How does natural selection work?

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Aug 26, 2016


Natural selection works by acting on existing variation within a population so that individuals with greater fitness survive.


Natural selection works with variation in traits that naturally exists within a population. Individuals with the greatest fitness survive. Fitness in ecology refers to an individual's ability to survive and produce viable offspring. Natural selection ensures that traits that are beneficial and increase fitness remain in the population while traits that are harmful and decrease fitness do not.

Let's look at the evolution of a giraffe's long neck. Giraffes did not evolve long necks because they needed to reach leaves higher in trees. This is NOT how natural selection works and is an example of Lamarckism. Instead, there was natural variability in the length of giraffes' necks that existed in the population. Those with longer necks were able to eat more food because they could reach taller branches, were healthier, and produced more offspring because of this advantage.

Over many generations of giraffes, those with longer necks are more successful and those with shorter necks are less successful and their numbers decline until all giraffes have long necks. The image below illustrates the idea:

It is important to note that natural selection acts on an individual's phenotype, or the expression of the gene, as opposed to genotype.

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