What is a species?

Trait distribution
Trait distribution. With the X axis showing the value of the trait and the Y the frequency in a colony of asexual organisms.

With the increasing focus on the role of intra tumour heterogeneity in cancer, figuring out the types of tumour cells in a cancer becomes more important. But whereas we have a good idea of what a species is when it comes to sexual organisms, this question is harder when dealing with asexual ones.
One approach is to look at the genetic differences between two cells. If the differences are small then we can say that they belong to the same species. The problem with that is that genetic differences may not mean much. From the evolutionary point of view what matters is the phenotype: the way the cell looks, behaves and interacts with other cells and the environment.
This is closer to what we do with sexual species. If two individuals of different gender can mate and have offspring that can also mate then they are considered to be part of the same species.
Tamir Epstein is a research scientist with a PhD in physics and whose work involves him jumping between Princeton and Moffitt. He raised this issue and suggested an approach that I am trying to explore here.
If you start a colony of cells in a petri dish starting from one single cell there will be a lot of traits where the cells will be slightly different from the original one. We can describe this with a distribution like the one in the figure at the top of this post.
So here is a possible definition of species for asexual organisms courtesy of Tamir and (and myself to a smaller measure through the discussion): two organisms belong to the same species if for a relevant number of traits they can lead to the same distribution. That is, if you take two cells, place them under the same circumstances and let them grow, the resulting distributions should be identical.
I am guessing this is a start. In reality it is quite likely that finding two cells that can lead to equal distributions of traits will be tough so a degree of flexibility migh be necessary.

3 thoughts on “What is a species?

  1. I like this definition, but I would make it less like sexual species and a bit more precise, by turning it around a bit. Instead of defining species based on some magic “relevant number of traits”, for any collection of traits you care about define a relevant species. I.e. these two cells might be the same species for motility, but a different species when we look at florescence, and that should be alright.

    My only real concern with this definition, is that as you said, phenotype is the relevant part for evolutionary dynamics. In particular, this means that when you place your original cell in the petri-dish, the final distribution of phenotypes is a function of two different things: (1) the original cell, and (2) the phenotype selected by the evolutionary pressures of the chosen environment. It could be that your environment is so biased toward certain phenotypes that as long as the cell even rarely produces that phenotype, that phenotype will end up dominating the population (kind of like convergent evolution). In that case, even if the two cells you were testing really were different, you wouldn’t pick that up as long as they each express this highly selected phenotype slightly.

    I am not sure how much power experimental have to do the following, but this is a modification that I would suggest. Suppose you want to see if cell type A and cell type B are the same species, then (1) add some sort of heritable genetic tracer alpha to cell type A and beta to cell type B (maybe infect them with some phage or introduce some plasmid), (2) grow a petri dish with both cell types (with tracer) present in it in equal numbers, (3) once the growth is finished, sort the cells by relevant trait phenotype (if possible), (4) sequence each batch of different trait to see that tracer alpha and tracer beta are equally-ish represented.

    If part (3) is impossible then we can replace it by three petri dishes, one with just type A, one with just type B, and one with both. Make sure all 3 have the same phenotype distribution (as in your method) but also sequence the double-type to make sure both tracers are about equally represented.

    Of course, if (1) is not possible then I would stick to your proposed method but make sure to do it in several different environments to try to minimize the risk of the final distribution of phenotypes just being determined by the environment.

    I would also suggest turning this post into a question and asking on the Biology StackExchange in case some folks there have insights.

    • As usual Artem’s comments are more elaborate and articulate than the post itself!
      In a comment I got from G+ (Randall Lee Reetz) I was asked about the motivation to define species at all! In cancer it is important to know both the tumour heterogeneity and how treatments can change that.
      As you say, picking the traits you find relevant can be used to define a species. And as you suggest (and also discussed a while ago with Tamir himself), the definition of species would need to be more fluid. They should be context and question dependent if they are to be useful. If we know that the tumour cells live in a particular type of (dynamic) microenvironment where certain traits are relevant then we might have enough to start talking about the different species of the tumour. If we are going to characterise the heterogeneity of the tumour (assuming we have all the experimental data) this would be a good start.

      About turning this into a question for Biology StackExchange: I was only aiming at getting the idea out first and sharing it with Tamir but I am guessing that your suggestion would be a logical next step. This would help us have an idea of whether steps (1) and (3) of your experimental approach are feasible.

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