In an animal population most organisms produce more offspring than can survive to adulthood. For this reason, there is competition with each other for resources, such as food, shelter and mates, in order to survive. Each population has individuals that differ from one another and some individuals have more advantageous characteristics than others. Those are the ones who are going to survive and reproduce passing this way their advantageous characteristics to the next generation. Those that are not as well adapted end up dying without producing any offspring. As a result of the above, there will be a gradual change in the animal population and more organisms will express the alleles with the more advantageous characteristics and the alleles that result in less well-adapted characteristics will get lost. This is how species evolve over time.
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