Stabilizing Selection

Question: Why some species are not showing any variation in their phenotypes?

Answer:

Stabilizing selection is viewed as most common selection in nature.  Above the graph of selection pattern is shown, from that one can see that the following generation the middle portion of the graph or the intermediate phenotypes will be favored. The intermediate phenotypes shows an advantage over the extreme ends of the particular traits. One can also view it as Heterozygous advantage which means the heterozygous traits are favored.

Left: Generation 1, Right: Generation 50

The graph shown above on the left is applied to the first generation of the organism which we want to experiment. But if we follow the generation and collect the data, we might end up with only the phenotypes shown at the mean value. That means the tails are cut off and organisms lose the extreme phenotypes. If we just pick up the animal let say a particular species of lizard and measure the length of the body in that species than we might see the body length of 10 +/- 3 cm. which may allow us to think that there was no selection in this species which is due to the explanation given above. The lizard in past may have body length of 3 cm and may measure up to 25 cm. But due to selection 10 cm which is relatively intermediate was favored and thus after many generation only 10 cm body length lizard is available.

I took evolution biology class at University of Houston in Spring 2011 with Dr Frankino and Dr Zufall. I learned more about this selection pattern in Dr Frankino's class.

Dr Frankino showed that with experiment one can push the organism to show the lost trait and we can achieve the extreme end of the phenotype trends.

Please see the link below for more information:

http://www.largedocument.com/52a0b71c/stabilizing_selection.pdf