In the previous post, I presented a histogram for the heights of a sample of 131 adult shells of an Albinaria species. Now here is a bivariate plot of shell heights (H) against shell diameters (D) for the same sample.
Height and diameter are about equally variable and they are also correlated with each other. What could this mean?
If the snails were under strong selective pressure to conserve their shell volumes, we would expect a negative correlation between H and D: when H became large, D would be smaller and vice versa. So a plot of D against H would look like this:
I obtained this plot by calculating a hypothetical value of D for each measured value of H at a constant volume. I obtained the volume from the mean H and D values for the sample assuming that the shell shape was a cylinder. Of course, in a real-life sample there would be scatter arising from biological variation and measurement error.
The positive correlation between H and D in my sample (1st plot) indicates that shell volume is not conserved. What is under selection is probably the shape of the shell. But what is the significance of shell shape?
Let this be tonight's food for thought. I will probably return to this topic in the future.