Purpose and desire and flower color

One of the crucial points I have tried to make in Purpose and Desire is that evolution is a phenomenon driven by cognition, not by the Neodarwinian model that focuses on gene selection. A question that has popped up repeatedly among the critics is that, while it might be possible to posit such things as cognition and intentionality in creatures with advanced and complex brains, surely it is impossible to imagine cognition as a driver in creatures that lack those brains—things like bacteria, or plants. Cognitive plants? You must be joking!

Well, no, I’m not joking. Of course, plants are cognitive beings, as I have noted elsewhere on this blog. No living thing cannot be cognitive and still be living. Nevertheless, the confusion persists. So much confusion persists these days.

Part of the confusion arises from the common mistake of conflating cognition with the (undoubtedly related but very likely much different) phenomenon of consciousness. I can say with metaphysical certitude that a plant is a cognitive being: it must poll its environment, and act upon this information. That is the very nature of adaptation.

I can say with similar metaphysical certitude that I am a conscious being. In some feverish delirium, I might entertain that idea that a plant also is conscious, but if it were, it would be a consciousness so alien to my own that I could not even imagine it. Consciousness is, in the most literal sense, metaphysical.

But what has this to do with evolution? As I argued in Purpose and Desire, cognition can be a driving force in evolution because cognition is an important aspect of that fundamental property of life, homeostasis, and it is homeostasis that confers the striving and intentionality that drives life and its evolution forward in time. As a counter to that idea, the standard Neodarwinist retort is that such things are unnecessary because the same apparent striving can be derived more simply from natural selection of genes. There is no goal, no intent, no purposefulness, just the relentless and repetitive selection of genes for good function—fitness. Occam’s Razor renders moot any metaphysical speculation about intentionality. It’s not necessary, therefore it does not exist. Never mind whether it is correct.

A classic case in point has been the co-evolution of flower color (and other signals, such as floral aroma) and animal pollinators, like insects, bats or birds. The standard Neodarwinist explanation has been a kind of mutual boot-strapping whereby a pollinator’s genetic predisposition to sense, say, a certain color, has benefited the reproduction of flowers that are genetically predisposed to express that color. The pollinator gets food out of the bargain and the flower gets a pollination service, and this confers a mutual advantage in fitness. Over many generations, natural selection will favor a convergence of genes for perception and genes for expression in both pollinator and plant. All wrapped up neatly with a pretty bow.

So, it piqued my interest when a paper on evolution of floral signals by pollinators, written by (the aptly named) Florian Schiestl and Steven Johnson, landed in my Mendeley feed this morning. Aside from being a very fine review of the problem, the paper blew up the tidy Neodarwinian explanation for the co-evolution of pollinators and flowers. And, even though I am sure this was not their intent, it supported the notion of evolution as a cognitively-driven, and intention-driven, phenomenon.

For example, they show that the basic explanation for co-evolution of pollinators and plants fails. The Neodarwinian model posits incremental small changes in genes for perception (in the pollinator) and expression of signal (in the flower). This model does not stand up. So, for example, bees developed and perfected color vision long before plants had pretty flowers for them to see: no co-evolution there. The well-known preference of hummingbirds for red flowers seems to be more a learned habit than something wired-in by genes for detecting red colors. Again, hard to see a Neodarwinian explanation in there. There are other disjoins, but you get the point.

Another important point is that the variation of flower color is vast compared to the variation in the genes that produce flower color. For example, a major group of pigments known as anthocyanins are responsible for much of the variation of flower color. There are just six genes involved in production of anthocyanins, and only a tiny fraction of the variation of flower color is explained by variation of these genes. Most of the variation of color is due to variation of how those genes are expressed: when and in what proportion the genes are switched on and off.

In short, the expression of flower color is an epigenetic phenomenon, something well-known to generations of plant breeders. The variation of flower color in morning glories, for example, is due to transposable elements in genes, those things that won Barbara McClintock her Nobel Prize. Transposons essentially define genes on the fly, a phenomenon exploited to great profit by breeders of ornamental flowers.

And, where you have epigenetics, you have a feedback of the environment back onto hereditary memory, modifying it, adapting it and demoting genes from being the specifiers of function they have been presumed to be since the 1920s .

The whole thing, therefore, is a cognitively driven phenomenon: the insects and birds sensing and responding to the flowers and the rewards they confer, and most surprisingly, the plants sensing and responding to the attention of pollinators. The genes, far from being the specifiers of all this, are swept along largely as spectators in the wake of the cognitive love-fest between pollinators and flowers.


Schiestl, F. P. and S. D. Johnson (2013). Pollinator-mediated evolution of floral signals. Trends in Ecology & Evolution
28(5): 307-315.

Streisfeld, M. A. and M. D. Rausher (2009). Altered trans-Regulatory Control of Gene Expression in Multiple Anthocyanin Genes Contributes to Adaptive Flower Color Evolution in Mimulus aurantiacus. Molecular Biology and Evolution
26(2): 433-444.

Iida, S., Y. Morita, et al. (2004). Genetics and epigenetics in flower pigmentation associated with transposable elements in morning glories. Advances in Biophysics
38(Supplement C): 141-159.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

w

Connecting to %s