Adaptations to Desert Environments

Selection Thinking

The true power of the process of natural selection is that it provides us way of thinking about the diversity in the world around us. If we expect that organisms will be adapted to the condition in their environment the we can think like engineers and ask the questions- how would I design the a trait in that environment? We call this approach "selection thinking". This approach has been highly successful in allowing scientists to understand aspects of physiology, morphology, life history, and behavior in all sorts of environments in all sorts of species. In my research I have used this approach to study behavior in sparrows, woodrats, and beavers and reproduction in deer and plants.

Often, ecologists use mathematical models (often models ripped off from economists and engineers) as tools to help them understand traits of organisms. Although explicitly using mathematical models to study adaptations is probably too advanced for most middle school and high school science classes, I do think that it is important for you as teachers to know that math is an extremely important tool for scientists and be able to express that to your students as often as possible. It is unfortunate that math and science are usually taught as separate topics. As a Zoology Major at UCSB I took calculus during my Freshman year because that is what my advisor told me to do. During the final quarter of my senior year when in a graduate level Reproductive Ecology the professor used calculus to solve a problem that I had the "Oh, now I understand why I was supposed to learn all of that math!" moment.

Using mathematical models forces scientists to do very important things. First, we must clearly state our assumption. Second, it forces us to formailze out logic. When scientists don't use mathetical models they are often forced to rely upon what we call "arm waving" verbal arguments (you should be familiar with these arguments because we see them all of the time when we watch politicians on TV). Often, conclusions that seem reasonable based on verbal arguments actually are incorrect because they are based on either unrealistic assumptions or faulty logic.

I came across an example of a faulty verbal argument while I was working on my Ph.D. I was interested in understanding how parents should invest resources to their offspring, specifically, how big should plants make their seeds. This is a relatively simple problem to think about. When plants reproduce they should be selected to make as many surviving offspring as possible. The number of surviving offspring should be the product of the number of seeds produced and the probability that a seedling survives after it germinates. The number of seeds produced depends on seed size; you can make fewer larger seeds or more smaller seeds. Because the size of a seed is influenced by how many resources that seed contains, the probability that a seedling survives is positively correlated with the size of the seed. The original models predicted that fitness would be maximized if a maternal plant made all of here seeds exactly the same size. However, when you actually measure sizes of individual seeds (and I measured tens of thousands of seeds during my Ph. D.) you see that there is a lot of variation in the size of seeds produced by the same plant. The focus of my Ph.D. research was to try to figure out why plants produced seeds of different sizes.

Several years earlier a scientist named Dan Janzen (a very famous tropical biologist) had published a theory suggesting that producing different sized seeds was an adaptation. His theory was based on a "hand-waving" verbal arguement. In a class I took in graduate school I developed a model to try to see if Janzen's arguement really made sense. My model suggested that Janzen's conclusions were wrong because the verbal logic he used was faulty. My professor suggested that I tried to publish my model. While I was writing that paper, another professor from Orgegon published a matehmatical model that came up with the completely different conclusions than my model. When I compared our two models, I saw that his conclusions were based on an unrealistic assumption and when you used the correct assumption in his model we drew similar conclusions.

Selection Thinking in Arid Environments

Because the environmental conditions in arid environments are particualarly severe, deserts offer an interesting location to study adaptations to local environmental conditions. Hopefully, the readings will give you a broad exposure to how natural selection can mold physiology, morpology, reproduction, and behavior in arid environments.

Powerpoint Presentation

Click here to see a powerpoint presentation "Introduction to Desert Flora and Fauna"
http://www.slideshare.net/secret/pw2UrKumkR7KRT

Expected Learning Outcomes

At the end of this course a fully engaged student should be able to

- identify and discuss the unique challenges associated with living in arid environments (TEKS 112.43 12C)
- explain adaptations of animals and plants for water uptake and water conservation (TEKS 112.43. 7B)
- explain adaptations of animals and plants for dealing with high temperatures (TEKS 112.43. 7B
- develop curricular materials to teach students about adaptations to arid environments TEKS 112.43. 7B)
- develop curricular materials to teach how animals or plants are adapted to a different (non-desert) environment ((TEKS 112.43. 7B & 112.43.12B)

Practice Assignment

To test your understanding of how natural selection affects traits, I suggest that you try to develop a lesson to teach your students how the traits that you observe depends on the environmental conditions. In about one page, outline the lesson you would use to explain how and why the same trait varies between two very different environments. I suggest that you choose an adaptation to life in the desert and compare that train in an very different environment such as a tropical rainforest (much wetter) or the arctic (much colder). If you post your answers here I, and hopefully your classmates, will provide you some feedback.