Define "species"

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Chance Offline
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Define "species"

Posted: Mon Jan 09, 2012 11:45 am



The love discussion and one of TVO's comments about "cats and dogs being easily definable" made me think of a question I wanted to ask the forum.

Define species.

I would argue that this is not an easily definable term. We're discussing classification and diversity of life right now in my biology classes, and inevitably I'm going to need to give them a good working definition of species. If I look to my textbook, which is rather old and outdated, it says "a group of organisms that can interbreed and produce fertile offspring."

Other than the obvious problem with this definition (that there are many examples of fertile hybrids being produced) evolution presents its own problems with the species concept. All living things are, after all, a continuum of change and at no time stagnant. I'll probably define species to my students as something like "a group of organisms that share a direct phylogenetic lineage and tend to breed with each other to produce fertile offspring."

I'm definitely open to better definitions. Those of you who've recently taken biology courses or who are just interested in this, how would you define a species? Or should we even define critters in terms of individual species?


TVOham Offline
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Re: Define "species"

Posted: Mon Jan 09, 2012 1:48 pm



In one of the Bio courses I took this year a species was defined the same way you mentioned.

When we were talking about hybrids and hybrid zones it was said that hybrids are included in species. For instance, if finch A can reproduce with finch B than they are members of the same species.
If there are fundamental differences between A and B then they'd be referred to as a "sub-species".
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Re: Define "species"

Posted: Tue Jan 10, 2012 6:07 pm



What are the examples of fertile hybrids?
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TVOham Offline
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Re: Define "species"

Posted: Wed Jan 11, 2012 12:17 am



Bengal tigers and Siberian tigers?
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Chance Offline
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Re: Define "species"

Posted: Wed Jan 11, 2012 6:09 am



The examples I know are confined to the herpetological world, owing to my strange fascination with reptiles I suppose. North American Milksnakes and kingsnakes are commonly hybridized in captivity and produce fertile offspring, but they're in the same genus at least (Lampropeltis). However, there are examples of two completely different genera hybridizing and producing fertile offspring, like the cornsnakes (Pantherophis) and Kingsnakes. Or two species from two completely different parts of the world, like the ball pythons from Africa and the Woma pythons from Australia - though admittedly their fertility has yet to be determined.

When it comes down to it, I think a person would have a difficult time drawing a stark species separation line. And really that should make sense, owing to the fact that all species on Earth are derivations of a single common ancestor.


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Re: Define "species"

Posted: Wed Jan 25, 2012 12:31 am



I think you should first define it the way you describe in your first post, giving well-known examples to reinforce this definition. Then discuss how evolution creates a continuum of diversity that cannot be locked into simple man-made classifications. As such, there are notable exceptions. One might be able to conceive of a system with no exceptions, but it would be so hampered by details as to render it near-useless. So we accept an imperfect system of classification because it is usable and useful, though not perfect.


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Re: Define "species"

Posted: Wed Jan 25, 2012 2:29 pm



I don't know why, but the first thing that pops into my head whe I hear "Species" is either a non-vegetive life form, or Counterpoint.


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