Natural selection is a pressure that causes groups of organisms to change over time. Animals inherit their genetics from their parents or ancestors, and the environment is constantly changing. Therefore, no organism is perfectly adapted to its environment. Therefore, natural selection constantly influences the evolution of species.
Natural selection overview
Even if a parent is perfectly adapted to the environment, the environment will change, leaving the offspring poorly adapted to the environment. Because there are so many animals and so few resources, only the best and fittest organisms can reproduce. Natural selection works against all organisms and can be viewed as the environment and the forces that act to prevent organisms from surviving and reproducing. Therefore, organisms that can survive can also pass their DNA on to the next generation. This “selects” for those DNA sequences.
Fortunately for all organisms, genetic variability makes each individual slightly different. These slight differences in performance can lead to differences in the amount each individual reproduces. By reproducing more, an individual creates more genetic variations that helped him succeed. The offspring of these individuals will also benefit from the genetic variations that enabled their parents to succeed. Organisms without these genetic adaptations will not reproduce as much, and thus their lines will one day cease to exist. Nature constantly exerts a selective force on the different genetic combinations that try to reproduce, and in this way, natural selection is the main driving force of evolution.
Examples of natural selection
Stabilizing Selection Example
To stabilize the selection, imagine a population of mice that lives in the forest. Some of the mice are black, some are white, and some are gray. If mice had no predators and no other force acting on their coat color, there would be no reason to change and it would only change randomly in response to certain mutations in DNA. However, that is not the case with these mice. They have many predators.
Foxes and domestic cats prey on mice during the day. At night, owls and other predators roam the dark for dinner. Either way, the mice are in a difficult position. But not all mice face the same risk at all times. During the day, black mice are much easier to detect, and predators eat more black mice. White mice stand out at night. This means that owls eat more white mice at night. Gray mice are the only ones that survive the longest during the day and night. For the next generation, there will be far fewer black and white mice to reproduce.
After a few generations of strong selective pressure, the entire population may turn gray. It depends entirely on the genetic makeup of the trait, but in some cases, a single trait is selected and the rest are lost from the population. In other cases, the colors of the black and white fur could become rarely seen features. Retaining traits can be an advantage when predators change. For example, if all night owls and predators disappeared, it would be more beneficial to be black. Black mice would take off and become more frequent in the population.
Directional Selection Example
It is important to consider different traits in the same animal population. Imagine again the population of mice that live in the forest. Instead of its color, consider a trait that runs on a continuous scale. Imagine that mice range in size from a normal-sized mouse to something much larger than a rat. Although the mice are of the same species, they grow in many different sizes. Predators, however, have a terrible time trying to catch and eat the largest of mice. Large mice not only weigh more, they can defend themselves. The smallest mice are mostly helpless and provide the perfect sized snack.
If this were the case, and nothing slowed them down, the mice would get much bigger. This is directional selection. This is probably what happened in the case of the capybara, a giant rodent from South America. Like our fictional rodents, the pressures of their environment have made them much larger than any other rodent known to man. Many rodents find different advantages in being small, which is why most rodents have remained at a certain size. These advantages could be as simple as the ability to hide or the availability of food, but animals of certain sizes do better for different reasons, and populations can change size over time.
Diversification Selection Example
Last time with the mice. But this time, consider a new trait in the population. Imagine that some of the mice begin to develop flaps of skin between their front and rear legs. Effectively, it creates a parachute that allows them to glide away from predators. Mice that have full skin flaps do this very well and can almost always escape predators. Similarly, finless mice avoid trees and open spaces where finned mice venture, and are much better at hiding from predators. The greatest selective force is against the mice in the medium.
Without the ability to glide, mice somewhere in the middle cannot escape predators at a rate that can help them reap the benefits of trees. At the same time, half fins make it difficult for them to run and hide from predators. Due to this weakness, many more of these mid-spectrum mice are eaten. This begins to divide the population into two distinct traits. Eventually, this can lead to mice becoming a completely different species.
It is possible that this is how bats became the only flying rodents. Just like in the imaginary scenario described, there are real rodents that don’t fly, some that can glide and bats. While the common ancestor among all of these animals might not have been called a rodent, they are all mammals. Like our imaginary scenario, diversified selection could have caused the population of the common ancestor to change and separate. In the real world, selective pressures are much more complex, and we can only guess at the exact historical relationship between animals.
Example of sexual selection
Look at a peacock. Try to imagine a functional use of that ridiculous tail. Perplexed? Scientists also did this until the mechanism of sexual selection was explained. This form of natural selection can sometimes select functional adaptations, but often produces strange adaptations that only serve to attract couples. In the case of the peacock, the colorful tail is used in a display intended to attract females. Males with larger tails and more dazzling colors are preferable to males with smaller tails. This peculiar preference seems to have no real relationship to the success of males in foraging and breeding, but due to the preference of females, all-male peacocks have large, colorful tails.
Curiously, this pattern of males that become the most decorated genera is valid for many species of birds. Male ducks, many male tropical birds, and even the male sparrow are much more decorated than their female counterparts. This is also seen in some reptiles. In fact, many animals have adapted strange displays or decorating methods so that their nest attracts couples. Selection can work both ways and mainly depends on which sex can be more selective when selecting a partner.
Predator-Prey Selection Example
The fastest land predator is the cheetah. The cheetahs were not extremely fast for no reason. The main prey object of the cheetah, the antelope, is also fast. Whoever got quick first will remain a mystery forever, but the fact is that these two species are mutually pushing each other to be faster animals
Faster cheetahs experience advantage over other cheetahs because they trap more antelopes and can support a much larger family. Eventually, the slow cheetahs will die, and the swift cheetah population will explode trapping antelopes. The antelope population, responding to the new selection, are also more successful when they are fast enough to avoid cheetahs. Therefore, the antelope population is also directionally selected for faster animals.
Scientists theorize that this give and take between predator and prey populations is responsible for shaping many of its defining traits. In fact, scientists were puzzled why the American Pronghorn, a species that resembles the antelope in size and speed, would exist considering the lack of cheetahs in North America. Without a predator fast enough to catch you, at some point, the extra speed is not a great advantage. Scientists remained puzzled until fossils of a cheetah-like predator were found in North America. Unlike cheetahs in Africa, cheetahs in North America did not survive human expansion, and pronghorn are left without a predator.
Principles of natural selection
There is an incredible variety of selective forces in the natural world, ranging from competition between species to predator-prey dynamics, to sexual selection between different genders. The defining characteristic of natural selection is that it is a force that allows some organisms to reproduce more than others. Natural selection does not always lead to the “correct” answer, as some people tend to think.
Natural selection is an imperfect process. You cannot create new DNA spontaneously, or change the DNA that is given to you significantly. You can only slow down or stop the reproduction of one DNA while allowing other DNA to persist. Each population has the opportunity to adapt, migrate to different conditions, or become extinct in the face of natural selection.
The natural selection process analyzes the DNA that is given to it, with the minor mutations and recombination that occurs during replication, and simply doesn’t let any of the DNA through. Sometimes the screen is random, like a lightning strike that kills a single tree. Other times, the screen is biased towards certain types of organisms, causing a selection to occur. This can be seen in the pine beetle invasion in North America. Pine beetles are being selected because they are exploiting a rich food source. Pines, on the other hand, are being selected for not having adequate defenses against beetles.
Types of natural selection
As animals diversify and fill different niches, the pressure exerted on them can change in many ways. The functional requirements for being a bird are very different from those required for being a fish. Their food is different, the environment in which they exist is different, and they must obtain oxygen in a different way. Natural selection, therefore, selects very different looking animals to fill the different niches of the ecosystem.
Regardless of the trait, natural selection tends to do one of the three things to a population. You can keep the feature the same, stabilizing the selection, moving the feature in one direction, directional selection, or selecting the extreme values of the feature, diversifying the selection. In addition to being classified by the effects it causes, natural selection can also be classified by the relationships of organisms that cause natural selection, and sometimes selection can be made by abiotic factors.
Most of the features in the animal kingdom can be described by a bell curve, in terms of their distribution. Most animals of a certain species tend to show the same trait or characteristic, of relatively the same size. There are always some exceptions to larger or smaller traits in certain individuals, but in general, most individuals sit somewhere in between.
Stabilizing selection is a form of natural selection that protects against outliers or trait exceptions. The screen prevents these animals from reproducing as much as “normal” or more normal individuals. More babies are born who are “normal” and fewer outliers are seen in each consecutive generation due to this bias. In this way, species can become very different from other species, however, all members of a species will look exactly the same.
Directional selection is a type of natural selection that occurs when one side of the spectrum of a certain trait is favored over the other. For example, if the smallest organisms are eaten and the largest organisms are fully protected, the population will tend to grow much more. If the opposite is true, the population will decrease in size over time.
It also artificially uses directional selection so that humans can create “miniature” animal breeds, which look like small copies of their larger counterparts. However, artificial selection only focuses on a single trait. This allows many negative traits to be present in the population, which would naturally have been selected against.
Like directional selection, diversified selection pushes the population to the extremes of the trait. This type of selection is also called disruptive selection. Diversified selection, in contrast to directional selection, pushes the trait both ways. This can happen in several ways, but it often leads to speciation because populations can become very different. However, if it only diversifies for short periods of time, selection can lead to a variety of traits that a species can share.
While you can classify natural selection in terms of the effects it has on the population, you can also see it as the interaction between organisms with different relationships. Sexual selection is a type of natural selection in which the different genera of a species exert forces on each other that change their appearance or traits. To humans, these traits often seem arbitrary, such as brightly colored feathers, the ability to perform a ritualized dance, or certain nesting traits such as decoration that do not appear to play a role in reproduction.
In many organisms, sexual reproduction is a highly competitive process. As such, organisms spend a considerable amount of time trying to choose a mate that will increase the success of their offspring. In some organisms, this boils down to the strongest or largest. However, many organisms have adapted complex mating rituals to identify potential mates. Sexual selection in these organisms can produce some strange traits, as seen in many birds.
When sexual selection is an example of intraspecific selection, multiple species can often exert selection pressures on each other, also known as interspecific selection. While this exists in many forms, one of the most common is predator-prey dynamics. Predators will always try to consume the easiest food source, making the prey evolve and more difficult to catch. In turn, the predator becomes faster and more agile. This cycle is continuous and predators and prey are constantly changing.
Other types of natural selection
Natural selection can come in an infinite variety of ways. Each organism will be more or less successful depending on which genes it carries and how those genes interact with the environment. Genes can cause new ways to process nutrients, allow different structures to form, and allow old structures to be reused. Completely different organisms that occupy the same niche often have similar structures. These structures were not obtained from a common ancestor, but only from the forces of natural selection. Natural selection is the main driving force behind all the different forms and functions of life on Earth.