Before we have shown how impossible it is to define species and draw borders between them. Now it is time to give detailed answers to some questions that our reader might have:
Q: I thought that species is a group of organisms capable of interbreeding. Hence the borders between the species can be defined between individuals that can no longer interbreed.
Unfortunately, it can’t be.
A test. I have collected a thousand of Lacerta agilis lizards from the wild. It’s gonna be the first generation of our breeding group (B1).
Now we apply selection to B1. Pick up 50 individuals with largest heads and narrowest hips. The remaining 950 we discard, and then let the selected 50 breed and produce the second generation of breeding group (B2).
With B2 we do something similar: select 50 most big-headed and wide-hipped and breed them to get B3. The remainder of B2, before discarding, we try to breed with wild, original Lacerta agilis individuals (NI). We succeed, which means that B2 is still Lacerta agilis.
Then, the same with B3: select and breed B4, the remainder – try with NI.
And so with B4, B5.
We get to B20 and see how different are they from NI – huge-headed and narrow-hipped. But they still interbreed with NI – they are still Lacerta agilis.
B99 – they are freaking big-headed! And yet they interbreed with NI.
We have witnessed the gradual change from Lacerta agilis into freakish big-headed and narrow-hipped L. agilis variety. And finally B100 failed to interbreed with NI. Success: we have bred a new species, let’s call it Lacerta unnislav to honour the author of the article
Ooops, there is a problem: what species are B1-B99 then? B99 looks like B100, and we could easily add it to Lacerta unnislav. But B99 interbreeds with NI, and adding it to Lacerta unnislav would make it Lacerta agilis. Perhaps, B1-B99 is a separate, intermediate species? But it interbreeds both with Lacerta unnislav and Lacerta agilis, and thus cannot be a species. The only way L. unnislav and L. agilis could be two species is if B1-B99 do not exist. Except they do. So what, just pretend that 99 generations of lizards never existed?
And also this: a hippo was born impotent. It interbreeds with no-one. Is it separate species then?
Q: That story of lizards was most entertaining, but I don’t think such a continuum could exist in real life.
Here’s an absolutely real, observable example: a continuum of arctic gulls from Larus genus. Their populations are spread from Europe to North America.
Seven populations are shown on the map. Populations 1 and 2 are considered L. fuscus species. They are virtually the same. Population 3, so called L. heuglini, is considered a separate species, but is very similar to L. fuscus, and the two interbreed with no limitations. Moving along: 4 and 5 are L. vegae. Not really different from L. heuglini, but quite different to L. fuscus. In other words, L. fuscus gradually flows into L. vegae through L. heuglini. Then L. vegae flows into L. smithsonianus(6), and finally, the last, seventh population – L. argentatus. All seven form a gradient, and any two neighbouring populations interbreed easily. Why they are separate species? Why 6 and 7 are different species, but 1 and 2 are the same? Because scientists wanted it that way, that’s why. It’s just names.
Since they interbreed easily, shouldn’t we make them a single species? Theoretically we could, but there is a problem. As you can see on a map, the sequence locks into the circle, and 7 “comes to meet” population 1. And they are extremely different from each other! 7 and 1 are obviously different species that do not interbreed. But there is a gradual flow between them, there, right in front of us. How do we define species in them with certainty? We don’t.
We call such gradients “ring species”. Not only gulls are ring species – there are other examples.
Q: Okay, there are exceptions to species. Those gulls, impotent hippos. It means nothing. Few exceptions do not disprove the rule.
The idea that “few exceptions do not disprove the rule” is acceptable only in daily business and various shamanic pseudosciences. In any true science that aims to describe the realities of the world an exception disproves the rule, instantly. The rule is either dumped or researched deeper/amended. If there is a single pair of real numbers, to which a+b = b+a rule does not apply, there won’t be any “single exception does not disprove the rule” talks – the rule fails.
To sum up, no organism can be unambiguously put into a species. Just like colours. Reddish orange is what: orange or red?
This is how the story ends…