Biology as tech/Biology as art III

Denton's Challenge: Are Leaf Shapes Adaptive?

Evolution News & Views

In his new book Evolution: Still a Theory in Crisis, Michael Denton provides a cavalcade of examples of non-adaptive forms in nature. On page 77, he points to angiosperm leaves as a case visible to everyone.

It is not only the unicellular world that abounds with what appear to be abstract formal patterns. Even on the most cursory and passing observation of some of the most familiar natural forms, such as the forms of leaves and the variety of phyllotactic arrangements that might be observed in any suburban garden, it is hard to resist concluding that a vast amount of botanical order serves no specific adaptive end. [Emphasis added.]

Our film Biology of the Baroque explores this further starting at 4:20, showcasing the wide variety of leaf shapes in a forest. Each plant has its own shape in the same environment, yet they live side by side. "But for Darwinian evolution to explain the shape of these leaves," the narrator states:

...there ought to be some reason why that specific shape caused one organism to live and another to die in a given environment. Yet there appears to be no functional reason why there are so many different leaf shapes. Much like Baroque architecture, these shapes seem extra, perhaps even decorative. They're not needed to survive. They are simply beautiful.

Enter Current Biology to the rescue of Darwinian functionalism. In "Evolutionary and Environmental Forces Sculpting Leaf Development," evolutionary botanists Daniel H. Chitwood and Neelima R. Sinha try to give adaptive reasons for leaf shape.

Leaf shape is spectacularly diverse. As a major component of plant architecture and an interface for light capture, gas exchange, and thermoregulation, the potential contributions of leaves to plant fitness are innumerable. Particularly because of their intimate association and interaction with the surrounding environment, both the plasticity of leaf shape during the lifetime of a plant and the evolution of leaf shape over geologic time are revealing with respect to leaf function. Leaf shapes arise within a developmental context that constrains both their evolution and environmental plasticity. Quantitative models capturing genetic diversity, developmental context, and environmental plasticity will be required to fully understand the evolution and development of leaf shape and its response to environmental pressures. In this review, we discuss recent literature demonstrating that distinct molecular pathways are modulated by specific environmental inputs, the output of which regulates leaf dissection. We propose a synthesis explaining both historical patterns in the paleorecord and conserved plastic responses in extant plants. Understanding the potential adaptive value of leaf shape, and how to molecularly manipulate it, will prove to be invaluable in designing crops optimized for future climates.

And so the debate is engaged. They want to explain the spectacular diversity of leaf shapes in an adaptive context. To accomplish this, though, they would have to relate unguided genetic mutations to selection at the molecular level. They admit up front that "The molecular mechanisms underlying such morphological diversity are even more poorly understood." 

Yet, patterns of correlation among leaf shapes across extant species, temperature and precipitation, and between fossil leaves and the paleoclimate, provide tantalizing glimpses into hypotheses unifying the development, evolution, and environmental plasticity of leaf shape with the possible functions it might confer.

They make the following suggestions for possible adaptive reasons for leaf shapes:

1. Plasticity: Shapes are not fixed in a particular species; leaves respond during development to environmental cues as they grow. "The leaves displayed by a plant at successive nodes reflect the developmental and environmental context of each leaf from initiation onwards."
2. Time: Leaf shape across geologic time is sensitive to the environment. "Relationships between leaf dissection and paleoclimates provide the most sweeping evidence of the responsiveness of leaf dissection to temperature and precipitation," they claim. As evidence, they point to studies that show "leaf serration can be used as a reliable index of the paleoclimate."
3. Climate: Leaf shape is plastic within one plant's lifetime and across geologic time, they say. "That plasticity within the lifetime of a single plant, extant species distributions, and correlations with the paleoclimate follow each other suggests an intimate relationship between leaf morphology and climate." 
4. Transpiration: "Paleoclimate correlations are focused on marginal serrations, which have been hypothesized as a constraint arising in thinner leaves more reliant on major veins, an adaptive feature allowing early season growth in deciduous forests, sites of increased transpiration, or that arise from hydathodes to maintain optimal leaf turgor." This is not true of all plants, however. "Similar plasticity in annuals observed in cold temperatures involves more than just serrations and cannot be explained by features exclusive to woody eudicots alone."
5. Sunlight availability: "Leaf dissection may simply reduce leaf area and affect canopy light interception...."
6. Economy: "....while also minimizing the distance between photosynthetic tissue and veins."
7. Heat transfer: "The potentially reduced leaf area in dissected leaves also facilitates heat transfer, reducing the boundary layer effect, but leaf shape likely also affects boundary layers independent of size."
8. Embryonic packing: "Leaf dissection may even reflect developmental constraintsrelated to packing in buds."
9. Compound leaves: "The possible functions of compound leaves have centered on the associated costs of producing a branch with many leaves vs. a complex leaf with many leaflets, and the premium put on vertical growth and 'branch shedding'."

All these hypotheses amount to little more than suggestions and just-so stories. They admit that none of them constitute a general law.

While hypotheses about the function of leaf shape are fascinating, they are numerous and cannot be generalized. Focusing on changes in shape common to evolutionary transitions into new environments, plastic responses in model organisms and patterns observed in the paleorecord may help narrow the field of relevant hypotheses to arrive at a more robust understanding of leaf morphology.

We ain't there yet, in other words. They leave the problem of leaf shape virtually unexplained, despite the fact that "Integrating natural variation in leaf shape with molecular pathways has always been a central goal of evolutionary developmental biology."

How much time to the Darwinians get? They've been thinking about this problem for 157 years at least. Must a functionalist story be served up, no matter how fraught with anomalies? Think again about the diversity in a forest, all these leaf shapes coexisting in the same environment. Where is a general law that would make them all trend toward the same shape? There is none. The diversity is still spectacular.

Denton's book is filled with other examples. "It's OK if it's just a maple leaf," he says in the video:

You can perhaps pass over the maple leaf, but if non-adaptive order, like the maple leaf, permeates the biological world, and if a lot of the taxa-defining novelties seem to be non-adaptive, you now have a nightmarish scenario when the fundamental assumption of Darwinism is that all these novelties in nature are adaptive suddenly looks very insecure.

Intelligence is a cause that can make things beautiful and diverse. After 157 years, its explanatory robustness is doing a much better job than fruitless attempts to seek "tantalizing glimpses into hypotheses" in a search for "possible functions" in the adaptationist program.