Friday, August 05, 2005

Speciation in a classroom petri dish

Many creationists and some Intelligent Design types accept that "microevolution" occurs within a species, allowing for small, gradual changes, but "macroevolution" from one species to another does not. Here's a typical statement:

"Speciation" in the Darwinian sense of one species gradually changing by selection into another has not been observed and no examples are known.

Of course, it's wrong, but even better than reading about examples in scientific literature would be to demonstrate speciation in a high school classroom petri dish.

So here's my idea - take a bacteria species that cultures easily and normally lives in pH-neutral environment medium, not very acidic and not very alkaline. A teacher could demonstrate this by transferring bacteria cultured on a neutral petri dish to higly acid and highly alkaline dishes, where the bacteria quickly dies.

Now the invention that someone needs to make - a petri dish shaped like a long rectangle that has an acidity gradient, so that it's very acidic at one end, gradually changes to a neutral pH in the center and highly alkaline at the far end. Students transfer bacteria to the neutral pH location, and then wait. Within a time period of days/weeks, the bacteria should, through natural selection, gradually evolve and grow into areas that are increasingly more acidic or alkaline than the parent population could survive in. At the two ends of the petri dish, the students will have created two different bacteria species that are reproductively isolated from each other because they live in incompatible environments. The students can prove that by transferring bacteria from the alkaline end to the acidic end and vice-versa, and they'll observe that the transferred bacteria soon dies. Speciation as described by biologists, for all the students to see right in front of them.

Several additional notes:

Biological education isn't my field, so there may be difficulties with this in practice, but it seems doable.

The idea of a species* is a little squishy when it comes to bacteria, but even if one doesn't accept the species concept for bacteria, the experiment does simulate what happens in nature and lets students see natural selection in front of them.

If some bacteria survive as one long colony of bacteria from the acidic end to the alkaline end, the students will have created a ring species, which is itself a fascinating example of speciation.

Some creationists now accept speciation, and say all the species biodiversity on earth could have developed out of the tiny number of species that could fit into Noah's Ark. They really believe in Noah's Ark. These people will not be affected by this bacteria experiment, but when creationists are forced to start saying the polar opposite of what they had previously been saying, they're going to lose some supporters. The Noah's Ark Darwinists also have all kinds of additional problems, from speciation in slow-maturing vertebrates to zero support in the fossil record.

*UPDATE: changed language from "biological species concept" to "species". See comments for details.

UPDATE 2: I'm about to lose Haloscan comments into the void, and since there a just a few here, I'm going to append them:

The biological species concept is a little squishy when it comes to bacteria...

The BSC doesn't apply to bacteria at all (or any asexually reproducing organisms); that's one of the biggest limitations of/arguments against the BSC. There are other ways to define species in prokaryotes (amount of genetic divergence, ability to exchange plasmids), but I don't think this experiment will produce species, so much as environmentally unique strains.

If some bacteria survive as one long colony of bacteria from the acidic end to the alkaline end, the students will have created a ring species, which is itself a fascinating example of speciation.

Technically a ring species must form a ring. What you are talking about is clinal variation along a linear gradient. Ring speciation (still a contentious topic) involves the meeting of two ends of a circular cline in which adjacent populations can easily exchange alleles, but the two ends have diverged enough to prevent gene flow when they come together to form the ring. Maybe if you did the experiment in a donut shaped container you could observe something similar to ring speciation.

It would be interesting to see if the strains adapted for the pH extremes could coexist in the neutral environment. If they can, I'd question whether or not we can actually consider this speciation. Of course, if one or both strains become so specialized to their new environment that they cannot survive in neutral pH, it could probably be considered speciation.

GravatarThanks RPM - I probably should have said "species" as a concept is a little squishy when it comes to bacteria, rather than the term BSC. Squishy maybe, but not meaningless.

The bacteria species chosen would be a much better analogue to eukaryotes if it were one that readily exchanged DNA information between cells, since that would perform much the same function as sexual reproduction. If the experiment is successful, you then get isolated gene pools in the bacteria groups.

I had also thought of using a donut-shaped petri dish (it would be helpful for educational purposes), but some ring species don't form complete rings, like the California salamander.

Interesting question if the bacteria from the two ends could coexist (and share genes) in the neutral environment. If so, you're probably right that speciation is incomplete. It would be complete in a natural enviroment, though, if that neutral habitat no longer existed. I also would not be surprised if the bacteria that stayed adapted to neutral pH outcompeted the two outlying groups and effectively kept them from coexisting.

Gravataryou mention petri dishes with a gradient in them. Which reminds me of experiments we were shown in a lecture while I was doing my Microbiology degree at the University of Wales, Cardiff.

They were taking samples from high salinaty environments like roadside grit stores and plating them on square dishes with a salinity gradient on them. They were looking for the ranges that bacterial fauna would grow.

I beleive these square gradient plates were pretty low-tech. You tip the plate slightly and pour in high-salt agar so that you get a layer shallow at one and deep at the other. Once it's set you tilt the other way and pour in low-salt agar. Diffusion/Osmosis take care of setting up the gradient for you.

I'm pretty sure that a quick search of the literature would find the original papers.

I shall quit rambling now. Just to point out that the equipment you want already exist.

GravatarThanks huw-l, that's an amazingly simple way to make it work. It seems like it might also work low-acidity and high-acidity agars.

key: science, politics, evolution

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.