Defining a species
A species is often defined as a group of individuals that actually or potentially interbreed in nature. In this sense, a species is the biggest gene pool possible under natural conditions.
For example, these happy face spiders look different, but since they can interbreed, they are considered the same species: Theridion grallator.
If two lineages of oak look quite different, but occasionally form hybrids with each other, should we count them as different species? There are lots of other places where the boundary of a species is blurred. It's not so surprising that these blurry places exist after all, the idea of a species is something that we humans invented for our own convenience!
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Happy face spider photos courtesy of Rosemary Gillespie, UC Berkeley; Streptococcus micrograph courtesy of Dr. Vincent A. Fischetti
Understanding Evolution © 2015 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California