Sexual selection is a major force in evolutionary biology and explains many of the widespread phenotypic characters, such as bright colors, melodious songs, and elaborate behavioral displays that fascinate biologists and non-biologists alike. It is therefore surprising that sexual selection receives so little attention in introductory biology laboratories (however, see Eason & Sherman, 2003). Here we describe a set of experiments investigating sexual selection in the fruit fly, Drosophila melanogaster, that require students to formulate hypotheses, conduct behavioral assays, employ basic statistical analyses to test hypotheses, and use critical thinking skills to synthesize findings. These experiments follow the inquiry based science teaching model described in Inquiry and the National Science Education Standards: A Guide for Teaching and Learning (National Research Council, 2000) (see Table 1). The experiments described in this article were used in a college-level introductory biology course, but we suggest that they could also be used in lab sections of advanced high school biology courses.
Background & Pre-Experiment Discussion
It is not difficult to engage students in a discussion on sexual selection. Students are inherently curious about the bright colors, elaborate displays, and traits that increase an individual's ability to compete for access to mates; but they may not understand the evolutionary explanation for existence of these conspicuous characters. Here we describe a series of experiments to expose biology students to the operation of sexual selection.
Understanding the process of natural selection prior to conducting these experiments is critical for students to understand the rationale for the experiments and the interpretation of results. By the time they do the experiments described here, our students have had several lectures on natural selection and evolution, and have had one lab with experiments that demonstrate natural selection in action. We start the pre-experiment discussion by briefly reviewing the requirements for evolution by natural selection:
* phenotypic variation
* heritability of phenotypic variation
* differential survival and reproduction based on phenotypic variation.
Then we proceed by introducing the concept of sexual selection; that phenotypic variation among individuals in a heritable character leads to variation in reproductive success (that is, fitness). Explain that sexual selection has two major modes: active mate choice (usually by females) and competition for mates (usually among males). Provide the students some examples of characters used in mate choice and others that are used in competition for mates (Figure 1). (Note: for a comprehensive table of traits and the modes of sexual selection affecting them, see Andersson, 1994, pp. 132-142). For a particularly salient lesson, have a good visual presentation-slides, videos, and/or actual feathers, antlers, animals-to present in class. Next, challenge the students to explain how something as elaborate and conspicuous as a peacock's tail evolves as the result of female mate choice. Explain that in species where males have elaborate sexual displays, like peafowl, females tend to prefer the males with the most exaggerated display traits as mates (Andersson, 1994). Using the peafowl (Pavo cristatus) example, if peahens prefer peacocks with the longest tails as mates, then long-tailed males will be chosen as mates more often than short-tailed males, and will therefore produce disproportionately more offspring. If tail length is heritable then both long tails, and the necessary genetic material (genes, alleles) that code for long tails, will increase in frequency in each subsequent generation of peafowl. If we define evolution as a change in allele frequencies over time, this example provides a hypothetical, though empirically supported (Petrie, 1994; Petrie & Halliday, 1994), example of evolution by sexual selection. …