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


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.

In: 50

I’ve had this same question my whole life and never got any good answer from a biologist. Why aren’t different dog breeds considered different species? Are different races of humans different breeds like dogs? Why are certain bird species that look alike, live together, and breed together, considered different species?

The answer I usually get is: it’s arbitrary. Curious to see what others say, but it’s not hardfast how they make distinctions.

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.

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.

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.

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.