Home > Biology, Uncategorized > What is a species?

What is a species?

Normally, an article about something will usually be intended to clear up the understanding of the topic. And so you might expect this article to clearly tell you what a species is. But this one does not do that, as you will see.

What do you think does a species mean? Go on. Try defining it. You might define it as a group of similar animals. But then, what does similar mean? Defining “similar” to be similar in appearance is very vague. Even within humans appearances vary throughout the world. If you say “similar” means “similar features” that too would be vague, since different species of animals might have similar features like different species of rats and mice.

You can try to be a bit more scientific, and say that there must be genetic similarity. But then again, even my brothers and I are not identical. So how similar should two individuals be genetically to belong to the same species. The fact that chimpanzee and humans share close to 95% of their genes makes it even more tough to define species.

Thinking about it a bit more, you might say, that a species is a group of animals that can interbreed. You cannot breed a tiger with a giraffe and expect a baby. Thus the ability to interbreed could be a good indicator of whether two individuals belong to the same species (Needless to mention that that the test of whether two males can be identified as being part of the same species, is whether each of them is capable of mating with the same female).

We seem to have made some progress, but then we are again met with examples of where a female horse and a male donkey can produce a mule thus violating our definition.


A picture of a mule (Source: Wikipedia)

(By the way, a male horse and a female donkey will produce a hinny) But donkeys and horses are separate species, as we know. We can then refine our definition a bit further to accommodate this violation. Mules are normally sterile. That is, two mules cannot mate and produce offspring. Thus if we define species as a group of animals which can produce offspring, that are in turn capable of reproduction, we will have accommodated such examples  too and we can all live happily for ever, right?

Wrong! I too was happy, till some time back, wrongly assuming that I know what a species means. The definition we have so far, is still widely used by many people since it has a lot of practical value. But this definition is not complete. Do you see what is wrong with the definition?

Now I come to the real intent of this post. That is to explain to you why the definition of “Species” is not clear. There are 3 reasons why we cannot define species in a clear cut manner.

1. Our whole definition depends on organisms being capable of sex. But then, many organisms reproduce asexually, especially single celled organisms, and some multi celled organisms. Our definition completely ignores these groups of organisms. Defining species in asexually reproducing organisms is tough, because in such cases we dont need two individuals to reproduce. We might have to fall back on definitions based on the word “similar” or to definitions based on genetics, both of which have problems as we saw before. This is one important stumbling block in defining species.

2. The second reason is that of unpredictability of hybrids. In the example of a mule that we saw earlier, I had mentioned that most mules are sterile. But not all of them are. Some indeed reproduce. The reproductive ability of hybrids too is variable. Thus we cannot definitely say that mules are always sterile. Which means that, as per our definition, horses and donkeys shoud be one species. And, that would also lead to mules being a separate species.

3. The third reason and the most interesting is this. As discussed in an earlier post on evolution, the way species evolve is by gradual genetic change of the organism. One species evolves from a different species over many generations. Every parent and its immediate child all along the evolutionary path of many generations, belong to the same species. But the beginning and end points, that is the first ancestor and the last child, clearly(?) belong to different species. Thus it is impossible to draw the line where one species became another species, and thus tough to decide who belonged to which species. If you say that it does not matter, since most of our ancestors are dead by now, let me point you to a similar living example, which had me stunned when I encountered it the first time.

I am referring to a phenomenon called “Ring Species“. It is best explained with an example. There are a group of birds called Larus gulls found all around the world. But each area has different varieties. Such varieties also exist around the arctic. But here, this varieties exist as if in a circle. That is, their geographies form a circular ring. They form a ring also in the sense that each variety, can interbreed with its immediately neighbouring varieties. Let us quickly see an illustration below.

Illustration of a Ring Species

In the illustration above, imagine each letter represents different “varieties” of Larus gulls. Before moving on, I must make it clear, that though the picture suggests otherwise, there normally will not be such clear cut demarcation of geographies of populations. There will always be areas of overlaps at the borders. What is strange about ring species is that, A can interbreed with B and E, its neighbours. B can do so with A and C, C with D and B and so on. But A cannot breed with C or D and similarly B cannot with D and E and C cannot with A and E. Thus it baffles us since we don’t know whether to classify the whole group as a species (since they can interbreed), or to classify each type as a species (since some types cannot interbreed with others). Just think about that a little bit. It challenges all our notions of a clear definition of species.

Thus we saw three reasons why species definition is very tough. If at this stage, you are confused as to what a species is, then I am happy, because that is what I set out to achieve. The definition of a species is fuzzy, arbitrary and the problem is not yet solved.

Trying to think of why it is so hard to define, I felt that this could be a problem that probably has no solution. It is like trying to define exactly terms like adult, child. Similar to trying to define country boundaries. The boundaries can only be artificial. There cannot be anything intrinsically different between this side and that near a national boundary (or for that matter even state boundaries). The lines are always fuzzy. Like how, we see a lot of Telugu and Tamil speaking people near the Tamil Nadu – Andhra Pradesh border. By just looking at them, you cannot say where Tamil Nadu ends and Andhra Pradesh starts. So is the case with Species too.

I am not asking you to stop using the word Species, any more than I would ask you to stop using country names, but just keep the complexities of the definition at the back of your mind when using the word. After all, that defining a species is hard, has in no way prevented discovery of new ones.

Some new species discovered in 2010, have been discussed here. Some pictures of more species can be seen at this National Geographic page. Just look at the incredible variety.

Notice the picture of the transparent frog in the NG page which is described thus:  “This so-called glass frog’s transparent body lacks pigmentation and reveals its organs in action—including a beating heart“. It will leave you with a glow on your face for the rest of the day.

ion below.
  1. Manikandan
    December 27, 2010 at 12:17

    I thought we would take all the factors into consideration. A good discussion though

  2. Madhav
    December 27, 2010 at 12:18

    But I think even taking into account all the factors into consideration, there is no clear cut way of identifying whether a particular bacterium belongs to this species or that. I think this also leads to the question of what an individual is. Why do consider each human body an individual. What about the numerous cells in our body? Each one could be an individual by itself 🙂

  3. Manikandan
    December 27, 2010 at 12:19

    Its about functionality, I think. Its like the difference between a society and an individual. Each cell can survive in the most primitive way but to perform complex tasks it should specialize which inevitably makes it dependent on other specialized cells, thus evolving into organ systems. Which brings us to the role of reproductive system: Though the single bacterial cell can reproduce the “whole bacteria” all by itself, none of the other nonspecialised cells of higher species ordinarily can perform the reproductive function. Thus they depend on the specialized “gametes” of the reproductive organs. So where does individuality start? “I am because I think” or “ I think because I am”?

  1. April 3, 2012 at 14:04

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