For a species to survive, it must reproduce. However, the population is limited by environmental factors and so remains more or less constant over time.
There is competition between individuals of the same species (intraspecific competition) or between members of different species (interspecific competition) for resources.
Since there is variation within a population, some individuals are less well adapted to a particular environment. The less well adapted are 'weeded out' as the selection acts on the phenotype of the individual.
These individuals fail to reproduce or die, the more successful ones reproduce and pass on their genes to the next generation.
Note: Adaptations are environment-specific; an advantageous characteristic can become disadvantageous if the environment changes in a particular way.
The change in adaptation that occurs is called evolution.
There are three types of selection that occur in nature:
- Stabilizing selection.
- Directional selection.
- Disruptive selection.
In each case we will use the illustration of a population of mammals and the characteristic being selected for or against is fur length.
In each situation, the population is normally distributed - there are a few individuals that have very short or very long fur length but most have an intermediate fur length.
Initially there is a wide range of fur length about the mean of 1.5cm.
Due to rapid breeding in either very cold or very warm weather, animals with extreme fur lengths survive.
When the temperature remains constant with little variation, the individuals with very short or very long hair become less numerous and are eventually eliminated from the population.
If the temperature falls, the individuals with longer fur length are at an advantage as they have better insulation against the cold.
There is a selection pressure favouring the animals with longer fur so these animals are more likely to survive and thus reproduce.
Over several generations, the average fur length increases as more young have inherited the genes for long fur. When the mean fur length has reached the most advantageous length, the selection pressure ceases.
If the temperature difference between summer and winter increases, long hair for animals being active during the winter or short hair for animals being active during the summer is advantageous.
Intermediate fur length is disadvantageous. Therefore, two sub populations are formed over time.