How many species 2: What is a species, and Why does it matter?


One of the questions asked in the comments of the previous post in this series is quite pointed, and very much on topic for this discussion, so I’m going to take a minute or two to answer it. I’ll give the answer to the example in another post, that will shortly follow this one. Karl asked:

So why are you asking that question? How is “species” defined. Does it really have a definition? Does it matter? Isn’t “species” just a modern reaction to the biblical term “kinds”

Now that I’ve taken a few minutes to think about it, I’m starting to remember why I was dodging that question. I could write a long, rambling discourse on the topic, but in all honesty the best I can do for a definition of “species” is to paraphrase Justice Stewart’s concurring opinion in Jacobellis v. Ohio: I might not ever be able to intelligently define the term, but I know it when I see it.

Read more (at The Questionable Authority):


The lecturers at the university I attended taught evolution usually in the context of a species being a reproductive entity in its own right. Obviously there are a whole series of riders. Horses and donkeys can cross, but mules are infertile, so horses and donkeys are distinct species. Chihuahuas and great danes presumably can’t cross in nature, but if human intervention can give them fertile offspring, they are the same species. I am no authority here, but what the lecturers said made sense. For people working with a microscope in genetics, one supposes the issue becomes clouded. The microscope has to be supplemented with observation in natural settings. I have been told - and I would value expert opinion on this - that sheep and goat DNA to date have not been shown to have any detectable difference. Observation proves them to be different, albeit very similar, species. With fruitflies or to a lesser extent, Darwin’s finches, difficulty of natural observation presumably means you fly by the seat of your pants to some extent. The geology lecturers emphasized this “Species Problem” because palaeontology is pre-occupied with it, for obvious reasons. It’s a bit difficult to get the fossils to get up and start breeding. Always the idea was to try for the theoretical ideal of the reproductive unit. So Darwin’s first (and fearsome) job with his isolated island finch aviary was to get them classified into reproductive units. One can understand his going on with his voyage and leaving that task to others. Has the task been completed yet? Someone may be able to enlighten me. Science advances by thourough research first, theorizing, second. I am not a biologist and I have great respect for biology’s achievements but one doesn’t need a post hole degree to see that those lecturers were on the right track when they defined a species ideally as a reproductively self-contained unit. Nature, zoological nomenclature, the man in the street, and, yes, the Bible, confirm this definition. Full-on Neo-Darwinism negates all these, and leads to the silly nonsense we had here in Australia not long ago where a misinformed academic got on the radio and talked along the lines of humans and chimp-like creatures getting into heavy dating, 5.1-odd mill. yrs ago. Even the reporter showed honest incredulty. When asked for evidence, he mumbled something about something that never was properly followed up, in Russian literature. Again, any takers on this topic? I have enough trouble with proper elocution in English.

I reckon “species” is somewhat less fuzzy than the “higher” taxonomic ranks (genus … kingdom). Two populations that regularly exchange genes would be in the same species, two populations that would very rarely (if at all) exchange genes (given the opportunity to do so) aren’t. Between those points on the scale it would appear to be somewhat subjective.


Re “those lecturers were on the right track when they defined a species ideally as a reproductively self-contained unit”

Except that regardless of what one uses as a definition, there are going to be borderline cases: during any speciation event, there’s bound to be a “before” period in which two groups are one species, and an “after” when they are different species.


”…there’s bound to be a “before” period in which two groups are one species, and an “after” when they are different species.”

This supposes that speciation occurs at one point in time. It doesn’t. Speciation is a process, not an event.

Re “This supposes that speciation occurs at one point in time. It doesn’t. Speciation is a process, not an event.”

Which is why I said that there are going to be borderline cases - there’s going to be cases that are fuzzy no matter what one uses as a definition.


I’m going to quote Brookfield :

The essence of the “species problem” is the fact that, while many different authorities gave very different ideas of what specices are, there is no set of experiments or observations that can be imagined that can resolve which of these views is the right one. This being so, the “species problem” is not a scientific problem at all, merely one about choosing and consistently applying a convention about how we use a word. So, we should settle on our favorite definition, use it, and det on with the science.

It was “get on with the science” of course, sorry for the typo.

re ‘get on with the science’.… sure, as a taxonomist i recognize the need for SOME sort of working definition. pragmatics aside, the question of whether or not our working definitions approximate ‘objective reality’ seems to be the rub for folks in this instance (the fruit fly example). i think it is even more interesting to question whether or not there IS an objective reality of ‘species’. the monist assertion is tantalizing but it’s rather much something like a zen koan… all things are different, all things are the same.

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This page contains a single entry by Mike Dunford published on May 30, 2006 3:16 PM.

The Nelson/Miller Saga Continues was the previous entry in this blog.

How many species 3: an answer, and some more questions. is the next entry in this blog.

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