Paper of the Day: Dudley and Schmitt 1996, “Testing the adaptive plasticity hypothesis”

Hi there! I am starting to get back into blog writing in the months leading up to starting grad school in the Fall of 2017 (don’t ask me where, I haven’t decided yet). One of the things I’d like to use this blog for is summarizing papers I’m reading. I hope it’ll give people a chance to access some science, even if it’s only a little bit at a time, without paying $20 for a pdf. Plus, it’ll give me some practice at translating science to a general audience. Here goes try number one!

Paper of the day: Dudley and Schmitt 1996, “Testing the adaptive plasticity hypothesis: Density-dependent selection on manipulated stem length in Impatiens capensis

Untangling some mumbo-jumbo jargon for you first – if you decide to read the paper yourself (which I highly recommend, if you can access it for free!), you would need to understand at least these terms:

Plasticity: the ability of an organism to change its phenotype in response to changes in the environment. (i.e. human bodybuilders changing their muscle mass and distribution)

Phenotype: the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment. (i.e. eye color, height)

Genotype: the genetic constitution of an individual organism. (i.e. all that DNA stuff that makes you uniquely you)

This paper tests the hypothesis that organisms adapt to the environment where they live, and therefore perform better than organisms that aren’t originally from that environment. Or in other words, “the phenotype evoked by each environment results in higher relative fitness than the alternative phenotype.” In biology, this is often referred to as the “home court advantage.” In this paper, the authors study the plant Impatiens capensis and how it responds to different environments.

impatiens-capensis
Impatiens capensis

In nature, Impatiens sometimes has to compete with fellow Impatiens plants or with other neighboring plant species for sunlight. How does a plant compete with other plants? Since plants can’t move, they have evolved many strategies to adapt to the environment where they’re stuck when they sprout from a seed. Many people have heard about the strategies that occur on a species-level, like cacti being able to draw on the water they hold in their bodies during dry spells in the desert. But, some species have populations spread across highly variable environments, and changes in the conditions plants face can occur from the top of a mountain to the bottom, or one football field-sized patch to the next, or at even smaller spatial scales, like one plant being next to a river and another stuck wedged between two rocks two feet away. To deal with this on an individual level, plants have evolved to be highly plastic – this means that each individual plant has the ability to adapt to its specific environment, and even if you put four genetically identical plants in different places, they could all end up looking and behaving totally differently.

That’s the basic premise of plasticity. BUT, the big question the authors ask in this paper is: is plasticity adaptive? In other words, is it a good strategy for that plant to adapt to its environment, or not? The overall goal of a species is to continue existing, so scientists determine adaptiveness by figuring out whether the adaptation contributes to that individual plant’s potential for future offspring (or “fitness”) – for example, by allowing the plant to grow bigger, make more flowers, and/or make more seeds that will grow to become the next generation of plants.

So, how does Impatiens compete with other plants for sunlight? It grows taller by elongating its stem and growing leaves higher than other plants. That way, it avoids being shaded by other plants. In this study, the authors manipulated the plants in two treatments by mimicking 1) an environment where the plants were shaded by other plants, and 2) an environment with full sunlight (not shaded by other plants). When plants were placed in the “shaded” treatment, they elongated their stems, and when they were placed in the full “sunlight” treatment, they did not elongate their stems.

Then, the scientists put the plants from each treatment, shaded or sunlight, into two different environments in nature: (1) plots where there were already lots of other plants present (high-density), and (2) plots where few other plants were present (low-density). In doing so, they were able to determine whether there was a home-court advantage, and they found that there was: plants with elongated stems performed better (or had higher “fitness”) in high-density plots than those without elongated stems. In other words, taller plants were more fit when there were other plants around for them to compete with, because they were able to reach the sunlight, whereas the shorter plants didn’t do as well because they were shaded by the other competing plants. On the other hand, plants withOUT elongated stems performed better than those WITH elongated stems in low-density plots. Why? The taller, more spindly plants tended to fall over if they were not physically held up by other plants crowding in on them (like in the high-density plots), so in the plots with only a few other plants, they were less fit than the short, squat plants that didn’t have any problem staying upright on their own.

The difference in fitness between the two environments (high or low density) that the tall vs. short plants displayed is an example of the “home court advantage.” The plants did better in the environment in which they adapted to live. But plasticity isn’t always adaptive! In a different study, some of the same researchers studied whether it was more adaptive for Impatiens to display this stem elongation type plasticity in sunny versus woodland habitats (Donohue et al. 2000). They did this because in a sunny environment, the plant can either experience sun when there are aren’t competitors, or shade when there are competitors – but in a forested area, the plants may experience shade from the tree canopy. As you can imagine, it’s not very useful for the plants to try to grow taller than the trees to reach the sun – so in this circumstance, their plastic response of elongating their stems was not as adaptive.

Why should you care about all this? It seems like it’s not important to the way people live their lives every day, but if we want to conserve and protect nature, we also have to understand all the nitty gritty messy complicated details that go along with nature. Understanding the fundamentals of plasticity itself could have enormous benefits to people when applied to medicine: imagine if you could trigger genes that activated increased production of melanin in your skin, getting a tan AND an effective sun blocker without having to expose yourself to harmful UV radiation at the same time? That’s a plastic response we could induce in humans if we knew how it all worked at a mechanistic level.

Thanks for reading! This was my first go at explaining a science paper to a relatively naive audience – my intended audience is anyone with high school-level biology or above. Send me a message or leave a comment if you have any constructive criticism!

References:

Dudley, S.A., and J. Schmitt. 1996.  Testing the adaptive plasticity hypothesis: density dependent selection on manipulated stem length in Impatiens capensis.  American Naturalist 147: 445-465.

Donohue, K., D. Messiqua, E. Hammond Pyle, M.S. Heschel, and J. Schmitt.  2000. Evidence of adaptive divergence in plasticity:  density- and site-dependent selection on shade avoidance responses in Impatiens capensis.  Evolution 54: 1956-1968.

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