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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 isn't everything
Organisms may appear to be alike and be different species. For example, Western meadowlarks (Sturnella neglecta) and Eastern meadowlarks (Sturnella magna) look almost identical to one another, yet do not interbreed with each other — thus, they are separate species according to this definition.
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| The Western meadowlark (left) and the Eastern meadowlark (right) appear to be identical, and their ranges overlap, but their distinct songs prevent interbreeding. |
Organisms may look different and yet be the same species. For example, look at these ants. You might think that they are distantly related species. In fact, they are sisters—two ants of the species Pheidole barbata, fulfilling different roles in the same colony.
Many characteristics can vary within a single species. For example,
the plant hydrangea may have pink
"flowers" — they're actually modified leaves — or
blue "flowers." But
that doesn't 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 not — but how distantly separated do they
have to be before we draw the line?
- Ring species are species with a geographic distribution
that forms a ring and overlaps at the ends. The many subspecies
of Ensatina salamanders in California exhibit subtle morphological
and genetic differences all along their range. They all interbreed
with their immediate neighbors with one exception: where the
extreme ends of the range overlap in Southern California, E. klauberi
and E. eschscholtzii do not interbreed. So where do we mark the point
of speciation?
- Chronospecies are different stages in the same evolving lineage that
existed at different points in time. Obviously, chronospecies present a problem
for the biological species concept — for example, it is not really
possible (or very meaningful!) to figure out whether a trilobite living 300 million
years ago would have interbred with its ancestor living 310 million years ago.
This trilobite lineage below evolved gradually over time:
Should we consider trilobite A as a separate species
from trilobite D, and if so, where should we divide the lineage into separate species?
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