Atlantic Forest

Exercise 3: Natural Selection/Evolution: Ecotypic Variation
Module 3: Natural Selection and Evolution

Your Questions

1. How are ecological gradients defined?
2. How do local populations respond to ecological gradients?
3. What are the origins and consequences of intraspecific variation?

Background

Ecotypes are local populations that differ genetically in adaptations or responses to environmental conditions. Ecotypes are also known as races or ecological races and the traits showing ecotypic variation can be either morphological or physiological. Not all differences between populations are ecotypes since some differences are simply the result of the response of a given genotype to different environmental conditions. Both genotypic diversity and phenotypic plasticity are important causitive agents for the variation observed within species.

Genotypic diversity is when there is a great deal of genetic diversity within a species, which in turn produces a great deal of morphological (structural) variation. This type of variation produces traits which can be passed on through the generations. Phenotypic plasticity occurs when a trait is influenced by the surrounding environment during the development of an organism. Because the environment plays such a large role in the formation of these types of traits, this type of morphological variation cannot be passed on through the generations.

Nonetheless, for species that live in habitats that vary spatially or temporally, both of these generative forces are essential for them to be able to cope with the environment. If the differences between or among populations are not genetically based, but due to plasticity of response of the organisms, then the populations are occasionally also referred to as ecophenes or phenecotypes.

An ecological gradient can be defined as a change in an environmental factor, such as water availability, soil pH, salinity, temperature, over a specified distance. However, this distance may be measured in centimeters or thousands of kilometers depending on the scale of the study and the organism used. In the site we will be studying we will use a gradient based on water availability.

Your Assignment

1. Compare two or more populations of the same species along an environmental gradient.

Methodology

1. Task A—Recognition of local environmental gradient
   • Identify a local environmental gradient
   • Select a species distributed along the gradient
   • Select traits for measurement
2. Task B—Measurement of effects
   • Establish sampling points along the gradient
   • Collect individuals at each sampling point and measure the traits previously selected.
3. Task C—Data analysis and write up
   • Data entry into computer
   • Analyze the data
   • Compare the populations
   • Speculate as to the generative force behind the variation
   • Summarize results for presentation (written and oral)

Objectives

1. Present concepts of ecological gradients
2. Introduce the concept of natural selection
3. Introduce basic techniques for statistical analysis of data

Key Skills

1. Learn how to identify environmental gradients on a local scale
2. Recognize differences among populations
3. Basic statistical analysis
4. Experimental design and interpretation

Timetable

Total time—1 day

1. Morning
   • Localize an environmental gradient
   • Identify a species with a distribution over the gradient.
   • Collect and measure individuals at various points along the gradient
2. Afternoon
   • Data entry and analysis
   • Write up report
   • Give presentation

Materials Needed

1. Field notebook and writing equipment
2. Tape measures (50 m)
3. Compass and GPS
4. Collection equipment for studied species
5. Flagging, stakes, string