https://plus.google.com/u/0/108510686109338749229/posts/apoxkhhLSva

Gut flora (bacteria) is really coming into its own. In our surprisingly germaphobe world, this is kind of surprising. Today, we see ads side-by-side for Activia yoghurt and pill-form probiotics to promote regularity. At some point, perhaps we’ll realise that bacteria are not all evil.

In the meantime, this post does a fantastic job of describing how the bacteria in your digestive system may be way more important than we ever dreamed–so important that they might split a species.
It’s not one of the traditional, canon ways that speciation is though to occur. Mostly, we talk about speciation as being due to reproductive isolation, so anything that keeps species from mating (or producing viable offspring) can cause a species to split.

Prezygotic isolation means that the egg and the sperm (or pollen or whatever) never get to fertilize. There are tons of ways this can happen:

• geographic separation (allopatric, parapatric, or sympatric speciation),
• behavioral separation (different mating times of year, mating dances, songs, preferences in what appearance is sexy, etc.)
• mechanical separation (the body parts just don’t fit, sorry)
• gamete isolation (the sperm can’t fertilize the egg)

Okay, so what if the egg and the sperm DO get together?  I mean, horses and donkeys make mules; lions and tigers mate, right?

That’s post-zygotic isolation.  In many cases, very few cross-breeds exist between species.  This is thought to be due to

• Dobzhansky-Muller incompatibility (big name.  Basically, it means that each species has some trait that’s beneficial to that species, but when you put them together, they spell disaster, and the offspring can’t survive the freaky gene combination)
• Haldane’s rule (this has to do with weird things with gender-determining chromosomes.  Usually, the female of a cross-breed seems to be fertile, so it has something to do with X and Y chromosomes)

 

All of this is based on the biological species definition, which defines separate species as being unable to mate successfully and produce viable and fertile offspring.

I cannot conceive of how gut bacteria can keep two species from mating and producing viable, fertile offspring.  But there are other definitions of species as well.  Go forth, my paduan learners, and figure out how this could work!