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Sample Discussion Thread
This discussion was selected from the AMNH online course Evolution. This is an excerpt from an actual course discussion, but learner names have been changed. Evolution is part of Seminars on Science, a program of online graduate-level professional development courses for K-12 educators.

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Week 4 Discussion: What is a Species?

The definition of what constitutes a species is a topic of hot debate in evolutionary biology. This week you've read about two different definitions of a species: The Biological Species Concept (BSC) and the Phylogenetic Species Concept (PSC). What are the differences between the two concepts? Discuss some of the implications for research and conservation. Based on what you understand about how species form, why do you think scientists studying different aspects of evolution would prefer different definitions of what a species is? Please give examples.


Sarah (initial post) 27 September 11:27PM
In Dr. Zink’s essay, he describes both the Biological and Phylogenetic Species Concepts. Under the Biological Species Concept, a species is defined as populations of organisms that can successfully reproduce. Under the Phylogenetic Species Concept, a species is defined instead as a group of individuals of similar evolutionary history and distinct features. The Endangered Species Act aims to protect not only species, but also subspecies and “distinct population segments” (Zink). Because the two species concepts define each of these terms slightly differently, scientists conforming to different theories tend to disagree on ways to approach conservation. Zink, who supports the Phylogenetic theory of evolution feels like Biological speciation often results in fewer total species but greater diversity within the species. This, he says, can result in a rather bloated estimation of the number of organisms within a species and thus delayed conservation efforts. A scientist who supports the theory of Biological Speciation would hold that conservation of a species should begin if population numbers fall to the endangered level, however scientists like Zink believe that closer attention should be paid to protecting the distinct population groups within groups defined as species under the Biological Species Theory.

The example of the California Gnatcatcher illustrates how scientists interested in different elements of evolution prefer certain definitions of species. Scientists interested in tracing the evolution of genetic material will look at the DNA sequences among various populations of Gnatcatchers and upon finding out that there is no significant genetic variation between the populations would prefer to define all Gnatcatchers as a single species. However, scientists interested in morphology, habitat and geographic distribution would likely be interested in categorizing the gnatcatchers on features like the climate they live in, the color of their coat, and their size.

Instructor: Jody (in response to Sarah) 28 September 9:42AM
Sarah, after completing all the readings and looking into the two species concepts in detail - what do you think? Do you think one definition or the other has more merit? If scientists are not unified behind the BSC, what do you think of its central role in school textbooks?

You make a great point about numbers. As soon as we narrow the definition of species, then we have fewer animals within each species. Should protection be all about numbers? Some people have suggested that conservation efforts should be focused on preserving whole habitats and ecosystems rather than boosting the numbers of specific species. Do you think this is a better way to go?

Maureen (in response to Jody) 28 Sept 9:06PM
I think the preservation of habitats and ecosystems is a plan that protects a greater number of species and promotes continued biodiversity by insulating the areas with the highest potential for speciation. The “lost world” in Papua, New Guinea is just one example of how a “protected” habitat can give rise to a host of new species. The earth’s rain forests are home to so many yet undiscovered species, not to mention those that we know to be in existence, and their importance is only beginning to be understood (i.e. medical treatments, species interactions). By protecting these ecosystems, I think we establish the means for a greater conservation impact overall. I don’t think this plan should be instead of, but in addition to, conservation efforts to preserve dangerously low populations.

Sarah (in response to Maureen) 4 Oct 11:29PM
Hi Maureen,
You make a good point about undiscovered species and how protecting habitats can unintentionally protect organisms that are yet to be discovered. I agree with you about protecting whole habitats and ecosystems. While conservation efforts should still be focused on protecting endangered species, they should also expand beyond species protection to ecosystem protection.

Melissa (in response to Jody) 29 Sept 11:28AM
"As soon as we narrow the definition of species, then we have fewer animals within each species. Should protection be all about numbers? Some people have suggested that conservation efforts should be focused on preserving whole habitats and ecosystems rather than boosting the numbers of specific species. Do you think this is a better way to go?"

If I might jump in, I think that conservation should certainly be focused on numbers, but not necessarily the number of total species. I think that, if push comes to shove, there are two considerations that should take priority over preserving just the number of species: genetic diversity and ecological significance, two measures that are necessary for biota to recover from the stresses that have driven them to this point. Ecological significance is relatively separate from the BSC vs PSC discussion, especially since it is usually discussed on the population level. Genetic diversity, however, is tightly linked to the debate. While comparing the genetic diversity of populations is relatively straightforward, if we're making species vs. species decisions, these two definitions might encompass groups with very different degrees of diversity.

Scientist: Andrea (in response to Melissa) 1 Oct 8:34AM
It does seem strange that biodiversity should be defined by number of taxa, but that is how it has been done traditionally in natural history and paleobiology. This is why the ecological definition of biodiversity, essentially a measure of disparity or niche breadth, is being taken up instead in conservation.

Kathy (in response to Jody) 30 Sept 11:41AM
This is a very interesting debate and I look forward to reading other's responses. My first thought is to think about the degree of influence humans have had over the species becoming endangered. Now this is obviously very hard to measure and I'd imagine that humans are in some degree responsible for all endangered animals, but animals become extinct for a variety of reasons. I would argue that scientists should try to conserve those populations that are becoming endangered as a result of direct human influence, i.e. chopping down forests to build houses.

Sarah (original poster, in response to Jody) 4 Oct 11:PM
Hi Jody,
Based on the information presented in the readings, I feel that the Biological Species Concept has more merit. The reason for this is that the BSC specifically defines a species as a group that can produce fertile offspring whereas the PSC leaves much room for interpretation as to what a defines a species. Simply defining a species as a group that has at least one unique feature does not seem stringent or specific enough to yield consistent results. Under the PSC, I feel that someone could define a group of cats with thicker coats as a different species than a group of cats with thinner coats even though the groups could be genetically similar but experience different levels of shedding due to environmental conditions. This, to me, does not seem like a very reliable method of defining species. However, I realize that everyone is entitled to their own opinion and because scientists are not uniformly behind the BSC, I feel that school text books should objectively present both theories and the individual students can decide for themselves which they accept.

To answer your question about conservation and numbers, I do not believe that conservation should solely focus on the numbers of organisms in a species. Instead, I think that efforts should constantly be made to preserve ecosystems. This would involve careful control of predator prey populations, restrictions placed on introducing non-native species into a habitat, and reduction of human impact on natural habitats. I do think that conservation efforts should occur in response to endangered species, but saving individual groups of organisms should not be the sole priority. Without healthy ecosystems, more and more species will become endangered as predator prey relations change and food webs are altered. Thus, by protecting entire habitats and ecosystems rather than simply focusing on species numbers, conservation efforts can be much more efficient.

I do agree that the PSC is more useful when trying to classify extinct organisms where only fossil evidence remains. In this case, one may not know if two organisms can interbreed successfully, and thus may need to use morphological data to draw conclusions. However, I also feel that defining species as groups that have a given genetic variation from other groups would be more effective than either BSC or PSC. Not only is genetic information available in fossilized organisms, but defining a specific degree of genetic difference required for two species eliminates subjectivity.

Mike (in response to Jody) 4 Oct 11:35PM
It is rather common, and I think appropriate, to teach simplified and/or older versions of a scientific idea, confident that we will clarify and elaborate later. Examples: We teach Newtons' Laws before clarifying the physics with quantum theory for the very small realm, special relativity for the very fast realm, and general relativity for the very massive realm. I teach the classical five kingdom model (always dropping the 'g' of course! -- no sexist language in my classroom) before the three domain model. The Bohr model is a great place to begin learning quantum theory, but it is not correct.

After this week's lesson I am quite convinced that the PSC has more general validity. However, I think I will initially define species with the BSC in my middle school classrooms. The difference is, I will return to the topic and introduce the PSC.


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