Ask Me Help Desk - What is a species?

What is a species? What makes a species the thing that it is?

Is species-talk just a way of grouping individuals that have certain specifiable properties in common? If so, which properties are relevant for the purposes of classification and how is that decided? If not, then what over-and-above a classificatory device is a species?

Suppose that only one organism ever existed. Would there still be a species, i.e. a set with only one member? If so, would the species and the individual be one thing or two?

I'm trying to figure out both what makes a species the thing that it is and whether species have an existence distinct from the organisms that belong to them. If a species is something other than a taxonomic device, what is it and how do I know one when I see one? Do I see "species" every time I see an organism?

The species Ursus arctos, or brown bear, consists of all the bears that are not other species of bears. They share common descent, like the members of your family. They share some distinctive alleles (gene variants) or combinations of alleles that set them apart from other bears. They share some distinctive physical and behavioral attributes and, likely, they share an ecological niche that is different from that of other bears. They may eat different food or breed at a different time of year.

In general, the most important unit in evolution and ecology is the population. A species may consist of a single population or of multiple populations. A group of populations my individually go extinct, allowing their particular geographical area to be recolonized by individuals from another nearby population. Although this may result in minor changes in gene frequency, the species remains intact. Even if some individuals die, and even if some populations die out, the species continues to exist.

In the same way, even if a few of our brain cells die or even if we lose an arm or leg, we are still individuals. Even if someone receives a hand transplant, he is still the same person. I think it's fair to say that in evolutionary terms, a species is comparable to an individual.

All of the individuals in a population share a series of characteristics and the potential ability of interbreed with one another. The same is true of the species generally. Occasionally, individuals can successfully breed with individuals from other populations that would normally be considered a separate species based on genetic or ecological characters.

What happens to such progeny is instructive. If they are fertile, either with themselves or with individuals from either parent species, and if the environment allows it, such hybrids may thrive. But if any one of these things is not true, they will die out quickly, reinforcing the boundaries between the two species even though they don't seem to have sharp reproductive boundaries. Generally, a harsh environment causes "stabilizing selection" around a phenotype that is functional if not perfect. In times of plenty, variants may arise. In harsh times, the variants (on the edges) and hybrids (between two species) are more prone to dying out. These are generalizations only. There will be exceptions.

When species are in the process of splitting into two or more separate species of if a hybrid is arising, these distinctions become difficult. But there are likewise situations where it is unclear how to define individuals.

Consider Embryologist Scott Gilbert's comments on twinning in humans.
"In humans, identical twinning can occur as late as day 12 [after conception]. Such twinning produces two individuals with different lives. Even conjoined ("Siamese") twins can have different personalities. Thus, a single individuality is not fixed earlier than day 12."
Developmental Biology 8e Online: Summary

Would we consider a partly split embryo two people or one? Is it truly one individual on day 11 and two on day 12?

I don't think that a single individual type could ever be considered a species by any definition--genetic, ecological, or reproductive (the biological species concept, or BSC). It would be considered an individual with major birth defects.

Even though I am discussing species in very abstract terms, as entities, I think they are quite concrete and real. Yes, they have fuzzy boundaries. But as you have pointed out elsewhere, most entities have fuzzy boundaries at some level--whether atomic, molecular, or cellular. Having fuzzy boundaries doesn't make a dust bunny an abstraction.

Individuals are made of cooperating cells, whose constituent molecules are typically entirely replaced during the lifetime of the individual. In addition, we harbor hundreds of commensal and symbiotic bacteria and fungal species whose combined numbers vastly outnumber our own cells. As individuals, we are cooperatives and communities.

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Originally Posted by Akoue

I'm trying to figure out both what makes a species the thing that it is and whether species have an existence distinct from the organisms that belong to them.

Of course. Just as you have an existence separate from that of your individual cells.
Edit: I could take a sample of your cells and grow them in a dish--for longer than you are likely to live--but most people would not consider that cell culture to be you in any sense.

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If a species is something other than a taxonomic device, what is it and how do I know one when I see one?

Because its constituent parts are dispersed does not make it unreal. The atmosphere is real even though you cannot see all of it at once, indeed often cannot see it at all.

On the other hand, you could certainly put all the southern elephant seals in the world in one place and then you could see them all.

Or you could spread out the cells of a single person flat so you could see them all, but then the person wouldn't be a person anymore. Seeing something as whole entity is not the only measure of its concreteness. Is a river real? Or do you consider it just an abstraction? If you cannot see all the water at once is it not as real as a lake?

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Do I see "species" every time I see an organism?

No. You see individuals.

I think I'm a bit confused. Is the species-individual a type-token relation or a part-whole relation? I thought it was a type-token relation.

The relation in which an organism stands to the cells that compose it is the relation of whole to part. An organism is a composite of parts.

But I thought that the relation in which the species canis lupus stands to individual wolves is not part-whole but type-token: "canis lupis" is the name for a type of organism and particular wolves are the tokens that fall under that type.

Am I misunderstanding?

I've been arguing against typology all along.
So, if I understand your terms, definitely part-whole.
Except "whole" could be misleading. Part-whole in the sense of water molecules in a lake, maybe. I haven't thought this through.

EDIT: What I mean is that the parts (individuals) make a whole (species). But if you take away a few parts (individuals), it is still a whole (Species). In that sense "parts-whole" could be confusing.

I don't think you are misunderstanding so much as experiencing a minor paradigm shift. Or at least I hope so!

I'm trying to steer you away from a type token understanding of species. At least if I understand the term "type-token" which I've never heard before.

By type-token I was thinking something like the following:

Man=type.
Ralph=token; Harry=token; Humbert=token.

Each of the individuals is a token of the type, man.

By part-whole I was thinking something like this:

Body=whole.
Arm=part; lung=part; kidney=part.

Each of the parts, together, constitutes the body. Take away enough of the parts and the whole ceases to exist.

Now, looked at in one way, the tokens of a type stand to it as parts to a whole. The tokens elm, beech, and oak are tokens of the type tree; and they also make up part of the extension of the concept 'tree'. By extension of the concept I mean that set of things to which the concept can be applied. So the concept 'tree' can be applied to elms, and beeches, and oaks. The concept 'elm' can be applied to specific, i.e. individual trees; the concept 'beech' can be applied to individual trees; the concept 'oak' can be applied to individual trees. And so on. In other words, the concept 'tree' is higher than the concepts 'elm', 'beech', and 'oak' because it is a more general concept. (I can, after all, apply the concept 'tree' to an 'oak' without recognizing it as an oak.

In that sense, in the sense that pertains to the extensions of concepts, I can see how species can stand to things falling under it in the relation of whole to part. A particular wolf is part of the set of all the individuals to which the concept 'canis lupus' can be applied.

But I wouldn't have thought that makes the species canis lupus anything other than a label, a general term or concept that is used to classify individuals on the strength of resemblance relations. In other words, I would have thought that the concept 'canis lupus' provides a criterion by which to differentiate wolves from non-wolves. That criterion may evolve along with our understanding of wolf biology, etc. but I would have thought that the species is not an organic whole itself (in the way that, say, a body or a tree is), but is rather a type standing to its instances in the way I adumbrated above.

Now I am wide open for a paradigm shift. But I'm not there yet, which is to say that I'm still not getting how the relation of species (canis lupus) to individuals (particular wolves) is not type-token.

I hope I at least made my question--and perhaps the nature of my confusion--more clear. If not, say so and I'll try again.

Thanks for the clarification on type-token, part-whole. That's what I thought.

To be honest, I would allow that species can be both. As far as I can see, there's nothing in what you've said that precludes that. But the token-type relation will not lead you to think clearly about biology. It will lead you astray every time, in my opinion.

I try to eradicate it from my thinking. In the important biological sense, a species is a physical entity and I would urge you to think of it that way.

Further, like an individual, it has an existence that extends through time as well as space. And yes the beginning and end are fuzzy, just as it's difficult to define the exact beginning and end of an individual life. Of which more if you ask.

There are also things called superorganisms, who are made up of individuals and it's not clear if they are individuals or associations of individuals. I say this to drive home the point, which I've been belaboring as a side issue, that individuals themselves are extraordinarily fuzzy entities, but it doesn't stop them from being actual. And I would say the same for species. Species are not mere abstract classes of objects. They are functioning units of life, without a doubt.

Cool!

I have to think about this a bit. I may have more questions anon. But, for the moment, it makes sense to me. (I'm keeping my fingers crossed in hopes that that doesn't disappear overnight.)