Why do males typically experience stronger sexual selection pressures than females?

Males experience stronger sexual selection because sperm is energetically inexpensive to produce, while eggs are costly and limited. This difference in reproductive investment means females are more selective when choosing mates, forcing males to compete intensely for reproductive opportunities. The operational sex ratio, often male-biased due to longer male reproductive lifespans, further intensifies competition among males for access to fertile females.

What is sexual dimorphism and how does it relate to sexual selection?

Sexual dimorphism occurs when males and females of the same species appear phenotypically different, typically due to sexual selection pressures. These differences, such as vibrant colors or larger body size in males, help individuals assess potential mates' quality. Sexual selection drives the evolution of these distinctive traits to attract mates or compete with same-sex rivals.

How do intersexual and intrasexual selection differ?

Intersexual selection occurs between males and females, where one sex displays traits to attract the opposite sex, such as bird plumage or frog mating calls. Intrasexual selection occurs within the same sex, where individuals compete for dominance and mating access, like male sea lions competing for rookeries. Both types shape the evolution of sexually selected traits in populations.

Why do sexually selected traits often come with survival costs?

Sexually selected traits like vibrant colors increase visibility to predators and require significant energy to produce. Traits linked to androgens can reduce immune function, while competitive behaviors between males are physically demanding and distracting. Natural selection and sexual selection often conflict, pulling traits in opposite directions, which prevents traits from becoming too extravagant and maintains average fitness in most individuals.

What role does fitness play in determining reproductive success?

Fitness is quantified by the number of offspring an organism contributes to the next generation. For sexually-reproducing organisms, competing for mates becomes critical to fitness because reproduction requires mating to pass genes forward. Sexual selection acts on traits associated with mating and courtship, directly influencing which individuals successfully reproduce and pass their genes to offspring.

How can sexually selected traits signal individual quality to potential mates?

Sexually selected traits like ornamentation and coloration signal health, genetics, and nutrition status because they are energetically costly to produce and maintain. Only high-quality individuals can afford to display such traits without compromising survival. These signals help potential mates assess fitness and select breeding partners likely to produce viable, healthy offspring.

What is the operational sex ratio and why does it matter for sexual selection?

The operational sex ratio (OSR) is the ratio of sexually mature males to fertile females in a population. An often male-biased OSR intensifies competition among males for access to females, driving stronger sexual selection pressures. The OSR can be influenced by parental care investment, mate bonding, and reproduction rates, all affecting the intensity of sexual selection in a population.

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.