In asexually reproducing organisms, evolution occurs by 2 processes, mutations and natural selection. Mutations are the change in genome and natural selection is the difference in reproductivity of phenotype of different individuals. Mutations tend to be so small that they can be neglected for certain purposes so it can be treated as though a species evolves entirely by natural selection. In sexually reproducing species, natural selection tends to have the same general effect because past natural selection of the genes made it that way because the individual genes don't get broken up during reproduction.
Simple selection
editSimple selection occurs when one's ability to reproduce depends entire on one's own phenotype. One phenotype is said to be fitter than another phenotype when it has a tendency to outnumber it more and more by natural selection. It turns out that the fitness of a phenotype is proportional to the expected number of children an individual will have. Individuals sometimes have an unpredictable environment and so for some phenotypes, the number of children is determined not only by the phenotype but also by the environment. One example of simple selection is the fitness of a bonobo as a function of how fast it climbs a vine. It turns out that a specific climbing speed is the fittest. That's because those that climb that speed will have the largest expected number of children because they burn the least energy: those that climb faster burn more energy because they climb faster and those that climb slower also burn more energy because they click for a longer time. As a result, natural selection selects just for that climbing speed, which is said to be an evolutionary stable strategy because that state of the species is destined to stay the way it is by natural selection.
Suppose a species has individuals of both handednesses and also has ambidextrous individuals but the ambidextrous individuals have a tiny bit lower motor skill with both their hands that the left handed or right handed individuals have with their dominant hand and the individuals with a dominant hand have extremely low motor skill with their dominant hand. Furthermore, suppose each individual has some tasks requiring skill with one hand that it has an evolutionary advantage in completing like chiesling a rock into a tool but other tasks requiring skill with both hands that it also has an evolutionary advantage to complete like climbing a vine with your hands and feet. Then those with a dominant hand are more likely to complete the task requiring skill with one hand but those that are ambidextrous are more likely to complete the task requiring skill with both hands. As a result, the species will evolve to have a dominant hand if the evolutionary advantage in completing the task that requires skill with one hand is stronger and will evolve to be ambidextrous if the evolutionary advantage in completing the task that requires skill with both hands is stronger. In this case since climbing a vine doesn't require much skill, the evolutionary advantage in being more likely to produce your own tool from having a dominant hand is stronger so the species would evolve to have a dominant hand.
Frequency dependent selection
editFrequency dependent selection occurs when the proportion of phenotypes affects how fit each phenotype is. Frequency dependent selection can also lead to an evolutionary stable strategy. For example, suppose everybody is right handed or left handed and nobody is ambidextrous. Furthermore, suppose everybody who hasn't yet reproduced is randomly pair with another person who hasn't yet reproduced to dump water out of a bucket like the one at http://cdnimg.webstaurantstore.com/images/products/extra_large/6124/105963.jpg and those that succeed at dumping it out together are more likely to reproduce. Since one has to use their right hand to dump it out from one side and the other has to use their left hand to dump it out from the other side, those that are randomly paired with somebody of the same handedness are more likely to fail to dump it as a result of one of them being stuck dumping it with their nondominant hand. Those with the rarer handedness are less likely to be paired with somebody of the same handedness so they're selected for. Therefore, the evolutionary stable strategy is for half of all people to be each handedness.
As another example, suppose a species of snail occurs in 2 forms that are mirror images of each other and each member of that species can only mate with its own form. Furthermore, suppose there's a species of snake with an asymmetrical jaw that comes in 2 forms that are mirror images of each other, each of which can only dig one form of snail out of its shell and eat it. Then for any initial state where neither form is snail is too rare, natural selection will select for the form of snake suited for the more common form of snail. As a result, the natural selection for the rarer form of snail to be suited for the rarer type of snake will be stronger than the natural selection for the more common form of snail to me more likely to find a mate because it's not that hard for the rarer form of snail to specially look for another individual of the same form to mate with. The following result would be that the snail proportion evolves to the other side of half and half less far away from half and half then the snake proportion evolves to the other side less far from half and half then the snail proportion evolves back to the side it started even closer to half and half and the state of the 2 species spirals into the evolutionary stable strategy of both species having 50% of individuals be each form.
That would actually only be an evolutionary stable strategy if the asymmetrical jaw was so much better at digging one form of snail out that it's fitter than any symmetrical jaw despite the fact the a snake with a symmetrical jaw can eat either form of snail. If the fittest symmetrical jaw can only dig part of each form of snail out, surely the fittest asymmetrical jaw is fitter because the disadvantage of getting way less energy from only getting part of each snail it competes with the other snakes for out is stronger than the disadvantage of wasting only a tiny bit more energy to spend twice as much searching time to only eat half the snails it finds and still eat the same number of snails.
It can also happen that a species is destined never to reach an evolutionary stable strategy. For example, the Side-blotched lizard exists in 3 states that outcompete each other in a way similar to Rock-paper-scissors. As a result, natural selection keeps on cycling from the species being mainly one type to mainly the type that beats it and so the species is undergoing continuous cyclic evolution repeating every 6 years. How can there not be an evolutionary stable strategy when clearly there exists a fittest phenotype? It's because as the proportions of phenotype change, which phenotype is the fittest can also change.
Population size
editSometimes a species in an evolutionary stable strategy also has a stable population size. The larger the population size, the fewer the expected number of children a parent has enough food to rear, keeping the population at a steady size.
Evolution of cooperation
editSuppose members of a species split into pairs each of which is given a resource that can be shared. Natural selection selects for individuals who fight back if the other individual tries to take all of the resource because that gives them a 50% chance of winning the fight and getting all of the resource. As a result, if the evolutionary advantage of starting a fight from the possibility of winning is smaller than the evolutionary disadvantage of starting a fight from the possibility of losing, natural selection will select for individuals that don't start a fight in the first place. When the resource is replaced with honour if the evolutionary advantage in starting a fight because of the possibility of gaining honour is strong enough, the species will evolve to have a social hierarchy but if the evolutionary disadvantage in starting a fight because of the possibility of losing is strong enough, the species will evolve to be very peaceful like bonobos.