The divesity of butterflies

Species diversity

Long before biology was established as a science people have segregated biodiversity into “species”, “kinds”, “varieties”, “races” – whatever. The idea that there are natural groups within biodiversity unified by common traits has developed by itself. People actually wanted such groups:

  1. It’s functional. If there are kinds of “bears” and “deers”, it’s easier to coordinate hunting, make food, maintain security of the community.
  2. It’s dictated by the way people think. The need for systematics comes from our heads, from our language.

Ever since biology was established, scientists strive to find the “absolute” groups: alter the system, study genes, anatomy, fossils. What are they striving for anyway? Are they gonna come somewhere? Are there the absolute groups?

Normally, the scientists try to systematize the factual biodiversity – those animals that are actually around us. Plotted factual biodiversity looks like this:

Factual dots

Now I explain how this plot was made. We captured a heap of different frogs from different places, then picked up two characteristics. Say, body mass and head width. And measured both in every frog. Then drew a graph, horizontal axis attached to body mass, vertical – to head width. Now for every frog measured there is a point on a graph. The points at the top right corner correspond to the heaviest and most big-headed individuals. Obviously, two characteristics are far from enough to encapsulate the diversity of frogs in its entirety. They have different colours, leg lengths etc. But we could have taken 20 chars. Or a million. And build a multi-dimensional graph (would be hard on flat paper, though). We could take any chars at all: length and thickness of every bone, skin colour, percentage of crickets in diet, longitude and latitude of the habitat. If necessary, we could pick the colour from every square millimeter of skin. And such multimillion-dimensional graph would have described all frogs in the world. For simplicity only, we stick to 2D graph in this article.

Group segregation seems natural in this case. There is a group of big-headed heavy froggies, then there’s a different group of lesser-headed frogs. And there is a gap between the two. Since the area of gaps is much larger that the area occupied by points, we can group our frogs – piece of cake:

Factual species

Nine species of frogs – not bad. Notice however, how uneven our system is:

  1. All species are of different size, and the gap between two species varies from large to almost nothing. Our system is not uniform, essentially, we use different systematic criteria to define different species. In one case, a small difference in skull size is enough to separate species, in other case – it looks more like subspecies. What kind of system is it then, that cannot even agree on its own criteria?
  2. Some species are connected. Like the two to the top left. How do we know some point near a border should not belong to another species?
  3. Some species have inside structure – at the central bottom, for example. There are five “subgroups”, can you see them? How could we be certain that that’s a single species with subvarieties, not five similar species? What if all points on the graph are actually a single fragmented species?
  4. Most important: what if tomorrow I find a frog that is outside of any of those species?

I have defined those species based on my individual perception and mood. My gut told me that those red borders should be like that, not a little wider and rounder. There is no objective or absolute way to define species. I can make up some criteria, but featured phrase – “make up.” Let alone that if history happened a little different, the points on the plot themselves would be at slightly different places. That’s the truth of the species.

Now let’s consider extinct animals as well. It gets even harder:

Extended factual dots

Blue points are extinct individuals, red ones are present. This plot can actually tell us the whole story:
Originally there was a population of rather small, but big-headed frogs – they occupy the area close to the top left corner. Those were specialized on eating large locusts, that’s why they had such a powerful skull. At some point, a virus has exterminated that locust species (not instantly, in hundreds of years). The remaining prey were even larger locusts, but also small crickets. The evolution of the original population then went in two directions: some of the frogs adapted to bring down gigantic locusts, which required that they increase their body size; some of the frogs switched to little crickets and large scull was lost over time.

If we ignore extinct frogs, we’ll easily systematize present individuals into two species: one species is large, big-headed frog that hunts down large prey; another species is small frog feeding on small prey. But if we take extinct individuals into account, our system miserably fails:

– How many species? Two? Three? Or perhaps, it’s all a single species? Also, can you see a “dead-end” species that branched at some point to the bottom-right? Perhaps there are four species then: 2 extinct and 2 present?
– Okay, we have decided on the qty of species. Where is the border between each of them then? There are no gaps, but the border must be at specific place, mustn’t it: we can’t have no-name species or animals of several species at the same time.

My six attempts:

Cleave extended into species

So what exactly systematicians are fighting for?

Let’s make things more difficult for them

May I remind you that so far we have concentrated on factual biodiversity – the organisms that we observe around us, which is nothing compared to the qty of potential organisms. Now to fundamental biodiversity, which includes all possible organisms. If we plot them, we get this:


That grey area is the infinite amount of points that gradually flow into other points in any direction. Points are everywhere, there are no gaps, infinite intermediate forms! How many species can you define here? Just one, or a billion?

What is fundamental biodiversity?

Q: The organisms to fill all those gaps do not actually exist. There are no living intermediates between a lion and a tiger. Why do you make stuff up?

On entire Earth there could be no stone that weights exactly 530.573923583 kilos, but it doesn’t mean that such a rock cannot physically exist.

Even if no person in  history has ever pronounced the number 8002474224.0192 aloud, it doesn’t mean it’s unpronounceable. You could try right now.

Same – with organisms.

That’s the magic of fundamental sciences: they describe even the stuff that no person ever has observed. Throughout the history people has solved a limited number of square equations, while there infinite equations possible. And yet, the method to solve them works even for those that were never actually tried.

Keep in mind

The systematic that is currently used in science is based on type specimen – there is a separate article about it. The concept is artificial, as I have explained, but it doesn’t mean that you should not use it. Species do not exist, there are no borders between them. Nature doesn’t care how you call its creations. But people do care – they have to call things somehow, that’s what systematics were invented for.

“The concept of a species is a concession to our linguistic habits and neurological mechanisms” (с) Haldane, 1956.


Further reads

Previous article: Non-organism Diversity
Next article: FAQ on Species Diversity
List of all articles about biodiversity


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