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What’s A Species?

Earlier in the Cells are Us module, you learned that humans are made up of huge populations of millions of cells.  Just like your body, ecosystems are also composed of interacting units called species.  How would you define a species? 

For our purposes, organisms are of the same species if they produce offspring that can reproduce.

Recent evidence, however, shows that this definition may be out dated.  For example, a cow and a bison can mate and produce offspring that can reproduce, but they are definitely different species.  These days, many scientists use genetic variation as the factor in determining which organisms are in the same species.  It is important to remember that how an organism is defined is mostly based upon an subjective interpretation of the data.

Let's look at the mule as an example:


                         What's A Species?
                    To Protect and Serve
Populations on the Genetic Level
Hardy-Weinberg Equilibrium
Creating New Species
Population Growth
The Wars of Succession
Predators and Prey



Do you know where mules come from? Well, when a male donkey and a female horse reproduce, a mule is born. Knowing that a donkey and a horse constitute two different species, does this violate the previous definition of species? No. Specifically, can the offspring also reproduce? In this case, a mule cannot reproduce so, a horse and a donkey are not of the same species. Interestingly enough, the first President, George Washington, is considered the father of mule breeding. (Learn more about mules

Now that you know the definition, can you name any observations you’ve had with species?   For example, are the Chihuahua and the Great Dane  different species of dog?  Why or not why not?

To Protect and Serve

Do species have a function?  The main thing a species needs to do is to preserve itself.  Of course, to do so, it must produce offspring that are able to reproduce in sufficient numbers.  When you consider the definition of a species, why does this make sense?

To recall your original analogy, if many cells make up a tissue, what do many members of the same species make up?  The answer: a population.   

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