HL 2 Biology

D.2a: Microevolution


Alleles: varieties of a gene

polymorphic: gene where more than one allele is common in a population

gene pool: all of the gene in an interbreeding population at a certain time

allele frequency: “a number between 0.0 and 1.0 that indicates the proportion of a specific allele to all of the alleles of a gene in a population”

sickle-cell hemoglobin: conformation (shape) altered in low-oxygen conditions; hemoglobin with functions impaired

balanced polymorphism:  if two alleles each confer some survival advantage on the organism, then it is likely that these alleles will persist in the population

transient polymorphism: not stable; one of the alleles is less advantageous; gradually removed from population by selection pressure


Selection pressures on Land Snails (p. 311)

1) It is evident that yellow snails are eaten more frequently than banded snails in the early 9 days of the experiment. Yet after day 10, the yellow snails have shown a decline in the number eaten to as close to zero, where as the banded snails had shown an increase in the number eaten after the 10th day. Also, on the second day onwards, we can see that the number of yellow snails eaten started to decline.

2) The variation in the number of brown banded snails eaten during the experiment can be due to the decrease in the availability of yellow snails as many have been eaten in the early days into the experiment, leaving many banded snails to be available. Also, the increase in the number of brown banded snails can be due to the change in the color of the background. Banded snails may have been able to camouflage more successfully than the yellow snails in the early stages of the experiment when the background color of the habitat was brown. As the experiment progressed and the background color changed into a green color, the banded snails may have stood out more than the yellow snails enabling an increase in the number eaten.

3) I predict the two forms of snail will interbreed as they are classified into the same species (Cepaea nemoralis) and  by following the definition for “species” as “a group of organisms that can interbreed and produce fertile offspring.” this supports this hypothesis.

4) This example illustrates balanced polymorphism because just as the textbook defines the two forms of land snails presents the “two alleles that each confer some survival advantage on the organism”.  The yellow snails are more successful when the environment is a green color than the banded snails. Yet when the background is in a shade of brown, it appears that the banded snails are more successful in living compared to the yellow snails. Thus, we can see how their alleles for the color of their shell provides an alteration in the advantage of survival on the organism.


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