Coevolution Theory: Autumn Leaves

Lac Du Flambeau, Wisconsin, USA

Photo by: Cindy Brintnall

Autumn Leaves: A Theory in Coevolution

Good Evening readers! 

As autumn is in full swing here in Australia, I can't help but miss all the bright colors I get to see back home during this season. I wanted to find a way to incorporate it into my studies, and it turns out, there is science behind it (who knew), AND it is relatable to coevolution! Consequently, this week I will be discussing a theory behind why the colors change in the fall. It turns out, it may be all because of bugs.

 The most popular belief today is that the bright autumn colors are just a result of cell deterioration in deciduous trees (Archetti, 2000). However, Archetti states that the trees might change color due an adaptive change driven by coevolution between insects and trees.

The theory states that the bright colors of the trees signal their fitness and soundness, and parasites will then pass the healthy-looking trees over as hosts. The advantage to the trees is less parasites and eggs laid on it, and the advantage to parasites is that they can more easily choose a suitable host.

According to Archetti, this theory follows the rules of Amotz Zahavi's "Handicap Principle." This means that the trees have evolutionarily adapted to use an expensive and honest signaling system. Trees in poor health cannot afford to produce such flamboyant signals, and fail to ward off the preying parasites. The Handicap Principle is usually used as an example in sexual selection, such as when fitter male birds have longer or brighter feathers that can hinder them, but their expense also rewards them. This is true with the trees as well, because the trees who can "afford" the price of brighter colors are the ones who will receive less damage by the parasites.

However, there are a few issues with this theory.
- The system would hold up under "normal" systems, but in a particularly poor or particluarly good year, it wouldn't work.
- Insects are known to favor the color yellow
-Aphid and other termites' migrations do not take place in the short leaf-change timespan  (Wilkinson, 2002)


What do you think? The idea has yet to have overwhelming empirical evidence, but it does have some merit. Let me know, as always,  in the comments below!

Thanks for tuning in, and have a phenomenal week!


D



References:

Archetti, M. (2000). The origin of autumn colours by coevolution. Journal of Theoretical Biology, 205(4), 625-630.

Wilkinson, D. M., Sherratt, T. N., Phillip, D. M., Wratten, S. D., Dixon, A. F., & Young, A. J. (2002). The adaptive significance of autumn leaf colours. Oikos, 99(2), 402-407.

Coevolution and Pollination

Coevolution and Pollination:

Ophrys : Bee Orchids

Hello readers,

Today the topic is Coevolution and Pollination, and I am going to introduce to to a fascinating Genus of plants called Ophrys, or "bee orchids."

The Bee orchids depend on bees for pollination, so evolutionarily, they had to evolve ways to keep the bees interested and ensure pollination. This should not come as a surprise considering that many plants attract bees to cross pollinate, however, the mechanism this genus uses is quite interesting.

Rather than evolving for  just smell or color, bee orchids actually evolved to display a flower in the same shape as the bee itself. The floral mimicry is a tactic to encourage a process known as pseudocopulation. This is when a male bee, seeing what he thinks is a female, will land, attempt to mate, and pollinate the flower. The flower is not only in the shape of the bee, but also releases mimic pheromones, called allomones, that smell like a female.

During pseudocopulation, pollen will stick to the bees head or abdomen. This pollen will travel on the bee until it becomes attracted to another "mate," where pollen exchange can occur and pollination will happen. 

I find this story of coevolution particularly interesting, and it brings up many evolutionary questions. Because the bee orchid species depends entirely on the bee, it has devoloped use of deception to suit its purpose. But once the bee has been duped, it may not revisit the flower or the same type of plant again. Do you think this species would have increased its success by developing some other traits? 

The coevolution is an extreme example of mutualism, because the bees need flowers just as the flowers need bees. However, mimicry is typically seen in coevolution between prey-predator animals. Do you think the bee orchids are trying to out compete other plants? Or are they just mutual-beneficiaries-turned-frenemies?  Do coevolutionary "arms races" occur between mutualistic species?

Please discuss in the comments below! I would love to hear what you think. 

Thanks for reading, and have a phenomenal day! 

D

Sources:

Borg-Karlson, A. K. (1990). Chemical and ethological studies of pollination in the genus Ophrys (Orchidaceae). Phytochemistry, 29(5), 1359-1387.

Schiestl, F. P. (2005). On the success of a swindle: pollination by deception in orchids. Naturwissenschaften, 92(6), 255-264.