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.


5 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.

  2. I was recently reading Fisher’s The Genetical Theory of Natural Selection. And it made me think of this old post of yours. Fisher starts chapter 6 with a contrast between sexual and asexual reproduction. In the process, he wants to have a working definition of ‘species’ for asexual organisms. From the perspective of a game theorist, I found his definition particularly interesting and I wanted to share it with you (pg. 121 of 1930 edition):

    The groups most nearly corresponding to species would be those adapted to fill so similar a place in nature that any one individual could replace another, or more explicitly that an evolutionary improvement in any one individual threatens the existence of the descendants of all the others. Within such a group the increase in numbers of the more favoured types would be balanced by the continual extinction of lines less fitted to survive, so that, just as, looking backwards, we could trace the ancestry of the whole group back to a single individual progenitor, so, looking forward at any stage, we can foresee the time when the whole group then living will be the descendants of one particular individual of the existing population.

    This definition stood out to me because unlike the reductive definitions we tried to give above, which rely heavily on observing particular phenotypes or genotypes, this one is much more closely linked to the definition of natural selection. For Fisher, two asexual organisms are part of the same species if they struggle for existence against/with each other. This is fascinating and I think a stark contrast to our approaches. It places natural selection first and everything else second. It ‘falls out’ of the abstract idea of “struggle for existence”.

    Having read a lot of philosophy in the years since my first comment (most relevant would be a very recent reading), the above definition of species seems almost equivalent to Millstein’s (2009) view of a population as a Ghiselin-Hull individual (and tries to cut nature at joints similar to those identified by Wells & Richmond, 1995). So it is interesting to see Fisher anticipating these views, and I will have to revisit these recent papers to see if they mention his conceptions of species. It also forces me to withdraw the proposal for a definition of species that I gave in my previous comments. As I realize now, what I was actually identifying there is the idea of a ‘type’ (which I used to call a ‘subpopulation’ before I discovered the power of the type-token distinction) and that is something distinct from a species.

    Of course, Fisher’s view as described above isn’t perfect for cancer. One big issue is understanding the requirements of the struggle for existence. If here we follow Lennox & Wilson (1994) — which I think is perfectly reasonable to do, since their arguments seem convincing to me — the ideal r-selection (i.e. an exponentially growing tumour) would not correspond to a struggle for existence and thus it would not allow us to define species. Given that in cancer we often care about rapidly growing populations, this might be an important conceptual hurdle.

    A practical hurdle is that the big benefit of Fisher’s definition of species in his first sentence is the application to prediction in the second sentence. However, as game theorists, we often care about frequency (or density) dependent fitness and co-existence, thus we care about cases where two types coexist with a cancer species.

    Still, I think that Fisher’s definition can be a useful one to reflect on. Although I am sure cancer biologists have already done this. Maybe we should ask Andriy Marusyk. Obviously, he might push back on our starting assumption: cancer as asexual. Fusion, fission, and those plasmid-pipeline-like systems do suggest a relaxation.

    Fisher, R. A. (1930). The genetical theory of natural selection: a complete variorum edition. Oxford: Clarendon Press.

    Lennox, J. G., & Wilson, B. E. (1994). Natural selection and the struggle for existence. Studies in History and Philosophy of Science Part A, 25(1): 65-80.

    Millstein, R. L. (2009). Populations as individuals. Biological Theory, 4(3): 267-273.

    Wells, J. V., & Richmond, M. E. (1995). Populations, metapopulations, and species populations: what are they and who should care? Wildlife Society Bulletin 23(3): 458-462.

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