1. Author:
Dr. H.B.D Kettlewell (1907- 1979). British Medical Doctor, Lepidopterist and Geneticist. He is well known for a classical study on natural selection using as a model organism peppered moths (Biston betularia). Kettlewell conducted a series of experiments in 1950 on the phenomenon of industrial melanism. His research was conducted in two type of woodlands in England: Birmingham (polluted) and Dorset (non-polluted; results are not presented in this paper ). His great contribution is divided in two. Firstly, he coordinated surveys of the dark morphotypes across England, which was a country wide survey that allowed to establish a reference study for further studies of the species. Secondly, he did provide evidence of bird feeding on peppered moths selectively.
Dr. H.B.D Kettlewell (1907- 1979). British Medical Doctor, Lepidopterist and Geneticist. He is well known for a classical study on natural selection using as a model organism peppered moths (Biston betularia). Kettlewell conducted a series of experiments in 1950 on the phenomenon of industrial melanism. His research was conducted in two type of woodlands in England: Birmingham (polluted) and Dorset (non-polluted; results are not presented in this paper ). His great contribution is divided in two. Firstly, he coordinated surveys of the dark morphotypes across England, which was a country wide survey that allowed to establish a reference study for further studies of the species. Secondly, he did provide evidence of bird feeding on peppered moths selectively.
2.Scope of this paper: To demonstrate experimentally
(aviary and woodlands experiments) how color variation is under natural selection. Due to
pollution generated during industrial revolution, dark morphos of Biston betularia (carbonaria and insularis)
were assumed to be fitter that the white types (typical) because of lower predation during daytime.
3. Methodology
3.1 Field experiments: Experimental release in an industrial area
Mark- release capture experiments were conducted in polluted sites close to Birmingham. In this experiments only males
were used. They were marked with a dots of paint. The release occurred at sundown. Two different types of trapping methods were used. In the fist, he used a trap made of perforated zinc, where only males were allowed to enter, but not escape. The second one used muslin cages. One virgin female of each genotype (carbonaria, insularis and typical) was kept in each trap. Basically they did this because they wanted to keep as many males nearby the forest (female’s pheromones avoid males migration out the woodlands). Otherwise they would have escaped. Mark recapture was done using mercury vapor lights
3.2 Aviaries experiments
The aviary experiments were conducted in Research Station , Madingley, Cambridge. Dark and light color trunks of different species were placed inside the aviary. The experiments used predators such as the Great Titis in order to measure predation upon the tree forms of bestularia, which were released inside the aviary
4. Results
4.1 Field experiments
Approximately 700 individual of the tree morphotypes were released.There was
a large number of recaptures for the melanic forms (carbonaria and insularis). Assumption was that the white morphos (typical)
could have been predated and that’s why there were not many recaptures.
4.2 Aviary experiments
For the very first time, selective elimination on incorrect
background was first demonstrated. Therefore, the conclusion was that the birds are selective agents
5. Conclusions
This study is the classical example of natural selection, which is explained in classical textbooks of evolutionary biology. The author spent a considerable part of his life studying the effect of industrial melanism on peppered moths. The results of the experiments supports the hypothesis that birds act as selective agents.
6. Question for discussing in class
1. Dark- melanic forms are less abundant in post-industrial Britain. Would you still consider the conclusions presented by Kettlewell well - founded? If you are supposed to measure the selection coefficient in nowadays. Do you think it will affect the conclusions presented by Kettlewell?
2. How about to take into account other selective agents than bird's predation, for example predation by insectivorous bats of the genus Pipestrellus. How would the results differ from the ones already presented in the paper?
I really enjoyed this paper. Not just because it focuses on arthropods and use of cryptic coloration but because to how well I feel the study was conducted. He benefited from knowing the models he used (the Geometrid moth) and his knowledge of the genetic factors controlling color variations seen. He used a person’s ability to identify the level of crypsis of these insects against a variety of natural backgrounds. He then tested bird’s ability to find the same insects in a controlled setting to make a correlation between our ability and a bird’s ability to locate a cryptically colored moth. In a field setting this experiment is harder to set up due to the moth’s ability to relocate and the inability to observe every moth for predation. His method of marking and recapturing was a good solution to this problem. In areas where predation appeared high he observed these locations for predation. In these situations he observed that birds not only seemed to prey on more conspicuously colored moths first, but then began a more directed search for moths allowing them to then locate some of the more cryptically colored individuals. This helped to not only show that conspicuously colored individuals where preyed upon more frequently, it also showed the learning behavior of the predators and help to show a reason that cryptically colored individuals tend not to aggregate to avoid predation. I also like the note that in the controlled setting, when other insects were released along with the model that the eyed hawk moths (batesian mimics) were avoided except when the coloration was defective, in a way, anecdotal evidence supporting the argument of this paper.
ReplyDeleteTo give my input on the second question asked, I would predict that the predation rate on these moths by bats would be consistent despite color differences and be dependent on the abundance of each color form. Unless you can show that the coloration of the moth has an effect on echolocation, or there is a behavioral component that would reduce predation. You can set up an experiment in a large open cage/ aviary with bats introduce set numbers of moths and see if bats were more likely to take one color over another. If no preference was shown one could assume the same percent of moths are being taken and although might affect the numbers, ratios should remain fairly consistent.
My question would be why you feel the insularia morph was in lowest abundance? I would attribute it to disruptive selection. Where the darker moths blend in really well on a dark background, but not on the lighter background, opposite true for the lighter colored moths, although the insularia (an intermediate form) might blend in with both backgrounds, they never do so as well as either of the other two color forms therefore are more conspicuous on either background. Do you agree or disagree?
This was an interesting read. I actually remember reading something about the Peppered Moth at some point. I particularly enjoyed this paper due to the implications of the impact humans through industrialization on the distribution of the different types of Peppered Moth. One thing I did not completely understand was how the "scoring" system worked in determining conspicuous versus inconspicuous moths.
ReplyDeleteIn an attempt to address the questions Carlos posed:
1. I am not up to speed on the current state of the vegetation of post-industrial Britain. If it is less covered in soot, I believe Kettlewell's conclusions would still be well-founded as he suggests that the darker variations would be more apt to be preyed upon when sitting on lighter backgrounds.
2. This is an interesting question, and one that I am not well versed on. It does stand to reason that nocturnal predators may be able to locate the moths at night when they are still active. Could other agents, such as disease have impacted the moths as well?
There were definitely some parts of this paper I found questionable. It seems that the conclusions reached are pretty cool either way. The demonstration of natural selection for individuals with advantageous coloring was really novel for its time. The scoring system seemed pretty arbitrary, but the description likely doesn’t do it justice. The aviary experiments also seem like they subject the moths to unnaturally high selection pressures. No natural moth would be contained in a small space with a predator for a large amount of time as the experiments did. Even introducing other prey for the birds still seems to over simplify the situation and greatly increases predation relative to natural levels. The field experiments seemed to reflect natural conditions better as field experiments often do.
ReplyDeleteTo attempt an answer to question 1, I wonder what has happened in modern industrial places like China that might now have “peak soot?”
The aviary experiments, to me, really showed this idea of it being more complicated that we think. What I mean is that they had this great system mapped out about moths being on "correct" or not backgrounds with measurements for how conspicuous they were. Using this system, they devise this experiment to see if birds will eat the more or less conspicuous moths but they failed to take into account the fact that birds might do different behavior than expected and so when they saw the birds starting to systematically search, they had to modify to promote more real world conditions. I just always find it interesting how ecology is always looking for that next level of complexity to be able to add to the system but it is at odds with the idea that you only manipulate one variable at a time.
DeleteThis paper was an enjoyable one. Most interesting, this is one of the first papers we've read where the focus of the study is regarding something manmade. This takes into account the effect of pollution on the evolutionary benefits of wing color in butterflies. The fact that the whole study revolves around something anthropogenic shows the strides the field of Ecology has made since the formation of the field (or since the first paper in this book). In regards to the results of the study, it is so interesting that the author proved that birds place the most selective pressures on the evolution of the butterflies. While the study was mainly to learn about the colorings in butterflies, they also succeeded in emphasizing the coevolution of butterflies and birds. These themes seem to be more and more common in more of the papers we're reading, and again show the progress that the field of Ecology has made.
ReplyDeleteI found reading this paper almost eerie. Although I've read some of the other papers before, this is the first paper that I really remember from my undergrad. Regardless, still a very interesting paper.
ReplyDeleteTo Eric's comment that the moths are put under abnormal amounts of selective pressure in the lab experiment; couldn't the same (or rather opposite?) be said for the mice that are presented a perfect grid of larvae to predate? Would that not screw results in the same way? I thought the experimental design was quite apt- although the moths might be put under an unnatural amount of predatory selection, it clearly demonstrates an uneven ratio of predation based on color morphs, which is exactly what the author was testing. Had the author been testing the moths' ability to escape predation altogether (as opposed to just eluding detection), then I would agree that there was some issue in the design.
Also an interesting thought about bats, Carlos, but I would have to agree that color would probably not come so much into play for a nocturnal predator.
I found reading this paper almost eerie. Although I've read some of the other papers before, this is the first paper that I really remember from my undergrad. Regardless, still a very interesting paper.
ReplyDeleteTo Eric's comment that the moths are put under abnormal amounts of selective pressure in the lab experiment; couldn't the same (or rather opposite?) be said for the mice that are presented a perfect grid of larvae to predate? Would that not screw results in the same way? I thought the experimental design was quite apt- although the moths might be put under an unnatural amount of predatory selection, it clearly demonstrates an uneven ratio of predation based on color morphs, which is exactly what the author was testing. Had the author been testing the moths' ability to escape predation altogether (as opposed to just eluding detection), then I would agree that there was some issue in the design.
Also an interesting thought about bats, Carlos, but I would have to agree that color would probably not come so much into play for a nocturnal predator.
This was a neat read, because like others, I remember hearing the story of industrial melanism in moths as an example of evolution early on in school. But of course I'd never read the paper and didn't realize there was a multifaceted experimental study behind it all - I'd assumed it was just observation. Each experiment seems to demonstrate one important facet in the chain of logic that explains melanism frequency as a function of selective pressure from predation (conspicuousness by the color rating system, observed selective predation in the aviary experiment, and selective predation in the field with the release, mark, & recapture experiment). Kettlewell also has good evidence against alternative explanation for his results (e.g. non-biased recapture rates). All in all I found his methodology pretty sound and convincing, though there probably are some aspects for which additional evidence would be helpful.
ReplyDeleteA lot of the difficulty with experiments in ecology is that even when something is demonstrated quite definitively in a lab or experimental setting, there is still the matter of demonstrating that the phenomena works similarly in an unmanipulated field setting; ergo Eric's questioning whether predation levels in the aviary experiment are realistic. To address this, as well as Carlos' question about the subsequent decline in melanic forms in post industrial Britain, I'd propose a long-term observational study: quantify the selection pressure on moth melanism over time by measuring both predator abundance (e.g. annual surveys of insectivorous birds) and moth habitat coloration (e.g. transects with tree color scoring & counts). If the proportion of melanic moths follows tree coloration, accounting for predator abundance, that would strengthen Kettlwell's conclusions. Could also substitute space for time (i.e. polluted v. unpolluted habitat), which it sounds like Kettlewell did subsequently, though quantifying predation could be important to ensure the selective pressure was similar. I think that observational studies and experiments are complementary and fit well together in ecology because they can often address similar questions but in way the other can't.
I think Matt's question about the insularia moth is really interesting, and I'm not really sure of the answer, though disruptive selection could certainly contribute. A related question I have, is why does the insularia variety persist at all? And why does the non-melanic form persist many generations after conditions have put it at such a severe disadvantage (or vice versa for the melanic form in pre- and post-industrial conditions)?
I find this idea of industrial melanism very interesting. Its amazing to see how fast a species can adapt when immense selective pressures are put on it. A student in our lab recently did his masters thesis on color morphs of the vermilion fly catcher. In Lima Peru the bright red color of these flycatchers are dulled way down to grey and brown possibly due to weather, pollution, and possibly other selection factors such as predation. It was unusual to see, like Dunbar pointed out, an intermediate form. Being worse at blending in when compared to the other morphs seems to put insularia at an extreme disadvantage. My only thoughts would be that because it is heterogeneous, this morph will always exist as long as the other two morphs exist.
ReplyDeleteThis comment has been removed by the author.
ReplyDeleteI was interested in how Kettlewell used the aviary experiment to establish reasonable guidelines for setting up the field experiment. I agree with Matt that he was able to show that bird sight is similar to human sight in ability to distinguish the different morphs against different backgrounds.
ReplyDeleteIt would have been nice if Kettlewell had given us some insight into why he used the scale of +3 to -3 to represent the various distances.
I thought it was great how Kettlewell systematically eliminated other major explanations for his striking results (62.57% survival of the carbonaria morph and 45.75% of the typical morph, supporting his hypothesis that the insects were taken selectively). I tried and failed to think of other alternative explanations that would diminish the impact of these results.
How do we know that this trait of melanism is a dominant, single-gene trait?
Definitely an interesting paper and one that has been a model for introducing students to selection for a long time. I think its also interesting that one of the models that we rely on for teaching selection is the result of anthropogenic habitat alteration. It makes sense, even though there are many natural systems suitable for studying selection (and many in New Mexico) these natural systems are generally not observable over the course of a human lifetime.
ReplyDeleteTo the question posed by Dunbar regarding the persistence of the insular variety, I would suspect that the mottled grey coloration is actually advantageous in habitats not composed of birch forest or soot covered surfaces (i.e. any forest with trees that have mottled grey or brown bark). I agree with Matthew regarding disruptive selection and predation by bats.
This paper had a lot of components that make ecology the delightful puzzle it is: genetics, Darwinian evolution, predator selection/foraging, plant communities, and anthropogenic interactions. Being written in the 1950s, Kettlewell would have a firm grasp on Mendelian genetics and how the genes could be arranged in the moths, but now I am interested in reading up on whether or not there have been molecular studies on the genetics of these moths.
ReplyDeleteI think the mentioning of how black moths increased in frequency due to the addition of the human-produced soot was the most interesting for me. It’s a common theme in this book (especially with Odum’s paper from last week) that humans have a profound impact on any type of system in nature.