Largest Kingdom

[davew]

This has been bugging me since the last podcast. The question about which kingdom (or maybe it was phylum) has the most species. I believe the answer was which ever contains arthropoda. There is a certain logic to this as there a massive number of athropod species, but surely they are dwarfed by the number of fungus and bacteria species. I did some research, but was unable to even locate estimates of fungus and bacteria species. What I usually run up against is "most bacteria have not been characterized".

I suppose I could also focus on the fact that yeast is a type of fungus and yeast makes beer and I could have a beer and just chill out.

[Nini Ponsaing]

The question was which phylum in the animal kingdom was the largest. As far as I'm aware fungi and bacteria aren't part of the animal kingdom. I am talking about stuff I don't know, so anyone should feel free to jump in here if I'm wrong, or with more information.

I like the logic used in your second paragraph.

[davew]

Ah. I probably didn't listen closely enough. It also doesn't help that between 1st form and college I learn four different ways to classify living things. Now it's all just a hopeless muddle between my ears.

[TinBane]

Okay, first up, the term 'species'.
I don't want to sound all wanky, but it's a "human construct".
There are MANY definitions for what a species is, each has benefits and weaknesses.

For example, a common one is "can it reproduce with X, if not, they are different species". Many bacteria and fungi don't need to use sexual reproduction, so it doesn't work for them. Further more, when they do (eg bacterial conjugation) they can reproduce with just about any other bacterium.

Nowadays, bacteria are often catagorised into operational taxonomic units (OTUs), and the definition is usually specified. For example, I might define whether two bacteria are in the same OTU based on the %age difference between their 16s gene (it's often used to identify bacteria).

Why don't species exist? Because a species is a distribution of genetic diversity. It's like a bell graph. Two species might be like two bell graphs with different averages. There might be overlaps between the two. In fact, maybe two individuals can reproduce together, but two others can't. Species just aren't as fixedly seperate as we like to believe.

In bacterial alone, there are at least three major catagorisations used to determine the taxonomy. Some are functional, some are based on how you grow them in a lab, and some are purely genetic. What I can say, is that there are thousands (tens of thousands) of bacterial groups, which are considered as diverse as say, the entire 'arthropod' section of the animal kingdom. A gram of soil, might have 60,000 bacterial OTUs, based on genetic estimates.

So you are right, any one of Fungi, Bacteria or Archea, would absolutely swamp arthropods based on any metric (genetic diversity, number of OTUs, and even total biomass). However, they aren't within Kingdom Animalia

[X-man]

Good stuff tinbane - we talk a little about the fuzziness of the concept of 'species' in the natural selection bit of the latest episode (ep 25).

For plants and animals, we essentially go with the definition TinBane has outlined: The reproductibility of individuals - if you can get it on with another individual AND make fertile offspring, then you're the same species (hopefuly you had a bit of an idea before you tried the experiment). But even this is a fuzzy definition because it's a probability function.