How do you tell when a new species has evolved?

March 2, 2004 10:19 PM

It's easy to tell when a species has gone extinct. How do you tell when a new species has evolved? Are there any criteria besides not being able to mate with the former species?
posted by ph00dz to science & nature (24 comments total)

Just a nitpick here: I think you mean not being able to have fertile offspring produced from mating with the former species. Donkeys and horses can mate and produce mules, but mules are all female and sterile and therefore not a new species. Same with "ligers", which are bred in zoos to be half lion/half tiger, and which I think also end up sterile, and thus aren't a new species either. I'm sure someone here will corect me if I'm wrong about this.

In other words, the X-Men really are homo superior instead of homo sapiens not because of their super-duper powers, but because they're fertile. (And if they're part of the Summers family, fertile and multiplying to a ridiculous degree...)
posted by Asparagirl at 10:43 PM on March 2, 2004

Yea, i'd say not being able to mate with the former species and being able to mate with each other, and there has to be some others.

In birds for example, people have recently found that there are groups that look the same and have the same physical features but have different mating calls. These used to be considered one species because people made up the species based on how they look, but actually since birds mate with mating calls, if you have 2 different calls that's really 2 species. This has caused a lot of new bird species recently.

Now i think the test is to play a call and see if the bird responds.
posted by rhyax at 10:59 PM on March 2, 2004

Oh, and to clear things up "not being able to" may be a poor choice of words, as many members from different species can mate, it's just that they don't. There doesn't have to be genetic or mechanical isolation, it can be behavioral, temporal, geographical etc.
posted by rhyax at 11:01 PM on March 2, 2004

re: bird calls -- isn't that rather much like a (wo)man who responds to "yo! baby! that's one sweet ass you got! let's get it on!" versus a (wo)man who reponds to something more elegant?
posted by five fresh fish at 11:12 PM on March 2, 2004

but mules are all female and sterile

Mules can be both male and female, and a very small number of female mules are fertile.
posted by biscotti at 11:42 PM on March 2, 2004

"yo! baby! that's one sweet ass you got! let's get it on!"

There's something more elegant?
posted by Pericles at 1:37 AM on March 3, 2004

If you dig deeply enough, the whole concept of species becomes somewhat conjectural, according to some.
posted by piskycritter at 5:21 AM on March 3, 2004

a very small number of female mules are fertile.

Likewise, some female ligers (and tigons) are interfertile with tigers and lions (but because male ligers and tigons are so-far invariably infertile, not with other ligers or tigons).

The second generation hybrids are ti-tigons, li-tigons, ti-ligers, and li-ligers.

Though to my ear a "litigon" is just a polygon with a number of sides that is subject to legal argumentation.

I don't think the standard for speciation is "can't possibly mate with each other." It's just "in the wild, don't normally mate with each other with fertile results."
posted by ROU_Xenophobe at 6:30 AM on March 3, 2004

> If you dig deeply enough, the whole concept of species becomes somewhat conjectural

The Linnaean habit of applying Latin binomials universally throughout nature makes the situation seem a lot cleaner and more orderly than it is, though there clearly are some good species (Homo sapiens doesn't do much interbreeding with its nearest relatives.) Plant species can be a much more of a mess than animal species.

Oaks (Quercus) are a classic example. Imagine a small interbreeding population of oak trees. Draw a circle around it. Now imagine a second population right next to the first, with guite a lot of gene flow between the two. These two populations are clearly not different species by the no-interbreeding criterion. Draw a circle around that second one.

Now repeat the process several times until you have N little circles lined up. Eventually you'll find a population that doesn't have any gene flow between it and the first. These two populations represent good species, by the non-interbreeding criterion. But where do you draw the line?

Now, make it even worse. Pull your line of little circles (interbreeding populations) into a ring, so that population 1 is next to population 2 and also next to population N, with gene flow occurring between both 1< -->2 and N< -->1. Considering that ring of populations, any given one will interbreed freely with its nearest neighbors, less freely with not-so-near neighbors, and not at all with populations on the opposite side of the ring. So where do you draw the species lines?

Finally, just for fun, shove all your little circles on top of one another, indicating that all these little populations actually grow intermixed in the same forest. That's the natural situation as the field biologists found it.
posted by jfuller at 7:03 AM on March 3, 2004

> "yo! baby! that's one sweet ass

That's what the horse says, just before you get a mule.
posted by jfuller at 7:14 AM on March 3, 2004

How is this not merely a question of how the word "species" is normally used?
posted by shoos at 7:16 AM on March 3, 2004

> How is this not merely a question of how the word "species" is normally used?

That's part of the question, but another part of the job of science is to refine its concepts. In this case would be to do an increasingly better job of adapting the use of species language to the natural facts that the language is supposed to describe. Therefore the question "What is a species?" in not purely synthetic ("What is the case about X?") nor purely analytic ("What do we already mean by X?") but a combination: "What is there that we might reasonably mean by X?"
posted by jfuller at 7:30 AM on March 3, 2004

Oops, missing a "that" in my last. "In this case that would be..."
posted by jfuller at 7:44 AM on March 3, 2004

Think of a species in terms of a family tree. Unless there's some degree of intermixing, the cousins way out on the end are going to start genetically diverging as some die off, new mutations pop up and certain characteristics start becoming more pronounced.

If these cousins become isolated from each other through geographic, cultural or other reasons, they may eventually diverge so much that they can't / won't interbreed. That's when you get new species.

On preview: jfuller said it much better.
posted by bshort at 7:45 AM on March 3, 2004

Anothing thing to consider is that since speciation appears to require rather an enormous amount of time to occur, asking when there's a new species is kind of like asking which day you became an adult. You know it happened in there somewhere, because when you go home everyone looks at you differently and, you know, you can't produce fertile offspring with them anymore.

Also, extinction isn't that precise either. Every now and again, "extinct" has turned out to mean "the few survivors hid really well for a long time."
posted by vraxoin at 8:09 AM on March 3, 2004

> "the few survivors hid really well for a long time."

I'm still hoping for a trilobite.
posted by jfuller at 8:20 AM on March 3, 2004

I don't see this as a case of science "refining its concepts." I see it as science saying to itself "oh crap, that word, 'species,' it turned out not to be so tidy. Why don't we get back that tidyness we had before by redefining the word."

What would ph00dz gain, in terms of knowledge about the natural world, if someone gave him an answer to his question? Nothing. He'd learn something about how English is used (or "should" be used according to some) today.
posted by shoos at 8:35 AM on March 3, 2004

> I don't see this as a case of science "refining its concepts." I
> see it as science saying to itself "oh crap, that word,
> 'species,' it turned out not to be so tidy. Why don't we get
> back that tidyness we had before by redefining the word."

A distinction without a difference (barring a certain grumpy flavor to one version, which may not be essential.) Because there is a steady accretion of what we are pleased to call facts and which we are pleased to consider related to the meaning of species, our previous account of the nature of species must constantly be extended and reordered to include systematic reference to the additional facts. That's another way of saying "redefine the word," though it emphasizes that our redefinition has grown in organizing/explanatory power.

> What would ph00dz gain, in terms of knowledge about
> the natural world, if someone gave him an answer to his > question? Nothing

I, on the other hand, don't see how one could fail to pass on knowledge of the natural world while teaching someone the language we use to deal with it, any more than I could fail to teach someone about wood while teaching them to use use a hammer and saw.

More concretely, I don't see how I could show you the use of terms associated with species, speciation and taxonomy without showing how they are applied in a great many specific instances of natural situations -- as for instance, my Quercus example above. And I wouldn't be convinced that you knew how to use these linguistic tools unless you could apply them appropriately in the context of natural examples. Or, to put that slightly differently, if you expect to pass your orals you better be able to cite lots of cases, in addition to having a list of buzzwords and abstract locutions.
posted by jfuller at 11:23 AM on March 3, 2004

A tangent question, can humans and any other species mate? Like humans and chimps? Aren't our genes like 99.9% the same as we have been told from Biology 101? Say if artificial insemination were used? Or are the chromosones different numbers (would this even be an issue)?
posted by geoff. at 11:40 AM on March 3, 2004

Species are artificial categories which break down at the boundaries. The real world isn't as neat as a taxonomy.

In addition to looking at it horizontally (modern humans and chimps) you can look vertically and see that at some point we must have had ancestors who were siblings or cousins of the chimps' ancestors and probably could mate. Was there a particular individual generation at which the species became unambiguously separate? Doubtful.
posted by callmejay at 11:51 AM on March 3, 2004

jfuller, I think generally speaking, we're in agreement, but I just don't think there's been any organizing/explanatory power added to the term/concept "species" just because its use has been adapted to account for new information about the world. I say we toss the concept - it's way too soggy. You yourself gave an excellent illustration of why that would be beneficial.
posted by shoos at 12:44 PM on March 3, 2004

Was there a particular individual generation at which the species became unambiguously separate?

I would think that whichever generation could no longer mate with a chimp would probably be the one. That much could be pinpointed.
posted by me3dia at 12:56 PM on March 3, 2004

A tangent question, can humans and any other species mate? Like humans and chimps?

Maybe, but you'd have to try.

I suspect that a human/chimp hybrid carried by a female chimp would just kill the chimp, since -- even if it conceived -- it would probably have a skull bigger than the pelvic passage. Which would mean no real possibility of birth, = dead chimpette and dead humanzee. This might also rule out that kind of human/gorilla and human/orangutan hybrids.

Going the other way naturally would probably be difficult too, since IIRC chimps have penises that might be too small to achieve effective penetration on a human woman. Also, you'd have a horny chimp fucking you, which would not be the highlight of anyone's day.

Or are the chromosones different numbers (would this even be an issue)?

We have 23 pairs of chromosomes and other great apes have 24. Shouldn't be a big deal -- horses and donkeys have different numbers of chromosomes, but can still make mules.
posted by ROU_Xenophobe at 2:36 PM on March 3, 2004

chimpanzees and humans have different numbers of chromosomes, yea. The mule thing is kinda a fluke, one different chromosome is actually a big deal, a lot of people are infertile because of balanced translocations, which is where 2 chromosomes break and switch places, this causes havoc in meiosis.

I mean, maybe a chimp human hybrid might be possible, but i doubt it. The 99% thing is good, and shows how close we are, but there are bigger issues about how the genes are stored on the chromosomes, you could have 2 species that were 100% matches but not be able to breed because they have those genes laid out in completely different ways.
posted by rhyax at 12:52 AM on March 4, 2004