Directional Selection favours an increase or decrease in the value of a trait from the current population average. This type of selection occurs when the environment favours individuals with more extreme variations of a trait. An organism will encounter new forces of selection when it enters a new environment or when aspects of its habitat change which may result in an observable change in a population. In species with large populations and short generation times many offspring are produces. In these populations, the amount of genetic variation from both recombination and mutation increase. Human activity also results in directional selection. The following graph illustrates the effect of direction selection within a population in terms of a specific trait.
Hummingbirds move to a new habitat that has long-length flowers, their mid-length bills are no longer suitable. The long-billed hummingbirds are better suited for the new environment and have access to more food and therefore contribute more offspring to the next generation.
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