andrea D. Wolfe, Nelson D. Young, Wayne J. Elisons, and Patrick A. Reeves. Disintegration of the Scrophulariaceae.
Am. J. Bot. February 2001 88:348-361. Web. Accessed April 25, 2016.