How do we decide what counts as a new species and what’s just a variation within a species?

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I grew up hearing that the main indicator of a species was that they only reproduced with one another. But Neanderthals and Humans cross bred. And they’re separate species. And in captivity Lions and Tigers can breed, but they’re obviously different species. Like I guess I’m just confused where the line gets drawn when deciding what is and isn’t a different species.

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Anonymous 0 Comments

“Breeding” is only part of it. “Breeding to create viable, fertile offspring that can reproduce with individuals similar to either parent or themselves” is a more accurate requirement.

Lions and tigers can produce offspring (known as ligers). But if they were the same species, a male liger would be able to mate with either a female lion, tiger, or liger to produce an offspring; and a female liger would be able to mate with either a male lion, tiger, or liger and birth a viable offspring. This isn’t true. The only viable offspring of ligers arise from mating a female liger with a male lion, and even those are barely viable.

This is the same with a donkey and a horse. While a jack and a mare can mate and produce a mule, mules have an odd number of chromosomes and are sterile. Mules cannot mate and produce offspring with either other mules, donkeys, or horses.

Compared to wolves and dogs, wolves and dogs are the same species (*Canis lupus*). A wolf can mate with a dog and produce offspring that are viable and fertile, being able to mate and produce viable offspring with wolves, dogs, or half-wolves. In humans, this is the level of difference between the races — in the same vein, designations such as “sub-Saharan African” or “Scandinavian” could be considered “breeds” of humans (this is typically considered racist so it’s bad to argue this, but that’s a social issue rather than a scientific one). We actually use words like those in breeds of dogs, such as “German shepherd”, “Siberian husky”, or “Rhodesian ridgeback”, named for the places they come from (Rhodesia is the colonial-era name for modern-day Zimbabwe)

In the world of fantasy, the rules of this change between universes. Most people have heard of a “half-elf”, the offspring of a union between a human and an elf. Typically these unions are human male/elf female, but in most universes it works the other way around too — the trend towards this is social and not biological, much like how most white/Asian pairings are white male/Asian female. Many universes don’t delve into whether this makes humans and elves the same species, but it can often be discerned using these rules. If the universe has any characters in the canon who are descended from half-elves, this typically means they’re the same species in that universe. If there are no named descendants of half-elves, that probably means they are same-genus and that half-elves are sterile.

Anonymous 0 Comments

The easy answer is, that there is no real answer. There are numerous definitions for the concept of species, some are exclusive and some are inclusive and they all depend on which trait you personally believe is most important.

If you take Genetics as a basis, you might have two populations which have striking differences in their genome, but look exactly the same and if you take Morphology as a basis, you might find two populations looking completely different but are genetically nearly identical.

Even the often used definition of „being able to produce fertile offsprings“ can be tricky. If two populations can produce fertile offspring but it will never naturally occur, due to e.g. different mating behavior or geological barriers, are they already different species?

Anonymous 0 Comments

In class, I teach the definition of a species is a population of organisms that can breed AND produce fertile offspring.

There are two statements here:

1) must be able to breed with each other
2) the babies must be fertile and able to have their own. In your example of the Liger…they’re infertile like mules and other hybrid animals. There are the occasional one-offs, but in general they’ll be sterile.

Anonymous 0 Comments

What is a blueish green and what is a greenish blue.

We set an answer because we have to, but its all on us

Anonymous 0 Comments

I’ve witnessed heated arguments on social media between biology professors arguing over whether to define something as a species or a subspecies. Even the experts don’t have an agreed definition for much of the taxonomy.

Anonymous 0 Comments

Of the two examples you mentioned, yes, they can interbreed, but the offspring are generally infertile, though not always.

Anonymous 0 Comments

One extra thing, just to confuse matters more. Biologists can change their minds and reclassify a creature as a different species. For example, you mentioned Neanderthals and modern humans being different species? At some point in the last twenty years, they changed things up. *Homo neanderthalensis* is now *Homo sapiens subspecies neanderthalensis*. Our cavemen relatives are part of the family now.

Anonymous 0 Comments

There really is no hard line. Because evolution takes such tiny steps normally, the changes between generations makes more of a spectrum than a series of distinct steps. With the advent of genetic mapping, the line has gotten even blurrier in some ways. We can count the exact number of genetic differences between two populations, but how many is the cut off to make them two different species?

The US Endangered Species Act of 1973 doesn’t bother with defining “species” and uses the phrase “evolutionarily distinct population” to cut through that argument. Even that, though, is subject to argument, especially when money is on the line and lawyers get involved.

The definition we’re generally giving in high school these days (with an asterix) is “reproductively isolated”. The isolation can be genetic (dogs can’t mate with cats), behavioral (spring maters don’t mate with autumn maters), or geographic (I’m not swimming the Mississippi just to mate). The asterix is of course that there are exceptions even to this definition.

In short, it’s a knotty problem because we’re trying to discretely categorize things that aren’t discrete.

Anonymous 0 Comments

A species is a group of similar organisms which can interbreed and produce fertile offsprings.

Not ONLY with another, but CAN at all. Like, horses and donkeys are each a species, but a mule cannot reproduce. The physiology is similar enough that a hybrid can be made, but different enough that the hybrid cannot reproduce. Horses are a species, but they do not ONLY mate with horses. Mules are not a species as they can’t continue the line.

Cats are weird and don’t follow normal rules. Ligers and Tigons can make more hybrids that further blur the lines between species but normally don’t do so outside of captivity.

Anonymous 0 Comments

The reality of it is that ‘species’ is a human-made concept that attempts to categorize/bin points that are continuous. It is easier to think about categories than complex continuous processes. The concept of a species is just something that makes it easier and more convenient to communicate groups of organisms.

With this in mind, there has been quite a lot of debate as to what defines a species, and with every definition, there is usually an example of a species that breaks the rules of that definition.