[Simon Waters]

Simon Waters  asked the Naked Scientists:

My mind was idling along pondering the world around me, and it occurred to me that leaf shapes seem very distinctive.

Oak leaves vary enormously, but most varieties have that rounded edging effect.

It isn't immediately clear to me what the evolutionary pressure for
recognisable leaf shapes is.

One would have thought that Maples and Oaks would have similar pressures on leaves as they often grow in the same area (my back garden for example).

I assume tree varieties with similar leaf shapes are genetically close.
i.e. that the term "oak" is a meaningful category, and not that we
lump lots of trees with similar leaves together that have arisen through
convergent evolution.

I can imagine that there is some sort of genetic inertia. In that if a
tree makes a particular leaf shape, related trees will have the same or
similar leaf shapes till mutations arise. But since trees don't have
eyes, they have no reason to prefer a particular leaf shape (unless it
is for the benefits of creatures in the ecosystem in which they live).

So do we know what the evolutionary pressures on leaf shapes are?

What do you think?

[Asyncritus]

As I recall, a very famous botanist J C Willis wrote extensively on this question.

He, like you (and me), couldn't see the reasons for the different leaf shapes, their arrangement on the stems and a whole host of other items.

I also seem to remember his saying that since plants can't move about like animals, they were very much more susceptible to  environmental changes.

Here's a comment about him, and the link.

A British Botanist

By any standards, J. C. Willis was a great botanist. Among the many distinctions conferred on him were Fellowship of the Royal Society and an honorary doctorate of science from Harvard University. The scientific establishment on both sides of the Atlantic felt obliged to honour him, even though he was a rebel.
http://www.wcg.org/lit/booklets/science/hayward2.htm

[RD]

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        The Evolution of Plant Physiology By Alan R. Hemsley, Imogen Poole, Linnean Society of London Palaeobotany Specialist Group
        http://books.google.co.uk/books?id=8tJSb7-mYGIC&pg=PA255&vq


Leaf shape can provide a way of measuring prehistoric temperatures...

Precise estimates of past temperatures are critical for understanding the evolution of organisms and the physical biosphere, and data from continental areas are an indispensable complement to the marine record of stable isotopes. Climate is considered to be a primary selective force on leaf morphology, and two widely used methods exist for estimating past mean annual temperatures from assemblages of fossil leaves. The first approach, Leaf Margin Analysis, is univariate, based on the positive correlation in modern forests between mean annual temperature and the proportion of species in a flora with untoothed leaf margins.

http://www.psjournals.org/paleoonline/?request=get-abstract&doi=10.1666%2F0094-8373(1997)023%3C0373:WALGTA%3E2.3.CO%3B2