The diversity of flower varieties

Where species fail

In the previous article I mentioned that all existing species theories are in fact fictional. But why exactly won’t they work? Where do they fail?

To start with, there are several methods of systematics and species division. Some define species within the biosphere based on DNA, some – based on anatomy. I’m about to crush-test each of those methods.

1. Reproduction theory

According to that one, a species is a multitude of organisms capable of interbreeding and giving viable offspring.

Although I don’t understand, how would you tell the species of infertile individual? How do I tell if two males of some beetle are one species? They obviously do not interbreed, and I couldn’t find any females. The biggest problem, though, are non-sexual lifeforms. Bacteria, for example. The notion of interbreeding does not apply to them at all – what to do in this case?

2. Morphophysiological theory

As its name states, this theory implies that organisms of the same species are those most similar morphologically and physiologically.
This one is actually the first method people has ever used: they could see that two deer look alike, and included them in the same species.

But a paradox: four-winged fruit-fly, born to ordinary one, is still a fruit-fly. There are also two-headed snakes. According to definition, it’s not even a vertebrate, let alone reptile (a vertebrate is an animal that has one head). And yet, it is considered a snake. Meaning that we can occasionally abuse the definitions when our gut tells us so. But how far can it get? Who’s to decide when a definition is a rule, and when it’s a mere formality. Fundamental science becomes philosophical…

From Wikipedia.

3. The theory of inherited species

According to that one, the species status is passed “from parents to children”. Just like a surname. An animal born to a lion is a lion, and no-one cares what it looks like. It could be albino, could be legless – it’s still a lion, with some pathology though. That’s why a two-headed snake is still a snake.

Two problems. First: we all are descendants to a paleozoic fish.

Взято с Википедии.

From Wikipedia.

Second: most of the time we don’t even know the family tree of an animal in question. Here is a deer walking. How do you know it was born to deer not to sheep? You haven’t witnessed its birth. You could assume (based on morphology, which is another theory), but what if you’re wrong? What if it was born from a tube in a lab? You have no idea, and yet, you’re supposed to put it into a species…

4. Genetic theory

AKA putting organisms into species based on DNA similarity.

Problem uno: DNA is not like my birth date, one for life. It’s a variable structure. Organisms can change their DNA, and even exchange it with other organisms. Some parasites really love to do so, e.g., a plant called Rafflesia arnoldii, which steals DNA from its host – another plant called Tetrastigma rafflesiae – and builds it inside its own genome. Essentially, a plant is born one species, but could easily end… another…
Problem duo is simpler: we know nothing about the DNA of most species. Pick some random frog species, for example – no-one has ever sequenced its genome.

For conclusion, there is a problem that applies to all of the theories. Assume some of the theories work, say, the genetic theory. Where the exact species border would be? How much common DNA should two frogs have to be considered one species? 99,5%? Why not 99,5000000001? What about 99,49999999999999? The same with reproduction, or with morphology. You could choose your criteria, but you will never find the exact border where one species ends and another starts.

A living example – the Larus gulls, mentioned in previous article.

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Further reads

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

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Stephen Maxwell

Species are hypotheses of causal relationships, they seek to answer why questions.

See Kirk Fitzhugh (2006) “The inferential basis of species hypotheses: the solution to
defining the term ‘species’ “