Open Educational Resources

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Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License
This work is licensed under a Creative Commons Attribution-NonCommercial-Share Alike 4.0 International License.

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Open Educational Resource


What is a species? Why are there so many species on the planet? The question of why we have different species of organisms was the key thesis of Charles Darwin’s most famous book, On the Origin of Species.While several definitions of what constitutes a species exists, the most common definition lies within thebiological species concept. This concept states that different species exist when two individuals of different sexes are unable to reproduce with one another to yield viable offspring in the wild. A major limitation of this definition is that it can only be applied to sexually reproducing organisms that are extant (i.e., not extinct). In sexually reproducing organisms, barriers to reproduction can occur before mating, after mating but before a zygote is produced, after production of a zygote, or even after an offspring is produced. A variety of evolutionary processes can come into play in the production of new species, including mutation, genetic drift, selection, and gene flow. Reproductive barriers can evolve in allopatry, when two groups of organisms do not encounter one another, or in sympatry, when the two groups live in the same area and can interact with one another. In this lesson, we will discuss definitions of species, processes leading to speciation, why some lineages have many species while others have few, how species are maintained despite gene flow, and what dictates the rates of speciation. The lesson includes a hands-on activity for which students read a scientific paper to identify processes leading to speciation in domesticated chicken breeds


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