Sir David Attenborough asked:
If you look at trees, often closely-related trees, growing in exactly the same ground in exactly the same climate they have different-shaped leaves. Why? People will say, “it’s the airflow over it in different particular circumstances or the way that the water drips off it,” that’s the reason. But the trees are growing in exactly the same places alongside one-another. Why do they have different-shaped leaves?
We put this to Ed Tanner, Senior Lecturer in Plant Sciences at Cambridge University
I think the answer to the question about why closely related trees growing in exactly the same ground and the same climate have different-shaped leaves is actually that they don’t. Because they’re closely related they are very similar. For example, all oaks have broadly similar-shaped leaves because they share most of their genetic information. Perhaps a more interesting question is why distantly related trees growing in the same ground and in the same climate have different shaped leaves. The answer is it doesn’t matter very much. As long as leaves are reasonably good at doing their job, which is fixing carbon dioxide in the atmosphere it doesn’t matter whether they are wavy at the edges or not wavy at the edges. They have to absorb the light and once they’ve absorbed the light they would fix CO2. As long as they put their competitors in the shade any reasonably functioning leaf will do the job. It matters where your leaves are in relation to other trees. If you’re an ash tree you’ve got to be above an ash tree or if you’re a beech tree you’ve got to be above an ash tree. It doesn’t much matter what your leaves are like.
I would guess this could be the result of a combination of different trees requirements and the insect and other animal residents and visitors to which each tree plays host.
You will also find that much of this is down to genetic variability. The internal coding of a particular species that would be inherited by its offspring (its genotype) holds all the instructions for the next generation. During the interpretation of this information various anomalies may occur and provide the variability that drives the evolution of the species. This is the phenotype, or the actual physical manifestation of the genotype. There does not actually need to be any environmental input to drive these "within group/species" differences in leaf shape and size at all - it can all be explained by this genotype ==> phenotype process. dentstudent, Wed, 18th Feb 2009
RD - My point is only that this interpretation of genotype into phenotype can also explain a degree of variability within a species. Even if there was only one individual say, that produced many offspring (as is the case in many pioneer species of tree, for example birch), then there would be variation within that group due to phenotype, and hence also a degree of within population leaf size variability, for example. The environmental changes do not alter the phenotype interpretation, only (as you say) those individuals who may benefit because they are better adapted because of their phenotype. dentstudent, Thu, 19th Feb 2009