Do new species always happen because of reproductive isolation?

So, I really like this question from Zach Weinersmith:

He later followed up the query with a further clarification:

This is a fantastic question, because there’s a lot of REALLY weird biology when it comes to how lineages diverge. Sometimes, it happens gradually. Sometimes, it happens instantly.

Sometimes, animals can even undergo speciation while still mating with each other.

Let’s start breaking this down…

What does the term species mean?

I mean, yeah. This is a term we need to define. Zach is married to Dr. Kelly Weinersmith, who’s a respected parasitologist.

Species concepts are…tricky. As Ainsley Seago puts it:

When we use the word ‘species’, we’re trying to figure out at which specific point we get new kinds of animals. While this is simple in concept, there’s a really big philosophical problem: How do you define this in a way that actually means something?

As Dr. Seago pointed out, this is complicated to the point of being contentious. In a lot of taxonomy classes, there’s an entire unit on species concepts. This entire post could be about how we define the term ‘species’, but it wouldn’t be as fun. So, instead of spending a lot of time defining the terms, we’re just going to use the biological species concept, the idea that organisms become new species once they can no produce reproductively viable offspring when mating with each other.

Is instant speciation a thing?

Yeah, believe it or not, it can be…and it’s quite common. There’s a few ways this can happen. The most common is via a process called polyploidy. It’s kind of complicated, so I decided to go with a chart to explain it.

Dandelions, the kind you have in your yard, are a really important model species for this kind of speciation. The dandelions we have in the US are all triploids (three sets of chromosomes), which reproduce by cloning themselves. However, in their home range, there are populations which have two (diploid) or four (tetraploid) sets of chromosomes. When one of the asexual dandelions fertilizes a diploid dandelion, 90% of the offspring made are formed from self-fertilization.

This results in some very odd population genetics. There’s evidence that the different dandelion populations regulate each other by effectively using sterile triploid creation as a way to hamper the other species. Some triploids even have their own very complicated genetic behaviors on top of all this, but that’s another post entirely.

Each one of these plants is capable of producing one million seeds. In a field of 10,000 Palmer’s Amaranth plants, you can expect 10 once-in-a-billion events to happen.

Is it possible to speciate while the populations are still mating?

Wasmannia auropunctata is a weird little ant. It’s got bizzarre genetics, it’s attracted to electricity, it’s extremely adaptable, and for some reason, it’s associated with eye disease.

Bees, ants, and wasps have a strange quirk because of their genes. A queen’s daughters are more closely related to their sisters than their fathers, and this slightly different reproductive potential doesn’t benefit the males. This sexual conflict led to a bit of a messy divorce, biologically speaking.

We’ve gone through how reproduction works in bees before. Female ants, both queens and workers, are produced from fertilized eggs. So the FEMALES have two genome copies. Males come from unfertilized eggs, which have one copy of the genome.

In the ant Wasmannia auropunctata, an invasive species here in the US, males and females are produced clonally. This happens because the male’s sperm kicks out the egg’s nucleus, taking over the cell to produce a male ant. The female’s egg develops into a female to produce a new queen. There’s even evidence that the females accommodate this odd mode of reproduction by producing eggs that don’t have a nucleus, or no female genes.

W. auropunctata seem to use both sexual and clonal reproduction, with sexual reproduction being very rare. Colonies are either clonal or sexual; you don’t see sexually produced reproductives in a clonal colony and vice-versa. However, based on the genetic evidence, there’s not a reproductive isolation happening between the sexes. The clonal colonies can be related to the sexually reproducing colonies, even if the sexual colonies are rarely formed.

For one androgenically reproducing ant, Vollenhovia emeryi, there is evidence that the males and females form different evolutionary groups. While I don’t feel that it’s particularly strong evidence, I do feel comfortable saying that the phenomenon of androgenesis does demonstrate that it’s hypothetically possible for male and female organisms to be completely different species.

So let’s just make this a little bit more weird…

Bacillus rossius, an  innocuous looking bug with really weird genetics.

The stick insects Bacillus rossius and B. grandi live in the Mediterranean, share a habitat, and are closely related. So, sometimes hybrids between B. rossius and B. grandii happen. These sorts of things are common, they regularly happen with other animals in this situation, and it’s no big deal.

The female hybrids have a defense mechanism to avoid further hybridization; they eliminate the B. grandii genome from the eggs. This way, only the mother’s DNA is passed along to the offspring. If one of these hybrids mates with B. grandii, the offending genome gets kicked out.

It turns out that this system isn’t perfect, though. In about 20% of the eggs, the mother’s genome is eliminated as well. So, if these eggs aren’t fertilized, and get laid, they won’t develop because they don’t have any DNA. Things get weird, because Bacillus males also have a way around this.

When an egg is fertilized by Bacillus males, multiple sperm enter the egg. Normally they’re eliminated after fertilization, but if there’s not a nucleus, the sperm cells can simply divide, fuse, and then create an embryo. Or, two different sperm cells of the same species can fuse. In either case, the sperm are essentially fertilizing each other.

The Bottom Line

Instantaneous speciation, like Zach described, can happen in a few ways. Genome duplication, or mating with other species to produce triploid individuals is the most common by far. If the triploid species can end up reproducing, like dandelions, it’s effectively another species.

Sexual conflict can also create entirely new species, which might be happening with a handful of ants. If the males evolve a way to kick the female’s genes out of the eggs, then they can just make a new male without the female DNA. If this turns into a preferred mode of reproduction (and again, the evidence for this is pretty weak), then that’s also a new species.

So all the weirdness of this post raises an interesting question: Is it possible for an entirely male species to exist?

It turns out that there’s a genus of clams, called Corbicula, where many species are entirely male. They survive by using their sperm to parasitize eggs of closely related clam species, kicking out the female nucleus and producing males.

Sometimes, this doesn’t work, and the female genome gets incorporated into the next generation. That’s all well and good, because they  somehow kick out the sex chromosomes, and incorporate the genes from the other species, creating polyploids. Creating new species this way seems to be common for Corbicula.

As you can probably imagine, the evolutionary history of Corbicula is a bit of a hot mess.

Works Cited

Verduijn, M. H., Peter J. Van Dijk, and J. M. M. Van Damme. “The role of tetraploids in the sexual–asexual cycle in dandelions (Taraxacum).” Heredity 93.4 (2004): 390-398.

Pigneur, L. M., Hedtke, S. M., Etoundi, E., & Van Doninck, K. (2012). Androgenesis: a review through the study of the selfish shellfish Corbicula spp. Heredity, 108(6), 581-591.

Kobayashi, K., Hasegawa, E., & Ohkawara, K. (2008). Clonal reproduction by males of the ant Vollenhovia emeryi (Wheeler). Entomological Science, 11(2), 167-172.

Komaru, A., Kumamoto, A., Kato, T., Ishibashi, R., Obata, M., & Nemoto, T. (2006). A hypothesis of ploidy elevation by formation of a female pronucleus in the androgenetic clam Corbicula fluminea in the Tone River Estuary, Japan. Zoological science, 23(6), 529-532.

Foucaud, J., Fournier, D., Orivel, J., Delabie, J. H., Loiseau, A., Le Breton, J., … & Estoup, A. (2007). Sex and clonality in the little fire ant. Molecular biology and evolution, 24(11), 2465-2473.

Van de Peer, Y., Mizrachi, E., & Marchal, K. (2017). The evolutionary significance of polyploidy. Nature Reviews Genetics, 18(7), 411.

Schwander, T., & Oldroyd, B. P. (2016). Androgenesis: where males hijack eggs to clone themselves. Philosophical Transactions of the Royal Society B: Biological Sciences, 371(1706), 20150534.

Image Credit

Dandelion image:
Sharon, via Flickr CC BY-NC-ND 2.0

Dandelion pollen image:

Dandelion ovule image:
Musiał, K., Płachno, B. J., Świątek, P., & Marciniuk, J. (2013). Anatomy of ovary and ovule in dandelions (Taraxacum, Asteraceae). Protoplasma, 250(3), 715-722.

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