Question

Contrast intrasexual selection and intersexual selection.

Short answer

Question: Explain the difference between intrasexual selection and intersexual selection and provide examples of each. Answer: Intrasexual selection refers to the competition among individuals of the same sex for access to mates or resources required for mating. This can lead to the evolution of traits that provide a competitive advantage in contests, such as size, strength, or aggressive behaviors. An example of intrasexual selection is in deer species, where males grow antlers and engage in combat with other males during the breeding season, and the winner gains access to more females. Intersexual selection, on the other hand, refers to the process by which individuals of one sex (usually females) choose mates based on certain traits in the opposite sex (usually males). This can lead to the evolution of traits that are attractive or desirable to the choosing sex, such as bright colors, elaborate courtship displays, or vocalizations. An example of intersexual selection is in peacock birds, where males have elaborate plumage and perform intricate displays during courtship, and females choose mates based on their displays, which is hypothesized to signal good genes or overall fitness.

Step-by-step solution

1. Define Intrasexual Selection

Intrasexual selection refers to the competition among individuals of the same sex for access to mates or resources required for mating. This competition can lead to the evolution of traits that confer an advantage in these contests, such as size, strength, or aggressive behaviors.

2. Define Intersexual Selection

Intersexual selection, on the other hand, refers to the process by which individuals of one sex (usually females) choose mates based on certain traits in the opposite sex (usually males). This can lead to the evolution of traits that are attractive or desirable to the choosing sex, such as bright colors, elaborate courtship displays, or vocalizations.

3. Differentiate Intrasexual and Intersexual Selection

Intrasexual selection involves competition among members of the same sex, while intersexual selection concerns mate choice by members of the opposite sex. Intrasexual selection typically results in traits that provide a competitive advantage in contests, whereas intersexual selection leads to traits that are attractive or desirable to the choosing sex.

4. Example of Intrasexual Selection

In many deer species, males grow antlers and engage in combat with other males during the breeding season. The winner of these fights typically gains access to more females, so there is a strong selective pressure for males to have larger and stronger antlers.

5. Example of Intersexual Selection

In some bird species, such as the peacock, males have elaborate plumage and perform intricate displays during courtship. The females then choose mates based on their displays, which is hypothesized to signal good genes or overall fitness. This leads to the evolution of increasingly elaborate plumage and courtship behaviors in the male population.

Key concepts

Intrasexual Selection

In the dynamic process of sexual selection within an ecological context, intrasexual selection stands out as a fascinating mechanism. It describes the competitive struggles among individuals of the same sex, often leading to the development of distinct characteristics that help to win these contests. For instance, in certain species such as beetles, the males may develop exaggerated mandibles (jaw structures) used to wrestle opponents during competition for a mate.
These traits are more than just adaptations for physical confrontations; they serve as signals to potential mates of a male's fitness and capability to dominate rivals. However, these traits can sometimes come at a cost, like increased energy expenditure or a higher visibility to predators. It's a fine balance between survival and the need to reproduce, exemplifying the trade-offs that are a central theme in evolutionary biology.

• Competitive advantages for intrasexual selection may include strength, size, or weaponry like antlers or horns.
• Strategic behaviors such as guarding territories or controlling resources that attract the opposite sex are also part of intrasexual competition.
• The resultant traits enhance an individual's chance of mating success but must also be weighed against their survival costs.

Intersexual Selection

In contrast to intrasexual selection's emphasis on competition, intersexual selection is characterized by selection based on mate preference. Typically, it's the females who are more discerning and select mates based on certain appealing traits, such as the mesmerizing tail feathers of a peacock or the melodious song of a nightingale.
Traits favored by intersexual selection are believed to indicate a level of genetic fitness or resource-holding potential. For instance, a bright-colored male bird may suggest he is healthy and free of parasites, thus being a desirable partner. Males might also evolve impressive courtship behaviors to woo potential mates, initiating a complex dance of attraction and choice.
These preferred traits in males don't necessarily contribute to direct survival; rather, they are signals that increase their attractiveness and thus their potential for successful reproduction. This fascinating interplay highlights the role of female choice as a powerful evolutionary force, shaping the very nature of species over time.

Evolution of Mating Traits

The evolution of mating traits is a compelling topic within sexual selection theories. It is the end-result of both intrasexual and intersexual selection processes. Over time, these selected traits become more pronounced within a population, often resulting in remarkable differences between sexes, known as sexual dimorphism.
Mating traits evolve due to the reproductive advantages they provide. For instance, the eye-catching tail of a male peacock comes from generations of females preferring mates with longer and more colorful tails.
Such evolutionarily acquired traits can escalate over generations, leading to what biologists call a 'runaway process'—where the desire for certain characteristics becomes a driving force in the selection of these traits. Nonetheless, there must be a balance, as overly elaborate traits can impede survival by attracting predators or requiring excessive energy to maintain.

• Sexual dimorphism is a direct reflection of the different pressures faced by males and females in the mating game.
• Runaway selection shows how preference itself can shape the evolution of mating traits.
• Courtship rituals and physical traits become part of an evolutionary race to win over the best mates.

Mate Choice

At the heart of sexual selection lies the concept of mate choice. This critical decision-making process is based on assessing potential partners' desirability. Different species demonstrate varied aspects of choice, such as direct benefits like food offerings in insects or protection in some primates, and indirect benefits, like good genes suggested by physical characteristics.
Animals use a variety of cues and signals to assess mates, from visual displays to chemical pheromones. Female mate choice can drive sexual selection, pushing males to evolve traits that are sometimes extravagant and costly. The complexity of these traits can be remarkable: from the intricate dances of birds of paradise to the synchronized light patterns of fireflies.
Throughout this selection process, individuals must balance the desire for high-quality mates with the risk of remaining unpaired. Species thus develop a range of strategies from being less choosy to broadening the types of acceptable traits, emphasizing the dynamic interplay between choice, competition, and survival.

• Mate choice profoundly influences the direction and intensity of sexual selection within a population.
• While beneficial in the short-term, traits selected through mate choice can have long-term implications for a species' evolution.
• The dynamic nature of mate choice reflects a critical aspect of species adaptation and their evolutionary journey.