D.2a: Origin of Species: Microevolution

28/08/2013 § 1 Comment

What even is an allele? I’ll tell you what it is, it’s a form of a gene, (thanks study guide). And see here, sometimes there are populations where there are two alleles in a gene pool, and that’s what we call polymorphic, kids. More specifically, if one allele is slowly replacing the other, then that particular population is showing transient polymorphism. One example of this is the peppered moth, Biston betularia, where dominant alleles control the characteristics of the moths’ wing colour.

But wait, wait, there’s something even better, oh my god. An even more intriguing – and possibly more well known – example would be sickle cell anaemia combined with malaria. Okay, okay, so we’re going to call it balanced polymorphism because it totally follows the law of equivalent exchange. What happens during balanced polymorphism is that the two alleles continually persist against each other in the gene pool of a population and transient polymorphism doesn’t occur as both traits are being expressed at the same time, and is therefore balanced.

With sickle cell anaemia, if an individual is homozygous with allele HbA, they won’t actually get sickle cell anaemia but may get malaria. In contrast, if an individual is homozygous with allele HbS, they won’t get malaria, but they’ll develop a severe case of sickle cell anaemia. And the lucky dogs who are heterozygous with both alleles will not develop sickle cell anaemia and are also resistant to malaria. They are therefore… superior.

DATA BASED QUESTIONS

Selection pressures on land snails, page 311

1. Outline how the number of each form varies during the experiment.

The yellow snails start off at quite a high number (over the 90s) between the first four days of the experiment but definitely decreases in the number of eaten snails over time. By 10 days, barely any yellow snails are being eaten and from then onwards, hardly any yellow snails are consumed by the birds. In contrast, the banded snails start off at a lower number of eaten snails than the yellow snails (around 15) but the number grows over time. While the banded snail consumption is in relatively the same area for the first 10 days, after the 10th day, the number of eaten banded snails increases from less than 5 snails eaten per day to 61 snails eaten by the 20th day. 

2. Explain two reasons for the variation in the number of brown banded snails eaten during the experiment.

We know that the original colour of the snails’ habitat was a mixture of brown shades. In light of this, then the brown snails could obviously blend in much better into their backgrounds than the yellow snails (which appeared green against the background). As the background changes from brown to green, the brown snails become more visible, therefore the rate at which they are consumed will rise. A second reason for the variation could be simply that the birds ran out of yellow snails.

3. Predict, with a reason, whether the two forms of snail interbreed.

They probably do because both snails are of Cepaea nemoralis, and considering that all the snails lived in the same population and were of the same species, it’s quite likely they did interbreed.

4. Discuss whether this example illustrates balanced or transient polymorphism.

This example therefore illustrates transient polymorphism. I said it at the top, transient polymorphism is when one allele slowly replaces the other and as the graph illustrates, the snails with the green allele eventually start to win and the snails with the brown allele eventually start to lose and die off more. Gradually, through time and the changing environment, one allele triumphed over (read:replaced) the other.

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