Sexual Selection and Mate Choice

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.