Selection is an evolution process in which certain genes or traits of an organism are specifically selected to be eliminated or altered and allowed to mutate in generations to come. This creates a genre of successful individuals or organisms with better traits or genes. Selection pressure is the intensity or force with which an organism evolves over generations with changes in genes or traits. Selection pressure refers to a host of factors like nutrition, living space, changes in weather, diseases and the environment. All these factors together form limits on an individual viz. human beings, animals, microorganisms etc and subject them to selection thereby altering their genetic makeup in generations to come.
Selection pressure is primarily classified into two types: Natural Selection and Artificial Selection. In Natural Selection, the ecology or nature selects the specific traits of an organism and acts on it to give an advantage to the next generation of these organisms. This is an important process which is part of evolution in which the entire species undergoes a change in traits and genetic makeup with changes in ecological factors like pollution, heat, landscape change etc. Natural selection is further classified as sexual selection, ecological selection, stabilizing selection, disruptive selection and directional selection. The most common form of natural selection can be described as the resistance to antibiotics in organisms. When the organism is first exposed to an antibiotic, a part of the bacteria present in it are killed. The remaining bacteria develop immunity to this antibiotic which thus tenders the antibiotic inefficient after a while. This is turn produces a generation of organisms which are totally immune to this antibiotic.
Selection happens in situations when the individual has diverse traits or characteristics that make it difficult to co-exist with nature or adapt to an environment. While selection does not always target a specific characteristic and makes it a strong point in generations to come, it sometimes becomes a weak spot making advantageous genes to disappear over a period of time and making the individual totally unfit to survive in an environment. When an organism does not possess any unique trait or gene, it is not fit for selection, or in other words, selection pressure does not work on such organisms.