Explain two examples of natural selectionThis is a featured page



Example 1: Antibiotic resistance in bacteria


http://www.youtube.com/watch?v=e03YH7cE46A&feature=related

Name of population: The tuberculosis bacterium

Characteristic under evolution: Resistance to the antibiotic Rifampicin

Environmental change: Exposure to an inadequate dose (or inadequate duration) of the antibiotic

Response 1: When patients receive an inadequate dose (or inadequate duration) of the antibiotic then some of the bacterial population may survive. Each bacterium that is killed by the antibiotic has a particular allele that codes for the particular protein that the antibiotic targets. And conversely, each bacterium that survives the antibiotic must be lacking the particular allele that codes for the particular protein that the antibiotic targets.

Response 2: When a surviving bacterium divides it passes on its antibiotic-resistant allele to its two daughter cells. Having inherited the antibiotic-resistant allele, the two offspring survive, reproduce and increase the antibiotic-resistance gene in the population. After several generations the population can become resistant to the antibiotic.


Example 2: Change of beak size in Galapagas finches


http://www.youtube.com/watch?v=l25MBq8T77w&feature=related

Name of population: The ground finch (Geospiza fortis)

Characteristic under evolution: Beak size

Environmental change: Change of food availability caused by change of rainfall.

Response 1 and 2: In 1977 a drought wiped out most of the small-seed plants causing most ground finches to die: some of the big-beaked birds survived because they were able to feed on large-seed plants. Within a few generations, the average beak size evolved to be 4% bigger.

Response 3 and 4: In 1983, the Galapagos islands were hit by heavy rains that triggered a recovery of the small-seed plants. With the return of small seeds, the ground finches with small beaks reproduced better than those with big beaks. As a result, the average beak size decreased by 2.5% within a few years.


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