Selective pressure is the force that drives evolution by pushing organisms towards advantageous traits. Natural selection occurs when favorable mutations increase an individual’s chances of survival and reproduction, leading to the prevalence of that trait in a population. Selection pressure can arise from various factors, including competition, predation, and environmental stresses. It operates on an individual level and has no intelligence or foresight. Examples include the domestication of foxes and the case of the peppered moth. Human influence can also lead to selective pressure, such as the emergence of antibiotic-resistant bacteria.
Selective pressure can be considered as a force that makes a particular organism evolve in a certain direction. It is not a physical force, but an interaction between natural variation in a species and factors in its environment that cause one form to have an advantage over others. This can be thought of as a “pressure” that pushes the evolution of that organism towards a greater prevalence of this variation.
Evolution and Natural Selection
When organisms reproduce, random mutations can occur, causing the offspring to vary somewhat from the parents. These changes can be harmful, but sometimes they can be beneficial. For example, a change that allows an animal to run slightly faster can increase its ability to catch prey or evade predators.
A favorable mutation can increase an individual’s chances of surviving long enough to reproduce and pass this new trait on to their offspring, and thus it will become more common. Eventually, all members of the species can have this trait. Unfavorable mutations disappear quickly, as they are less likely to be passed on to the next generation.
These changes in the populations of different forms of a species are known as natural selection: the form of a species that is best adapted to its environment is the one that survives. This is sometimes referred to as “survival of the fittest”. The term “fittest”, in this context, does not mean the strongest or fastest, but the variant that best fits its environment. Strength and speed may play a role, but other factors, such as intelligence or color may be more important, depending on the circumstances. Natural selection is the result of selective pressures and drives evolution: as favorable mutations accumulate, organisms evolve into new species.
How selection pressures work
A selection pressure can arise from virtually anything, as long as it acts relatively consistently over reasonably long timescales and actually affects a species’ reproduction or survival rates. Potential pressures can include prey availability, the presence of predators, environmental stresses, competition with other species, including humans, and competition between members of a species. In the eyes of evolution, the probability of reproduction is all that matters: if, for example, a certain predator consumes only old animals that are already incapable of reproduction, the predator will have no impact on the evolution of the prey species.
The color of an organism can affect its chances of survival. For example, insects with colors that blend in with their surroundings are less likely to be seen by predators such as birds. A mutation that produces coloration similar to an insect’s usual background, such as a green color in a species that spends most of its time eating plant leaves, will increase its chances of successful reproduction and, over the course of a certain number of generations, this will increase become the normal form. Mutations that produce a different color will quickly disappear from the population.
It is important to note that selection pressure has no intelligence, foresight, rhyme or reason. Selection operates on an individual level, not on a species level. A new adaptation does not appear “for the good of the species”: it becomes fixed in a population only if it is good for each individual that possesses it, even if collectively it worsens the life of the species.
New adaptations can be partially self-defeating, as long as their net effect promotes the fitness of the organism. For example, Komodo dragons bite their gums with their sharp teeth when feeding, seemingly increasing their likelihood of lethal infection. But this also gives an advantage because the blood-saliva mixture is an ideal environment for the bacteria that infect their prey when they bite; the lizard can track an injured animal until it dies of the infection or is too weak to escape.
Selective pressure can operate faster than one might think, and this is especially true under selective breeding conditions, when pressure is intelligently applied by humans. One of the most striking examples is seen in a series of experiments by scientist Dmitri Belyaev that took place in the Soviet Union. The goal was to domesticate the silver form of the red fox, and it was achieved in just 10 generations of selective breeding. These foxes lost their distinct musky odor, wagged their tails like house dogs, and showed no fear of humans, even licking their hands to show affection. Related experiments also yielded a group of highly aggressive foxes that ferociously leapt against the walls of their cage when humans passed by.
Examples of dial pressure
A classic example of selective pressure at work is the case of the peppered moth. Until the mid-19th century, almost all specimens of this insect were light in color. It spent a lot of time resting on tree trunks and blended in well with the pale lichens that grew there. In urban areas, however, industrial pollution began killing lichens and darkening tree trunks with soot. A dark form of the moth that was better camouflaged quickly became more common, until nearly all specimens collected in urban areas were dark.
Attempts by humans to control unwanted organisms can sometimes result in selective pressure that leads to new forms that are resistant to the methods used. For example, the emergence of insecticide-resistant insect pests and weeds that are unaffected by herbicides has been observed. Some other examples of human influence are more worrisome. The widespread use of antibiotics has led to some disease-causing bacteria evolving into strains resistant to many of these compounds.
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