Explain two examples of evolution in response to environmental change; one must be antibiotic resistance in bacteria.

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Antibiotic resistance in bacteria is a common problem. It results from the transfer of a gene that gives resistance to a specific antibiotic usually by means of a plasmid to a bacterium. Some bacteria will then have this gene and become resistant to the specific antibiotic while others will lack the gene and so will die if exposed to the antibiotic. Over time, the non-resistant ones will all die off as doctors vaccinate patients, but the resistant ones will survive. Eventually, the resistant ones will be the only ones left as a result of natural selection and so a new antibiotic must be created. However, this has to be done on a regular basis as the bacteria keep evolving and become resistant to multiple antibiotics.

 

Example 1:  Bacteria Staphylococcus aureus (associated with a variety of conditions, including skin and lung infections)

Variation:  Antibiotic resistance (some strains have a drug-resistant gene ; other strains do not)

Environmental change:  Exposure to antibiotic (methicillin)

Response:  Methicillin-susceptible S. aureus (MSSA) die, whereas methicillin-resistant S. aureus (MRSA) survive and can pass on their genes

Evolution:  Over time, the frequency of antibiotic resistance in the population increases (drug-resistant gene can also be transferred by conjugation)

 

The Peppered Moth is another example of evolution in response to environmental change. There are two types of these moths; one species has a light color while the other one is darker. When Britain begun industrializing, the soot from the factories would land on trees and so the darker moths then had an advantage over the light ones as they could easily hide from predators. Before the soot, both types of moths were eaten by predators however now that the darker ones were able to hide the lighter ones got eaten more often. The population of the darker moths rapidly increased while that of the lighter ones rapidly decreased until only the dark moths were left. All the lighter moths were less adapted to the environmental change and so they could no longer survive in that new environment.

 

http://www.techapps.net/interactives/pepperMoths.swf

 

Example 2:  Peppered Moth (Biston betularia)

Variation:  Colouration (some moth have a light colour, while others are a darker melanic colour)

Environmental change:  Pollution from industrial activities caused trees to blacken with soot during the Industrial Revolution

Response:  Light coloured moths died from predation, whereas melanic moths were camouflaged and survived to pass on their genes

Evolution:  Over time, the frequency of the melanic form increased (with improved industrial practices, the lighter variant has become more common) 

Zana Katarina L. IB Biology tutor, IB Maths tutor, IB Psychology tutor

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