A reminder that the biological species concept is an arbitrary method of determining “species” that falls apart when you apply it to non-tetrapod groups or to deep time, and in fact all species concepts are arbitrary ways for humans to categorize and classify life in a way that makes sense to them.
Heck, even within mammals it falls apart when you consider camelids and Panthera.
In a super-nutshell: the biological species concept holds that a “species” is a group of organisms that can breed and produce fertile offspring. That is, each species is reproductively isolated from all others. Following this, hybrids between two species under the BSC should be unable to produce fertile offspring.
But this falls apart when hybrids are fertile. Ligers and reportedly jaguleps are fertile, as are hybrids between dromedary and Bactrian camels (and theoretically also camel-llama hybrids, but the latter has never been tested). Does this mean that lions, leopards, jaguars, and tigers are all the same species? Likewise with llamas, dromedary and Bactrian camels? These are arguments for why the BSC doesn’t always work as intended, and shouldn’t be used as an across-the-board species concept.
It applies to SO many vertebrates. There’s a lot of documented hybridization with fertile offspring in birds- for example, many falcons can interbreed and produce fertile offspring, and wild Western Gulls and Glaucous-winged Gulls have produced the EXTREMELY successful fertile hybrid “Olympic” gull. SO many other birds also interbreed in the wild! I think it’s estimated that something like 15% of wild bird species have been documented to hybridize?
It also calls to question, for example- if Homo sapiens and Homo neanderthalensis could interbreed and produce fertile offspring (and we know they did) can they be considered different species? What about dogs and wolves? Are a chihuahua and a gray wolf the same species? They can interbreed, after all. What about coyotes? We know there are fertile coydogs and coywolves. Domestic cats- we have cross bred them with TONS of different small wild cat species and gotten fertile offspring.
And these are just a few examples. Species across the board are churning out fertile offspring all the time. And this is why there’s a species problem- with no clear place to draw the line between species, how do we decide what a species is? Because at the end of the day, every lineage is a gradient, without clear divisions between populations, and “species” is just a word we came up with to try and break things into smaller, more digestable pieces
ifesbob 