Sexual Selection: Intrasexual and Intersexual
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You should be comfortable with the basic mechanism of natural selection, specifically how differential survival and reproduction drive changes in allele frequencies, and you should understand the distinction between stabilising, directional, and disruptive selection.
Why this matters
Sexual selection explains why many organisms possess traits that appear to reduce their survival fitness.
While natural selection generally favours traits that enhance an organism's ability to survive in its environment, sexual selection operates through a different mechanism: differential mating success. This process is responsible for the evolution of elaborate ornaments, aggressive weaponry, and complex courtship behaviours. Understanding sexual selection is crucial because it resolves a major paradox in evolutionary biology: why do traits that seem costly, dangerous, or conspicuous persist in populations? By distinguishing between competition for mates (intrasexual) and choice of mates (intersexual), we can predict how reproductive strategies evolve in different ecological contexts and understand the trade-offs between survival and reproduction.
The Core Concept: Mating Success vs. Survival
In standard natural selection, we often focus on survival. An organism that avoids predators, finds food efficiently, and withstands harsh weather is considered "fit." However, survival is only half the equation for evolutionary success. An organism must also reproduce. Sexual selection is the component of natural selection that acts specifically on an individual's ability to secure mates. It is distinct from survival-based selection because it can favour traits that are actually detrimental to survival. For example, a male peacock with a massive, colourful tail is more visible to predators and expends significant energy growing and maintaining that structure. If survival were the only factor, such a tail would be selected against. Yet, these tails persist because they increase mating success. This happens because females prefer males with larger tails, or because males with larger tails outcompete other males for access to females. This divergence between survival fitness and reproductive fitness is the hallmark of sexual selection. It is important to note that sexual selection is not a separate force from natural selection in the physical sense; it is a subset of natural selection where the selective pressure is driven by reproductive competition rather than environmental survival. The outcome is the same: changes in allele frequencies. But the driver is different. In sexual selection, the "environment" includes the preferences of the opposite sex and the competitive abilities of the same sex. This means that a trait can be "good" for mating but "bad" for living. The net evolutionary outcome depends on the balance between these two opposing forces. If the mating advantage outweighs the survival cost, the trait will spread. If the survival cost is too high, the trait will be eliminated. This dynamic creates a constant tension in the evolution of secondary sexual characteristics.
Intrasexual Selection: Competition Within the Sex
Intrasexual selection, often referred to as male-male competition, occurs when individuals of the same sex compete directly for access to mates. It is most intense when one sex (usually females, because they invest more in each offspring) is the limiting resource, so that members of the other sex (usually males) compete for access to them. The mechanism is straightforward: individuals who are better at competing win more mating opportunities. This competition can take many forms, ranging from physical combat to territorial defence. In many species, this leads to the evolution of weapons such as antlers, horns, tusks, or enlarged canines. These structures are costly to produce and maintain, but they provide a direct advantage in physical contests. For instance, in red deer, males with larger antlers are more likely to win fights and thus gain access to harems of females. The size of the antler is correlated with the male's body size and strength, making it an honest indicator of competitive ability. Intrasexual selection also drives the evolution of aggressive behaviours and heightened territoriality. Males may defend specific patches of habitat that contain resources attractive to females, such as nesting sites or food sources. By controlling these resources, the male indirectly controls access to mates. This form of selection often results in significant sexual dimorphism, where males are larger and more robust than females. The intensity of intrasexual selection is often linked to the variance in reproductive success among males. In species where a few dominant males sire the majority of offspring, selection for competitive traits is strong. Conversely, in species where mating is more evenly distributed, intrasexual selection is weaker. It is important to recognise that intrasexual selection is not limited to physical combat. It can also involve chemical signals, vocal displays, or social manipulation. The key feature is that the trait evolves because it helps an individual defeat rivals, not because it attracts the opposite sex directly.
Intersexual Selection: Choice by the Opposite Sex
Intersexual selection, or mate choice, occurs when individuals of one sex (usually females) choose mates based on specific traits. This process is driven by the preferences of the chooser. The traits that are favoured are often secondary sexual characteristics, such as bright plumage, complex songs, or elaborate dances. The classic example is the peacock, where, in some studies, females preferred males with more eyespots on their tails (other studies have not found this, so the case is still debated). But why do females have such preferences? There are several hypotheses. One is the "good genes" hypothesis, which suggests that females choose males with exaggerated traits because these traits indicate superior genetic quality. A male that can survive despite having a costly, conspicuous tail must be particularly fit. By mating with him, the female passes on these "good genes" to her offspring, increasing their survival and reproductive success. Another hypothesis is the "Fisherian runaway" process, where a preference for a trait evolves simply because the trait is attractive. If females prefer long tails, and long-tailed males have more offspring, the next generation will have both longer tails and a stronger preference for long tails. This positive feedback loop can drive the evolution of extreme traits, even if they are costly. Intersexual selection can also be driven by sensory bias, where females have a pre-existing preference for certain stimuli (e.g., red colouration) that males then exploit. In this case, the trait evolves because it triggers a neural response in the female, not necessarily because it indicates quality. Intersexual selection often results in traits that are highly specific to the species, as preferences can vary between populations. It is also important to note that mate choice is not always passive. Females may actively court males, or they may assess multiple males before choosing. The outcome of intersexual selection is that males with the most attractive traits have higher reproductive success, leading to the spread of alleles for those traits in the population.
The Trade-off: Survival vs. Reproduction
A central theme in sexual selection is the trade-off between survival and reproductive success. Traits that increase mating success often decrease survival. This is known as the "cost of sexual selection." For example, bright colours may attract predators, and loud calls may reveal an individual's location. The evolution of such traits depends on the balance between the mating benefit and the survival cost. If the mating benefit is high, the trait will evolve. If the survival cost is too high, the trait will be selected against. This trade-off is often mediated by the ecological context. In environments with high predation pressure, the cost of conspicuous traits is higher, so sexual selection may be weaker. In environments with low predation, the cost is lower, so sexual selection can be stronger. This explains why sexual dimorphism is often more pronounced in species with lower predation pressure. The trade-off also affects the life history of the organism. Individuals that invest heavily in reproductive traits may have shorter lifespans or lower survival rates. This is known as the "reproductive effort" trade-off. In semelparous species such as Pacific salmon, both sexes die after a single breeding season. In these cases, the trade-off is extreme: all resources are invested in reproduction, and survival is sacrificed. In other species, the trade-off is more subtle, with individuals balancing mating effort with self-maintenance. Understanding this trade-off is crucial for predicting how populations will respond to environmental changes. For example, if predation pressure increases, the cost of sexual traits increases, and the population may evolve less exaggerated traits. Conversely, if predation pressure decreases, the cost decreases, and the population may evolve more exaggerated traits. This dynamic interplay between sexual selection and natural selection is a key driver of evolutionary change.
The concept
Sexual selection
Traits favoured through mate choice and competition.
A common misconception is that sexual selection always leads to the evolution of the most extreme traits possible. In reality, the evolution of sexual traits is constrained by the trade-off with survival. If a trait becomes too costly, it will be selected against, even if it is attractive. This means that the optimal trait size is a balance between mating benefit and survival cost, not simply the maximum possible size. This constraint is often overlooked in simplified models of sexual selection. C
Worked example
Problem. Consider a population of birds where the mating success of males is proportional to the size of their tail feathers, but the survival rate decreases as tail size increases. Let \( m(t) \) be the mating success and \( s(t) \) be the survival rate, where \( t \) is the tail size. Assume \( m(t) = t \) and \( s(t) = 1 - 0.1t \). The fitness \( W(t) \) is the product of mating success and survival rate: \( W(t) = m(t) \cdot s(t) \). Find the tail size \( t \) that maximises fitness.
- Write the fitness function: \( W(t) = t(1 - 0.1t) = t - 0.1t^2 \).
- To find the maximum, take the derivative of \( W(t) \) with respect to \( t \): \( \frac{dW}{dt} = 1 - 0.2t \).
- Set the derivative to zero to find the critical point: \( 1 - 0.2t = 0 \).
- Solve for \( t \): \( 0.2t = 1 \), so \( t = 5 \).
- Verify that this is a maximum by checking the second derivative: \( \frac{d^2W}{dt^2} = -0.2 \), which is negative, confirming a maximum.
Units check. The tail size is a dimensionless relative measure in this model, so the result is consistent with the input parameters.
Three quick tests
- What is the main difference between intrasexual and intersexual selection?
Answer
Intrasexual selection involves competition within the same sex, while intersexual selection involves choice by the opposite sex. - Why might a costly trait evolve under sexual selection?
Answer
Because it increases mating success, which can outweigh the survival cost. - How does predation pressure affect the evolution of sexual traits?
Answer
Higher predation pressure increases the cost of conspicuous traits, potentially leading to less exaggerated traits.
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Lecture notes checked line by line by the editor; every number in the worked example was recomputed.
Concepts cited: T-021 · Threads: evolution