Infertile humans are still humans?

Someone who just took a biology final is telling me that infertile humans are technically a new species. She pointed to the definition which states

as further proof of this fact that being able to reproduce is an important part of being a member of a species.

I haven’t taken biology in almost a decade but that doesn’t seem right. I’d think that they would still be human because of their human parents. Also, doesn’t the term ‘human’ refer to the genus rather than the species? So even if they were a new species, wouldn’t they still be human? I’d just like an official explanation. Thanks for your time.

Well assuming she is thinking of the creation of new species by genetic mutation, wouldn’t genetic infertility be in the category of mutations that couldn’t possibly create a new species?


Species are often refered to by their genus and species, i.e. homo sapiens, but sapiens is the name for the species.

The key here is that species is a group definition (“related organisms”). An organism can only be a member of a species, it’s can’t be a species unto itself. There is a minor exception for infertility due to crossbreeding, say if a human and a monkey could produce an offspring that was inherantly infertile. But the organism itself wouldn’t be a new species, it would just be part of a new species of which youcould make other members by crossbreeding. Infertility for medical reasons does not affect species membership.

That’s more of a layman’s definition. Many organisms in different species are capable of interbeeding (wolves and coyotes, for example). Take a look here for a definition that actual biologists use. Note that there isn’t one simple definition, but a variety of methods that are used.

The infertile person isn’t capable of interbreeding, and so can’t be a new species. By her own definition.

And is human, by all the accepted definitions.

I think she’s thinking that each member of a species has to be able to breed, when in actuality, the species as a whole has to be able to breed, to be considered a species.

I don’t know how to explain it, exactly…

Species are defined by characteristics of populations, not individuals.

As an aside, am I correct in thinking that an individual cannot change species?

(This is something of a definitional issue, I suppose.)

No, with a but. Or yes, with highly improbable maybe

As John Mace already so eloquently pointed out, a species is defined by the population not the individual. What could happen is that an individual could be assigned to one population and then later be dicovered to be part of another population. That’s not an uncommon occurence and in that sense the oindividual has changed species, although it nevre physically changed.

On a more physical level it is theoretically possible for an individual to undergo a mutational change such that it is no longer fertile with its original disignated species but is fertile with another species. In animals of course this is ridiculously improbable and would require some sort of speading mutation in the reproductive organs. That’s essentially impossible in mammlas but in other species where the reproductive organs complete devlopment later in life it’s slightly more plausible.

For plants it’s more probable though not real likely and has never been observed. It’s common inplants for mutations to develop in one bud and thence spread to the whole shoot or even the whole aboveground plant. Technically if a rose were to develop such amutation such that all flowers were now only fertile with strawberries but never with roses then the plant would have changed species. Pretty improbable but not impossible.

So ask them this: If someone should become infertile (through disease, radiation, whatever) do they suddenly lose their “human” status? And if they’re not human, then what are they?

Or ask them of people of the same sex are the same species. It’s easy to take that “layman’s definition” to ridiculous conclusions.

Infertile or marginally fertile animals created by crossbreeding animals of two species (the donkey, for instance) are considered hybrids and are not properly a species at all.

It bears stating that the whole notion of species, and indeed, taxonomic nomenclature is a human invention. Genes don’t care about species boundries; they only care that multiplying themselves. A new species of animal or plant is simply one which has diverged sufficiently that it is genomically or cladistically distinct from existing species, even though it may be able to interbreed. (See the discussion on [post=6042638]ring species[/post] and [post=6048435]parapatric and sympatric speciation[/post] in that thread.)

It’s worth noting that (it is thought) viruses frequently insert themselves into (and occasionally extract themselves from) the genome (usually harmelessly, in an inactive section of chain). You’re carrying a bunch of seperate species inside the nucleus of every cell in your body. Bloody freeloaders!


Asuuming that you think of a virus as a species, which almost nobody does. Viruses are often not even considered alive and never designated as species. They’re just molecules.

Indeed it does. People often forget that species designations aren’t some fundamental aspect of nature, but just the way we humans have chosen to order things. And even then, biologists disagree all the time about where to draw the lines.

Excellent point.

Hmmm… An interesting discussion. I’m curious about group of infertile humans. The species link to Wikipedia posted by John Mace mentioned that the ability to breed is used in some definitions. Two of the definitions are;

I’m a little confused here. Does this mean that the offspring of a horse and donkey insn’t a species? If it isn’t any species, then what is is classified as?

So if a group of organisms don’t recognise one another as potential mates, then the there isn’t 1 species, right?
My question here is what about those who say homosexuals are created thru genetics? (Sorry for no cite about this, perhaps new research has proven this wrong, but at the moment my browser seems to be preventing me from looking at anything with that topic. Maybe when “GeGe” isn’t watching :wink: ) I’m certainly not saying homosexuals aren’t of the same species as their parents, however these are organisms who do not search out other individuals who are potential mates. Also, homosexual organisms and heterosexual organisms “do not normally interbreed in the wild” to speak of. If left to their own devices, these two groups would never interact sexually. Any ideas on why the mule and the homosexual aren’t classified the same way?

It’s a hybrid. In scientific terminology it’s Equus caballus X assinus.

By that definition, yes. Of course that’s not a widely used definition for obvious reasons, not least of which that it obviously can’t apply to plants or passively fertilised animals, fungi etc. It’s really not very useful.

Because they aren’t in any way equivalent.

Mules are hybrids. They result form the union of two distinct species and only form the union of two distinct species. You will never find a mules spontaneously thrown by a population of horses or a population of donkeys. Both horses and donkeys always bred true. Only by uniting those two distinct true-breeding forms will you produce a mule. Hence mules are hybrids belonging to neither of those true-breeding populations.

Homosexuals are normal variants on the human species. You routinely find homosexuals spontaneously thrown by a population of heterosexuals. Heterosexuals apparently never breed true. Any lineage of heterosexuals will inevitably throw homosexuals. The homosexual form is simply a variant, whether it results from environment or genetics. Hence homosexuals aren’t a hybrid, They aren’t the result of the union of two true breeding populations. They belong to one true breeding population, humanity, and are a variant within that population, not something outside of it.

I get the impression that what’s causing most of your confusion is that you aren’t fully considering that species is a self-sustaining population. Donkeys form self-sustaining populations, as do horses. They seek mates almost exclusively within those populations. Homosexuals and heterosexuals are not self sustaining populations. There isn’t an island somewhere that is home to only the last genetically pure true-breeding heterosexuals. We don’t have homosexual reserves where they all run wild and free. Sexuality is simply a trait within a population. These aren’t by any stretch two distinct groups that don;t interact., They are the same group.

The other problem is that heterosexuals and homosexuals routinely interact sexually. Many people have been propositioned by or propositioned people from the ‘other team’. Perhaps usually this won’t resulting anything, but usually that’s the case for the average heterosexual man at a bar. But it’s by no means unheard of for heterosexuals and homosexuals to engage in intercourse, with either party switching teams temporarily. Very few people bat for only one team their entire lives and even they will likely have intercourse with someone who switches who has had intercourse with the other team. So there’s a continuity of sexual congress within the population. I may not personally be able to bonk Jodie Foster but I could probably bonk someone who she has. Ergo by your own standards there’s no separate population.

Of course all this is rather academic because mate selection is really just shorthand for gene selection. If homosexuals aren’t reproducing it’s a non issue. And if they are reproducing then we have the same. Unless you are proposing that gays only reproduce with lesbians and vice versa.

Not yet, but with your generous financial support . . .

Considering that new species evolve from genetic mutations over many generations through the very process of reproduction, how do you propose that a new species of infertile “human” evolve in the first place?

We have “infertile” vegetation, such as seedless oranges and watermelons, but those genetic mutations would have ended with the death of that individual plant if we hadn’t prolonged its existance through mechanically splicing severed parts - an unlikely option for a highly evolved mammal.

And speaking of mammals - mammals have hair. Yet there are genetic mutations that leave people completely devoid of hair. Did their parents produce a non-human baby?

People are people. Trying to dissect the species into subgroups in this way only serves to tear us apart. (Sorry - this is more of a semantic debate rather than a true scientific question, so I’ll park the high horse.)

So, according to their statement post menopausal human females are a species unto themselves? (just another brick in the wall)

**Blake **answered your other questions, but let’s just make sure we’re clear on the above subject. No one knows the cause of homosexuality. There is no direct evidence that homosexuality has a genetic basis. It could be genetic, it could be environmental, it could be determined by conditions in the womb of the mother, or it could be a complex combination of many different factors.

Studies of identical twins suggest at least some genetic component, but that evidence is indirect No one has yet discovered a “gay gene”.