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Biological Species Concept
The biological species concept defines a species as members of populations that actually or potentially interbreed in nature, not according to similarity of appearance. Although appearance is helpful in identifying species, it does not define species.
Appearance isnt everything
Many characteristics can vary within a single species. For example, the plant hydrangea may have pink flowerstheyre actually modified leavesor blue flowers. But that doesnt mean that we should classify the two forms as different species. In fact, you could cause a blue-flowered plant to become a pink-flowered plant just by changing the pH of the soil and the amount of aluminum taken up by the plant.
Adding to the problem
We already pointed out two of the difficulties with the biological species concept: what do you do with asexual organisms, and what do you do with organisms that occasionally form hybrids with one another? Other difficulties include:
What is meant by potentially interbreeding? If a population of frogs were divided by a freeway, as shown below, that prevented the two groups of frogs from interbreeding, should we designate them as separate species? Probably notbut how distantly separated do they have to be before we draw the line?
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Spanish translation of Understanding Evolution For Teachers from the Spanish Society of Evolutionary Biology.