Why even ask? My purpose here is to try to pin down what "colony" and "organism" might mean. As with most terms, there are quite a few choices, once you start looking.
If someone speaks of, say, a city as an organism, there's a strong element of metaphor. Yes, a city can be said to collectively eat, and breathe, and even make decisions, but a city isn't actually an organism. It just has enough of the features of one to make for interesting comparisons and analogies.
On the other end, there are stands of aspen (and other plant species) that appear to be individual organisms, but are actually connected by a common root system and are genetically identical. Technically, this is a clonal colony, but we would generally think of each tree as an individual organism.
We might similarly think of a cluster of mushrooms as consisting of several organisms, but in fact mushrooms are just reproductive organs. It's the mycelium, a web of root-like structures in the soil, that carries on the day-to-day activities of a fungus, whether or not any mushrooms are evident. Since mushrooms are temporary structures, analogous to flowers on plants, and don't survive on their own, it seems reasonable to think of the mycelium and any attached mushrooms taken together as an organism.
On the other hand, trees are permanent structures and it's normal (depending on the species) to find a tree living independently, or next to other trees of the same species that aren't genetically identical. This probably makes it less intuitive to say that a stand of aspen is a single organism, so we hedge and say clonal colony.
Banyan trees are an interesting case. As their branches spread, they drop aerial roots, which eventually grow into the soil and support the further spread of the branches. Banyans can grow to cover several hectares (or several acres, if you prefer). Since everything is connected in plain sight, it's easy to speak of a single large tree, even though it may not be immediately obvious that all the "trunks" in what might seem to be a grove of youngish trees are actually roots of a single tree. If the aerial roots and the low branches they drop from were below the ground, though, would it then be a clonal colony?
To a large extent this is just a mater of nomenclature. What matters more is whether the pieces are connected or not, and whether they are genetically identical or not. All four combinations are possible:
- A banyan tree or aspen grove is connected, and the parts are genetically identical
- The trees in an apple orchard are separate but genetically identical. That is, they are clones (strictly speaking they collectively make up a clone -- we've been genetically engineering plants for millennia). It's also possible for a clonal colony like an aspen grove to be split into disconnected parts.
- Lichen -- which Wikipedia calls a "compound organism" -- is a symbiosis of a fungus and a photosynthetic partner, generally either an alga or a cyanobacterium. They are physically intertwined and the one could not survive without the other, but they are quite different genetically.
- Typical stands of forest consist of physically and genetically distinct trees, and this is the normal pattern for plants and animals that we distinguish as individuals.
Where does that leave our ant colony? Clearly ants are physically distinct. Genetically, the picture is a bit more complex. Ants, along with other hymenoptera and a few other species, are haplodiploid. Males carry only one set of chromosomes, rather than the usual two, while females carry both, because males develop from unfertilized eggs. Further, the queen of a colony generally mates with only one male over a given time period, and only one female in a colony (the queen) is fertile (or at least only a small portion of females are fertile). This has a number of interesting consequences:
- A male gets 100% of his genes from his mother
- A male has no father and cannot have sons, but does have a grandfather and can have grandsons (This one is worth working through in slow motion. All the clues are in the paragraph above)
- A female gets 50% of her genes from her mother and 50% from her father, as usual, but has 75% of her genes from the same source as her sisters and only 25% from the same source as her brothers.
- Lethal and highly harmful genes get weeded out quickly, since they'll kill off the males that carry them. With only one set of chromosomes, there's no place to hide.
"From the same source" is distinct from "the same". If the mother and father carry the same version of a gene -- the same allele -- then it doesn't matter which source it comes from. But if (to take a human example) mom has blond hair and dad has brown hair but a blond mother, then on average half the kids will have mom's blond hair, with a blond gene from both parents, and half will have dad's brown hair, with a blond gene from mom. They all have dad as the source of one of their sets of hair genes, but they don't all have the same hair genes from dad.
Selection cares about the variations, so it will tend to act the same on genetically identical individuals, and more and more differently on less related individuals. Workers in an ant colony are much closer to identical than ordinary siblings. This probably helps explain why ants and related species tend to be eusocial, that is, so socially cooperative that individuals will routinely act against their direct self-interest.
In particular, eusocial species typically have entire castes of sterile individuals. This makes no sense in the narrow sense of individuals competing to pass on genes, but more sense when you look at the overall picture of which genes are liable to survive. It's not as simple as a sterile soldier ant dying to save two of her sisters, though. If the sisters are also sterile, this makes no direct difference to which genes ultimately survive.
Probably being 75% related to one's sister makes it more likely that an altruistic behavior will take hold. That is, an instinct to protect the queen and eggs is more likely to work if one's relatives in the colony share it. Seems plausible, but the details are complex, and I haven't looked up what real biologists have to say on the topic. The question here is: If a fertile female has a large number of offspring, significantly more closely related than normal siblings, under what conditions are the queen's genes (and her consort's) more likely to be passed on by children who mostly forego reproducing in favor of one or a few fertile siblings, as opposed to by children who look after themselves?
In any case, haplodiploid genetics don't explain naked mole rats, which are genetically normal rodents, but eusocial nonetheless. But there can be multiple causes for the same effect. Naked mole rats are the only known eusocial mammals (or non-insects, I believe). Perhaps they just happened to be the one diploid organism that developed eusocial behavior far enough for it to remain stable.
Besides being head-hurtingly counter-intuitive to reason about, haplodiploidy, or anything that tends to make behavior more uniform and focused on protecting a small group of fertile individuals and their eggs, tends to make the group look less like a bunch of individuals and more like a single organism. And I think that's probably where we have to leave the original question. An ant colony is just that: a colony of individuals which, collectively, has some qualities analogous to those of an organism, and has more of those qualities than groups in many other species. It is not, however, an organism per se.
But just what is an organism? In particular, what is a multi-celled organism? Leaving aside the question of the microbiome -- the microbes living on and inside us that are nearly as different from us genetically as can be, and collectively outnumber our own cells handily -- a multicellular organism is a collection of individual cells, genetically identical (with exceptions like the germ cells -- sperm and egg -- which have a single set of chromosomes instead of a pair).
Most individual cells have specialized roles, and most of these cells are limited reproductively. In most cases they can divide and reproduce, but not without limit, or at least not in a healthy organism. Real reproduction, at least in sexually-reproducing organisms. is handled by a small set of germ cells which the other cells, it may be said, act to protect.
In particular, eusocial species typically have entire castes of sterile individuals. This makes no sense in the narrow sense of individuals competing to pass on genes, but more sense when you look at the overall picture of which genes are liable to survive. It's not as simple as a sterile soldier ant dying to save two of her sisters, though. If the sisters are also sterile, this makes no direct difference to which genes ultimately survive.
Probably being 75% related to one's sister makes it more likely that an altruistic behavior will take hold. That is, an instinct to protect the queen and eggs is more likely to work if one's relatives in the colony share it. Seems plausible, but the details are complex, and I haven't looked up what real biologists have to say on the topic. The question here is: If a fertile female has a large number of offspring, significantly more closely related than normal siblings, under what conditions are the queen's genes (and her consort's) more likely to be passed on by children who mostly forego reproducing in favor of one or a few fertile siblings, as opposed to by children who look after themselves?
In any case, haplodiploid genetics don't explain naked mole rats, which are genetically normal rodents, but eusocial nonetheless. But there can be multiple causes for the same effect. Naked mole rats are the only known eusocial mammals (or non-insects, I believe). Perhaps they just happened to be the one diploid organism that developed eusocial behavior far enough for it to remain stable.
Besides being head-hurtingly counter-intuitive to reason about, haplodiploidy, or anything that tends to make behavior more uniform and focused on protecting a small group of fertile individuals and their eggs, tends to make the group look less like a bunch of individuals and more like a single organism. And I think that's probably where we have to leave the original question. An ant colony is just that: a colony of individuals which, collectively, has some qualities analogous to those of an organism, and has more of those qualities than groups in many other species. It is not, however, an organism per se.
But just what is an organism? In particular, what is a multi-celled organism? Leaving aside the question of the microbiome -- the microbes living on and inside us that are nearly as different from us genetically as can be, and collectively outnumber our own cells handily -- a multicellular organism is a collection of individual cells, genetically identical (with exceptions like the germ cells -- sperm and egg -- which have a single set of chromosomes instead of a pair).
Most individual cells have specialized roles, and most of these cells are limited reproductively. In most cases they can divide and reproduce, but not without limit, or at least not in a healthy organism. Real reproduction, at least in sexually-reproducing organisms. is handled by a small set of germ cells which the other cells, it may be said, act to protect.
You don't have to squint very hard to see this as similar to the case of a eusocial colony. To be sure, there are some important differences. Cells in a multicellular organism are basically 100% related. They are generally unable to survive on their own for any significant length of time. They tend to reproduce in a fairly well-established pattern. That is, the organism grows coherently, and consistently from generation to generation.
Should we consider a multi-celled organism really to be a colony of one-celled organisms? Well, that's one way to look at it, but because those cells act so coherently and consistently, and because they're simple units (Shh! Don't mention mitochondria and other organelles!), and they're not viable on their own, and I'm sure for a number of other reasons, it's not useful to push this too far, much less claim that's "really" what's going on.
Nonetheless, I think it's still a useful comparison to study.
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