Biosphere2 

Exercise 3: Intraspecific Vigor and Diversity along a Resource Gradient
Module 3: Natural Selection and Evolution

Notes on the Exercise

A transcript of the discussion between Danoff-Burg, Kittel, and Hoylman about Exercise 3 is available here.

Lecture

• Establish the relationship between ecology and evolution and the importance of including an evolutionary perspective when studying ecology in that it would help to determine the appropriate type of questions to ask 
• Relationship between Natural Selection (mechanism) and Evolution (outcome) 
• Five observations that Darwin made that led him to NS — finite amount of resources, unfettered species would grow exponentially indefinitely, species tend to outstrip their resources, competition and death happens within a species, and there are individuals that are more successful in garnering resources and creating offspring 
• Ideal parts of a species range would be the area that is most in demand by the species and thus the greatest competition occurs there 
• Less successful species are forced to the fringes of the desired range and suffer as a result 
• Briefly summarize the Little Greenbul paper from Science [add citation] here and talk about the possible importance of fringe habitat in species production

General point: Ecotones as being generative forces for speciation and origin of community diversity - using the paper by Smith et al. In Science magazine June 20, 1998 (276:1855-1857)

Ecotones have usually been thought of as merely being the edges of most species ranges and thus would be the more stressed regions with less fit organisms occupying that region. Also many have pointed out that ecotones have much lower species diversity than the surrounding biomes and shouldn’t be such important areas to conserve. However, this paper points out that these regions are key in generating new species diversity.
 

1. Summary of main points 
• Interested in looking at the diversity within a bird species called the little greenbul (Andropadus virens) in Cameroon 
• Had an ecotone (which was what?) between two biomes (Savanna/Sahel and Equatorial Rainforest), each of which should present different selective forces on the birds, as rainfall and other variables differ tremendously between biomes and ecotone 
• They measured and compared the rates of migrations of birds between each locations within the ecotone and those in the rainforest using allele frequencies on the basis of 8 microsatellite loci 
• They measured morphological divergence in 5 morphological characters that have a close correlation with feeding ecology, flight, and fitness (wing length, weight, tarsus length, upper mandible length, and bill depth) 
• Results: 
• All but upper mandible length significantly differed between the populations in the ecotone versus the forest (generally ecotone had larger values) but not at all between ecotone-ecotone and forest-forest comparisons 
• Gene flow between populations differed and was quantified using the microsatellite data 
• Main Result: despite varying Nm values (effective migration rates), the ecotone-forest morphological divergence still existed up to a point — when Nm got to be huge the difference was swamped out (Fig. 2) 
2. Conclusions 
• The observed morphological divergence (Table 1) was due to selection for those species that had the optimal values for each trait 
• The magnitude of the difference between ecotone and forest greenbul populations was similar to that observed when many different species were compared (Fig. 3) 
• Because divergent selection as observed here will often (usually) lead to the production of new species, ecotones may be integral to the production and maintenance of biodiversity in tropical rainforests (rather than just within-forest vicariance mechanisms) 
• Suggest that for the long-term good, ecotones should be preserved as well, even though they may have fewer species in them (and hence lower diversity index values as we’ll talk about next time), they should be retained because they are such important evolutionary machines 
• What would be the predicted outcome if we were to look at a similar phenomenon on a much smaller scale — like along a resource gradient within a species? Prediction: that the heart of the range should have a smaller intraspecific range of diversity and vigor than in the fringes 

Approach

The exercise as written up here reflects an approach where two questions are presented, where the first is likely to give positive results and to lead to questions about genotypic vs phenotypic differences.  The second is the original question regarding heart vs periphery of range diversity

See file Ex3_ApproachDiscussion.pdf for more details regarding this discussion.

Hypotheses arising from exercise

1. For Question 1 (Do organisms exhibit morphological differences along local resource gradients?)

• H1a: Morphological differences along local environmental gradients are an expression of changes in genotypes along that gradient (ecotypes), or at least changes in population gene frequencies.
• H1b: Morphological differences are an expression of phenotypic plasticity.

2. For Question 2 (Does this intraspecific diversity differ between the heart of a species’ range and in the more peripheral areas of its range?)

• H2a: There is no difference in intraspecific diversity between the heart of a species’ range and the more peripheral areas of its range 
• H2b: Intraspecific diversity is greatest in the heart of a species’ range 
• H2c: Intraspecific diversity is greatest in the periphery of a species’ range

Independent variables in the experiment

• Location of individuals — along an environmental gradient and whether within the heart of the range or on the periphery

Dependent variables that they could use in their experimental design (see next section)

• Height of selected species 
• Number of branches per unit height 
• Approximate leaf area index 
• Absolute sizes of leaves 
• Approximate reproductive output per individual (if that tree species is fruiting) 
• Others

Implementation of exercise at B2

Students measured morphological characterisitcs from 10 trees for each site.  Trees were sampled along line transects until 10 individuals had been sampled.  For each tree, they collected 10 leaves from the outer and south-facing part of the canopy (to control for possible sun vs shade leaf differences).  Tree trunk (stem clump) diameter close to the ground was also measured to control for tree size (age) if needed.  Back in the lab, students measured the following leaf characters:

• Petiole length
• Number of pinnae
• Pinna length
• Subleaflet length, for middle subleaflet on pinna
• Subleaflet width, for middle subleaflet on pinna

• Number of subleaflets/pinna
• Composite measures:
• Pinna outline area: estimated by elliptic area equation using 0.5 pinna length and middle subleaflet length. 
• Middle subleaflet area: estimated by elliptic area equation using 0.5 subleaflet length and 0.5 subleaflet width. 

Variances appeared to be unequal for several characters, but was not tested.  For 2001, F-tests could be used to evaluating whether variances were different between sites, and hence if morphological diversity differed (Question 2). 

Other implementation notes

1. This will also be one of the first exercises during which students will be collecting scientific data of their own, so care should be exercised in the field to ensure that the students are rigorous and thorough in the data collection.

1. Data analysis was a demanding part of the exercise in 2000.  Plenty of time and plenty of assistance with Excel is needed to complete the task.

1. Communication among the 3 classes of the day's progress will be needed to coordinate the shared part of activity.  

Evaluation

• Report should include the results of the statistical tests and a summary of the discussions with the students in other biomes.
• Students should integrate the results of all three biomes into a consensus explanation for why they found the results that they did.

Molecular Alternative - not implemented for 2001

• This would be my preference for this exercise, as the students would acquire yet another valuable skill and it would introduce them to ecological genetics, a subject not otherwise included (maybe for those sites without allozyme capabilities, we could use molecular laboratory simulations like Fly Lab online) 
• Have been consulting with Deb Smith (Univ. Kansas), who thinks that it would be possible to do the exercise in one day, and certainly two days if we were to do some legwork ahead of time (like collecting the organisms, prepping the buffers and stains, pouring the agarose gels that the students will use [however they should also pour some, but since many will not work, they need to have backups immediately ready, and for time reasons they may not be able to pour the gels] 
• This alternative would be much more rigorous and interesting for the students, but may necessitate having the TA collect the samples before hand, to avoid needing a full second day to be set aside for the exercise, as training in these molecular ecology skills would be required 
• If this were done, we would not have the students do field work for this activity

Data from Previous Years

An Excel spread sheet with the Prosopis sp. intraspecific diversity data from Biosphere 2, SEE-U 2000

All Materials Copyright © 2001 by J. Danoff-Burg, T. Kittel, and A. Hoylman
All Rights Reserved. 

Rev. 4/20/01