Sexual Selection and Mate Choice

Concepts Instructor Prep Student Protocol

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Herbert Spencer first coined the phrase survival of the fittest in 1864, to describe Darwin's observations of natural selection. One way to quantify the fitness of an individual is in the number of offspring that it can contribute to the next generation - so an organism can improve its fitness by producing more offspring that are then able to successfully reproduce themselves. Reproduction is a critical element of fitness, and sexually-reproducing organisms must mate in order to pass their genes on to the next generation. This means that for sexually-reproducing organisms, competing for mates becomes a key part of fitness.

This pressure to find a mate leads to a phenomenon known as sexual selection - a type of selection that acts upon traits associated with mating and courtship. Typically, sexual selection is more intense for the sex that invests the least amount of energy in reproduction. Since sperm is energetically inexpensive to produce, males generally bear the brunt of sexual selection pressures. Conversely, eggs are energetically expensive to produce and then gestate. This means that females are expected to be more selective when choosing mates, and males must invest substantial resources in attracting females. This observation is related to the operational sex ratio, or the OSR - which is the ratio of sexually-mature males to fertile females. Often the OSR is male-biased, due to their longer reproductive lifespans.

Due to the often-intense sexual selection pressures males face, many species of animals display extravagant ornamentation that distinguishes males from females. When males and females of the same species appear phenotypically different, we call this sexual dimorphism. Sexual selection, however, isn't limited to only male ornamentation. There are two types of sexual selection, intersexual and intrasexual. In intersexual selection, one sex, typically the males, will display a certain trait or behavior with the goal of attracting and mating with the opposite sex. Intrasexual selection, on the other hand, occurs between members of the same sex. For example, male sea lions compete for dominance over rookeries of females. Here, intrasexual selection acts on the physiology of the sea lion, to make it large enough to compete with other males for a group of females. But with attributes like vibrant colors and larger size come costs. Like increased visibility to predators, or decreased immunity due to the overproduction of androgens. This means that natural selection and sexual selection are often at odds with each other, pulling traits in opposite directions. Because of this, individuals with exceptionally high- or low-quality scores are rare. Instead, individuals of average quality are expected to make up the majority of a population.

In this lab, you will perform simulations of mating scenarios in which females and males can select their mates based on varying amounts of information on their potential mate's quality.

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Determining Roles and Making Gamete Packages for Class
- First determine the quantity of materials needed based on the class size. This activity can easily be scaled up or down. In Table 1, options are listed for 30, 20 or 10 students. Click Here to download Table 1 NOTE: You may easily create a custom number by following these simple guidelines.
  - Ensure an equal male-to-female ratio.
  - Designate the majority as average quality individuals.
  - Make the number of high- and low-quality individuals equal.
  - Ensure that there is always an even number participating in the matings - If there is an odd number of students, have one student help tally the matings each round.
- To prepare the information badges that the students will wear, first acquire 10 clip-on plastic ID holders.
- Using a permanent marker, number the holders one through 10 in the upper and lower right corners.
- On the left side, write a Q in the upper corner and an A in the lower corner.
- Next, use table one as a reference to draw dots next to the Q and A for each of the 10 quality and attractiveness value combinations.
- Cut strips of white paper large enough to cover either the Q or A sections of half of the ID holders.
- Put one strip of paper along with three paper clips into each male ID holder.
- Next, prepare the strips of paper that will represent gametes. Pink and blue paper works well to represent eggs and sperm respectively, though any two distinct colors can be used. See Figure 3 for a cutting template. Click Here to download Figure 3
- Each student playing a female in these activities should receive four strips each. With a permanent marker, number sets of four pink strips with each female individual number.
- Each student playing a male in these activities will receive 16 strips of blue paper and one strip of white paper each. Number sets of 16 strips with each male individual's number.
- Finally place the strips of paper into each corresponding ID holder.
- Next, before the lab begins, establish an open mating area and a desk or group of desks as a fertilization area a little distance from the mating area where students may place paired gametes.
- Then, place at least 20 paper clips on the desk within the fertilization area to allow students to clip paired gametes together.
- Make sure a computer is readied with Spreadsheet 1 open as during the mating activity, the instructor or helper will keep time and record which matings occur. Click Here to download Spreadsheet 1
- Note: The example data sheets are originally formatted for a group of 30 students. For larger or smaller groups, the formulas for average number of matings and average reproductive success will need to be modified to include the correct number of individuals.
- Once the lab simulations are completed, you will provide the tallied class data from this class spreadsheet for the students to prepare graphs. Instead of providing the full data sheet, copy and paste the relevant data as “values only” into Spreadsheet 2 and give this to the students. Click Here to download Spreadsheet 2

Transcript

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Sexual Selection

Gamete Packages

Quality Attractiveness

Mating Simulation

Classroom Activity

Experimental Design

Data Collection

Reproductive Success

Spreadsheet Analysis