A phenotypic view of evolution Evolution in Structured Populations

What is an individual (Part 1)

It seems to me that the bad-boy question of evolution that sneaks around the edges, and nobody seems to notice is what is an individual? Think about it: My definition of evolution is change in the distribution of a population due to the gain or loss of individuals. I use “individuals” as if that were a known and obvious entity. I am not alone in that. Futuyma defines evolution in terms of “individual organisms”. Other writers are less explicit, but the implication is that evolution is defined in terms of individuals, which are assumed to be organisms.

Normally, our concept of what an individual is makes sense. That is, we see a human: that is an individual. We see an oak tree: that is an individual. As usual in biology, most of the time our intuition isn’t bad. Where it falls apart is when we get to the fuzzy edges. When we see two identical twins, are they one or two individuals? What about a stand of aspen trees (which clonally reproduce), or even worse, a huge fungus that covers 2,384 acres (http://www.scientificamerican.com/article/strange-but-true-largest-organism-is-fungus/)?


The worlds largest organism is a fungus, but is it an individual? (From http://www.scientificamerican.com/article/strange-but-true-largest-organism-is-fungus/ )

So, at the macro level, we have the question of whether genetic identity is enough to make two things (twins, a cutting off of a plant) the same individual, or do they need to be physically connected. Either answer raises a bunch of issues. For example, if we only require genetic identity, then I can take a cutting of your Christmas cactus, and take it with me to the other side of the world, and they are still the same individual, even though they have absolutely no contact with each other. Similarly two identical twins would probably be at least slightly offended to learn that they were the same individual.

Lindsey Lohan twins

Are they one individual or two. In this case, we can definitively answer the question, it is one individual, Lindsey Lohan, playing identical twins, but in cases of real identical twins the answer may not be as obvious. (from http://une10.blogspot.com/2011/08/visboo_29.html)

The other possibility is also difficult. With aspen clones some ramets (individual trees) are connected, others may not be. Thus, we really cannot know how many individuals are present without excavating them.

Aspen Clones[6]

How many individual aspens are in this picture? Is it three or is it hundreds, or somewhere in between? (http://watchingtheworldwakeup.blogspot.com/2008_09_01_archive.html)

It becomes even more difficult if we consider the other side of the equation. That is, it is tempting to define an individual as perhaps a set of genetically identical cells of the same species, perhaps requiring them to be physically in contact, and physically separated from other sets of cells. This is a made up on the spot definition probably won’t satisfy anybody, but it is close to the intuitive notion. However, this definition has a host of problems. Starting at an issue that nobody will lose sleep over: are mitochondria part of the individual? Mitochondria are a symbiosis between two cells, presumably an Archea like host cell and a Bacteria like symbiont, that has coevolved to the point that we no longer consider them separate entities. Instead the coevolution is so complete that we consider them to be their own (synthetic?) kingdom Eukarya.

eucaryote origins

The well established endosymbiotic theory of the origin Eukarya. (from http://www.nature.com/scitable/topicpage/eukaryotic-cells-14023963).

That is something of a straw man argument, but consider that in our bodies well over half of the cells are not human cells. Included in that are some symbionts that are passed from parent to offspring at or before birth, some that are acquired through social interactions with family and other important people in our lives, and finally some are acquired from the environment (I mean “symbiont” in the classic sense that includes both mutualists and parasites). Importantly, some are life-time residents, such as much of our gut fauna, and intra-cellular parasites, and others, such as pathogens, are temporary visitors using our bodies as environments in which to breed.


“Wolbachia bacteria, green, infect the ovaries of the malaria-transmitting mosquito Anopheles stephensi.” Wolbachia are maternally transmitted, and thus effectively evolutionarily similar to mitochondria. Should they be considered part of the individual? (http://www.npr.org/blogs/health/2013/05/08/182339563/using-bacteria-to-swat-malaria-inside-mosquitoes)

In short, it seems that the boundaries of what is an individual, or even an organism start to become fuzzy when we start poking at the edges. We should hardly be surprised by this. After all, one of the hot topics in biology has been the issue of “major transitions”, which, for example, Maynard-Smith felt was sufficiently important that it deserved to be the subject of one of his last books, and (shameless self plug here) there has been a recent a volume on evolution and emerging individuality edited by Bouchard and Huneman. If individual cells evolved for group living, then eventually became an “individual” then we should expect our concept of individuality and organismality to be difficult, this is simply the nature of biology.

A final issue that we have to consider is that if that transition from groups to individuals is actually took place in the past, we should be seeing that occurring in the present as well. This goes both directions. Going downward, when should we consider a cell in a metazoan to be an individual? And going upward, when should we think of colonies of insects as individuals? Should we ever consider herds of mammals, or communities of humans as individuals?

lazy or sick

We absolve the man on the left for staying in bed calling him “sick”, whereas we blame the man on the right for being “lazy”. In both cases the result is the same: they are lying in bed, only in the second we assign causality to the person. ( left: http://donlonpharmacy.com/flu-season-is-not-over/ right: http://www.cc-chapman.com/2011/social-media-has-made-you-lazy/ )

There are a lot of questions surrounding what we mean by an “individual”, however, interestingly, we have a good intuitive notion about what an individual is. Consider the example I use in my chapter in the Bouchard and Huneman book in which I imagine two people, one who is in bed because they have the flu, and one who is in bed because they don’t feel like getting up. The first person we might excuse because they are sick with the flu, and we do not assign blame to them, since they are “sick”. We might label the second as being “lazy”, and blame them for their failure to get up, or in other words, we assign the choice not to get out of bed to them as an individual. In thefirst case we assign blame to the disease, and absolve the individual, in the second we assign blame directly to the individual. In both cases the outcome is the same: The person stayed in bed. The question becomes why is laziness a property of the individual in the second case, but not the first? I think that the phenotypic perspective, and multilevel selection have a lot to say about this question, and it is something I will be exploring in the next week or two.

Join the Conversation


Your email address will not be published.

Skip to toolbar