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.
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
Sexual Selection
Artificial Selection
Modes of Selection
Directional Selection
Stabilizing Selection
Disruptive Selection
