The term “species” is probably one we’re all familiar with. It’s a deceptively simple idea – species are groups of organisms that are different from other organisms. Humans are humans, live oaks are live oaks, water bears are water bears. Duh. But deep down in the nitty gritty, defining species is a lot less straightforward. Living things don’t always behave like we expect them to, and this includes who they mate with and how they fit into our mental boxes. Despite the exceptions, many general trends exist.
A pretty standard lecture in Evolution classes is one on speciation. The most common mechanism goes like this:
There are plenty of exceptions to the above narrative – like basically a billion exceptions. However, it’s a straightforward way to think about how different (large, sexually reproducing) species arise in the case of allopatry, which is the scientific term for the speciation-by-physical-separation I just described.
Thinking about speciation in microorganisms, especially obligately asexual microorganisms, is a real brain cramp for someone who has long thought about speciation in vertebrates (i.e., me). But it’s a really fun cramp. Without sexual recombination of genomes, without pre and post-fertilization ways of allowing only suitable babies to live, what unites microbes into cohesive units? And how do we identify them? Some have posited the environment decides: “ecospecies” are organisms in the same environment that perform basically the same function. Morphology is basically useless since there are very few shapes that microorganisms have. Others rely on the genetic makeup of an individual to cluster it with other very closely related genomes, er, organisms. However, empirical data are frequently unable to identify obvious genetic cutoffs for when two sequences belong to the same or different microbial species. This poses the question: What is a microbial species?
A recent paper by Rossberg et al. answers that question with another question: Do species even exist for organisms smaller than 1mm? They use a theoretical framework to outline this basic idea:
In this case, drawing circles (or delimiting) species is literally impossible because there is just a smooth gradient of genomes. Furthermore, the same types show up in our two disconnected environments!
Rossberg et al.’s theory comes from a biological model that boils down to two important parameters: mutation rate (u) and carrying capacity of a population (K). The concept of niche space (or a suitable environment for an organism to live) is also important. Specifically, the product of mutation rate and carrying capacity (uK) needs to be below a certain threshold for species to form. This is because there needs to be a small amount of variation relative to the amount of niche space available or no clear “best” type will emerge that can outcompete all the other types quickly enough to become established. If mutation rate is high, there are too many available types. If carrying capacity is high, there is no way to limit who’s there at all. Many other things are happening with this paper, but their big conclusion, put plainly, is that if there is too much variation, differentiation cannot occur.
At first, I thought they were using 1mm as a kind of arbitrary cutoff that separates “big” small things from “small” small things. However, the authors use published data regarding the population size and speciation rates in combination with estimates of body size for the various groups to show that something magical happens around the centimeters to millimeters body size. They narrow the threshold size down to millimeters (instead of centimeters) by looking at a lineage through time plot of some empirical data and concluding the centimeter sized organisms don’t fit the expectation for things above the uK cutoff whereas those smaller than a centimeter do. Microorganisms (generally speaking) have short generation times (which can lead to excess mutations) and large population sizes – it is almost believable that they are unconstrained enough to not produce discrete species.
One final note: are there really no species smaller than 1mm? “Species” feels like such a fundamental concept in how we understand the world around us that the immediate response to this question may be one of consternation. I’m personally not convinced that there are no “species” smaller than 1mm. But even if there aren’t species at that size – even if all we have are genomic gradients – we certainly can’t eliminate the concept. To conduct research we need a way to classify the world and organisms around us and “species” does that. BUT! Getting into the nitty gritty and challenging basic concepts is also needed and I’ll take a mental cramp over a physical one any day.