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Selection

Evolution relies on differential reproductive success. Typically (but not always...), it is caused by selection - non-random mechanisms that result in individuals with a particular phenotype having a greater relative fitness than others.

A selective pressure is a factor that can lead to the differential fitness of individuals, which can impact the evolution of a species. These factors can be abiotic or biotic, such as predation, climate, or limited resources.

Types of Selection

Natural Selection

The most well-known type of selection, natural selection is when the individuals who are most suited to survive in an environment will survive and reproduce. "Survival of the fittest" is often used to describe this, where evolutionary fitness refers to those who are the most reproductively successful.

Natural selection is what was observed with the classic example of evolution, Darwin's finches, where the birds evolved different beak size/shapes based on the food that was available to them.

Adaptations

Natural selection is adaptive. Adaptations are variations that provide an advantage in a particular environment, increasing the organism's fitness.

Species are not perfect, however, and neither are adaptations.

Adaptations arise by modifying existing structures. If you look at the image to the left, the recurrent laryngeal nerve loops around the aorta in both humans and giraffes, despite it not remotely being the most efficient path for us, let alone the giraffe.

Traits that are adaptive in one environment are not necessarily in a different environment, and may even be detrimental.

Sometimes there are fitness trade-offs, where one trait may negatively impact another.

In addition, mutations - which cause new variation and new traits - occur randomly and by chance.

Natural selection is not goal-driven and does not strive toward a particular phenotype. Fitness in each generation depends on its present environment, not the past environment, and not a future environment.

As evolution works by making traits that already exist more of less common, it cannot "create" new needed phenotypes. And due to this, more genetically diverse populations are more able to respond to disturbances as they are more likely to have something that can survive or withstand the change.

In addition, not every trait is an adaptation - or even has an impact on fitness. There are traits that can be passed on without having any impact on fitness, such as those that drift randomly across generations. There are also what are known as spandrels, which arise as a byproduct of another trait, not because they themselves increase fitness. They are named after architectural spandrels, which is the triangular shape above an arch that is required to make an arch.

Kin Selection

Not all evolutionary advantages come from helping oneself survive and reproduce. Kin selection refers to when an individual behaves in a way that helps close relatives reproduce, even if it comes at a cost to themselves. This can still increase the individual's overall inclusive fitness, which includes both direct fitness (one’s own offspring) and indirect fitness (helping relatives produce offspring). Because close relatives share many of the same genes, helping them survive and reproduce increases the chance of shared genes being passed on.

Kin selection helps explain behaviors like alarm calls in meerkats, where one individual puts itself at risk to warn the group. These behaviors make more sense when the group members are closely related and share a large portion of their genetic material. Genetically, a sibling is just as related to you as your own child!

Sexual Selection

While natural selection focuses on survival, sexual selection focuses on traits that increase the chances of attracting a mate and reproducing. These traits may not necessarily improve survival (some actually make it more difficult to survive), but they provide a reproductive advantage.

Sexual selection occurs in two main forms. In intersexual selection, individuals of one sex (usually females) choose mates based on certain traits. These might include bright colors, an elaborate mating dance, or specific behaviors that indicate genetic quality or the ability to provide resources. In intrasexual selection, individuals of the same sex (usually males) compete with one another for access to mates. This can lead to the evolution of features like large antlers, body size, or aggressive behaviors.

Traits influenced by sexual selection can include appearance (such as plumage or coloration), courtship behaviors (like dances, songs, or nest-building), and even territorial displays or gifts of food. These traits evolve because they increase mating success, not because they directly help with survival.

Sexual Selection Resulting in Different Types of Species

Not all species experience sexual selection in the same way. The type of mating system a species has can strongly influence which traits are favored and how competition plays out.

In tournament species, individuals (usually males) compete for access to multiple mates. This leads to intense intrasexual selection, where physical traits like body size, antlers, or strength are selected for because they help individuals outcompete rivals. As such, there is often a high degree of sexual dimorphism, where the males and females look distinctly different. In these species, there is often a lot of variation in reproductive success - the top males will have almost all of the kids.

Because the stakes are so high, tournament species often show extreme forms of competition that go beyond fighting or showing off. Some of these include:

Competitive Infanticide - In some species, like lions and mountain gorillas, a dominant male's rein may be shorter on average than the female's nursing period. As the female be able to reproduce while she is taking care of her kid, the male will kill the babies so that he can have his own kids sooner.
The Bruce Effect - This is also known as "olfaction-induced miscarriage." Exposure to the scent of unfamiliar males will cause females to terminate their pregnancies, likely as an adaptation to avoid the risk from competitive infanticide.
Pseudo-estrus - Species like langur monkeys will display external physical signs of ovulation, even if they're pregnant, when a new dominant male takes over. This basically tricks the guy so that he believes the kids are his and therefore prevents competitive infanticide.
Coercive Kidnapping - This is basically "mess with me and your kid is gonna get it." Species like savanna baboons, which live in large troops and have strong dominance hierarchies, will often have more dominant males attack lower ranking males. As the more dominant males are the ones who most likely had kids, the lower ranking males will sometimes grab an infant and hold it to his chest to deter the attack.
Sperm Competition - Male and female wants don't always align. In house flies, females are fairly promiscuous and will mate with a large amount of males. They tend to have sperm from various males inside of her at any given time due to this. Males have evolved to produce a toxin with their sperm that kills other sperm (which also shortens female lifespans).

In pair-bonded species, individuals form more equal or long-term mating partnerships. Both parents often invest in raising offspring, and mating success tends to be more evenly distributed across individuals. Sexual selection still occurs, but it often favors traits related to cooperation, parenting ability, and mutual attraction, rather than dominance or aggression.

Artificial Selection

Artificial selection is a process similar to natural selection, except the selection pressure is the whims of mankind. In artificial selection, humans choose which individuals reproduce based on desired traits. Over generations, this can dramatically change a species.

Examples include the wide variety of dog breeds all descending from a common ancestor, or the way crops like corn have been selectively bred for sweetness, size, or resistance to pests.

The traits that are selected depend on human preference rather than environmental pressures. This is an important distinction - humans can be a selective pressure for natural selection. It's only artificial selection if we are consciously choosing and selecting for the trait. This is selective breeding, not something like pests becoming resistant to pesticides because of our repeated use of it.

Modes of Selection

Directional Selection

In directional selection, one extreme phenotype is favored over the others. This causes the population’s traits to shift in one direction over time. For example, if larger body size helps individuals survive better in a certain environment, then average body size will increase in future generations. This mode is common when environments change or when a new challenge or resource becomes important.

Stabilizing Selection

In stabilizing selection, individuals with the average or intermediate phenotype are favored, while those with extreme traits are selected against. This reduces variation and maintains the status quo. Human birth weight is a classic example: very low or very high birth weights are associated with higher risks, so babies born around the average weight tend to have the highest survival.

Disruptive Selection

In disruptive selection, individuals with extreme phenotypes at both ends are favored, while the intermediate form is selected against. This can increase variation and may even lead to the formation of two distinct groups. An example might be a population of birds where small beaks are good for eating soft seeds, and large beaks are good for hard seeds, but medium-sized beaks are not effective for either.

Balancing Selection

Balancing selection occurs when multiple alleles are actively maintained in the population because no single allele has a consistent advantage. This can happen for several reasons, such as heterozygote advantage (where individuals with two different alleles have higher fitness) or negative frequency-dependent selection (where the fitness of a trait increases the rarer it is).

An example of heterozygote advantage is the sickle cell trait: people with one copy of the sickle cell allele are more resistant to malaria, and don't display the sickle cell phenotype, giving them an advantage in areas where the disease is common.

An example of negative frequency-dependent selection is P. microlepis - a scale-eating fish with a mouth that opens either to the right or left side of its body. They sneak up on other fish and bite off their scales. If right-mouthed fish become too common, the prey learn to guard that side, giving left-mouthed fish an advantage - and vice versa. As a result, the frequencies of left- and right-mouthed fish oscillate over time, maintaining variation in the population.

Negative Frequency Dependent Selection

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